Effect of Fertilizer Rate, Substrate, and Container Type on Greenhouse Production of Sandhill Milkweed

Authors:
Gabriel Campbell-Martínez University of Florida, West Florida Research and Education Center, 5988 Highway 90, Building 4900, Milton, FL 32583, USA

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Mack Thetford University of Florida, West Florida Research and Education Center, 5988 Highway 90, Building 4900, Milton, FL 32583, USA

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Debbie Miller University of Florida, West Florida Research and Education Center, 5988 Highway 90, Building 4900, Milton, FL 32583, USA

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Sandra Wilson University of Florida/Institute of Food and Agricultural Sciences, PO Box 110670, Gainesville, FL 32611, USA

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Celina Gómez University of Florida/Institute of Food and Agricultural Sciences, 2543 Fifield Hall, PO Box 110670, Gainesville, FL 32611, USA

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Abstract

Sandhill milkweed [Asclepias humistrata (Walter)] is important for monarch butterfly [Danaus plexippus (L.)] conservation efforts, yet precise cultivation practices are largely not available. We tested the effects of three fertilizer rates and four substrate types and four container types on the performance of sandhill milkweed during greenhouse production. Seedlings fertilized with a high (0.90 g per 48-cell container) controlled-release fertilizer rate of 15N–3.9P–10.0K (15–9–12 Osmocote® Plus) had reduced performance compared with low and medium fertilizer rates (0.34 and 0.56 g per 48-cell container, respectively). Seedlings grown in large containers (∼175 mL including standard 32-cell liners and tall tree-tubes) outperformed seedlings grown in small containers (∼100 mL including standard 48-cell liners and short tree tubes). A transplant ready plant can be produced for spring within 16 weeks when seeds are sown in early January. Although sandhill milkweed seedlings can be grown under various fertilizer rates and in various containers and substrates, seedlings grown in tall tree tubes in a peat-based mix (Sunshine Mix) outperformed a nursery standard substrate and two wood fiber substrates. We recommend growing plants in a peat-based substrate within tall tree tube containers and applying a medium fertilizer rate.

Milkweed plants (Asclepias) are herbaceous perennials of high importance in monarch butterfly (Danaus plexippus) conservation, butterfly and pollinator gardens, and ecosystem restoration planting projects (Borders and Lee-Mäder 2014). More than 130 milkweed species are native to North America, and all begin growth in the spring and senesce during the fall, emerging each year from below-ground carbohydrate-rich storage tissues (Fishbein et al. 2011; Woodson 1954). Milkweed populations are in decline throughout North America for several reasons, including habitat loss and herbicide use (Boyle et al. 2019; Hartzler 2010; Hartzler and Buhler 2000; Pleasants and Oberhauser 2013). Approximately 21% of eastern US milkweed stems, ∼1.49 billion stems, disappeared between 1995 and 2013 (Flockhart et al. 2015).

Milkweed are vital for the first generation of specialist eastern populations of North American monarch butterfly larvae that feed mainly on their leaves throughout the United States and Canada during their annual migration from central Mexico in the spring (Brower 1961; Malcolm et al. 1987; Thogmartin et al. 2017). Monarch butterfly populations are in decline, in part because of the reduction of milkweed populations throughout North America (Boyle et al. 2019; Flockhart et al. 2015; Wilcox et al. 2019). For example, during the last decade, the eastern population of monarch butterflies has diminished by 80% (Flockhart et al. 2015; Semmens et al. 2016). Additionally, monarch populations are in decline because of environmental stresses, a lack of availability of nectar plants and quality overwintering sites, and infections from parasites (Agrawal 2017; Agrawal and Inamine 2018; Inamine et al. 2016; Ries et al. 2015). Models indicate that mass replanting is necessary to restore milkweed populations and the monarch butterflies that rely on them (Thogmartin et al. 2017).

Tropical milkweed [A. curassavica (L.)], native to South America, is the most widely commercially available milkweed currently on the North American market and has become a widespread weed in the southeast (Borders and Lee-Mäder 2014; Faldyn et al. 2018; Satterfield et al. 2015). Widespread availability of tropical milkweed is attributable to its ease of propagation and production compared with native milkweed, which are known to be more difficult to cultivate and require longer to produce sellable plants (Borders and Lee-Mäder 2014). However, the use of tropical milkweed for monarch butterfly conservation has become a contentious subject, with increasing evidence of the negative effects it has on monarch butterflies (Faldyn et al. 2018). For example, the planting of tropical milkweed reduces monarch butterfly migration and encourages year-round breeding for eastern North America monarch butterfly populations, which increases the occurrence of the protozoan parasite OE [Ophryocystis elektroscirrha (McLaughlin & Myers)] (Batalden and Oberhauser 2015; Satterfield et al. 2015). Migration is discouraged because tropical milkweed remains aboveground longer than native milkweed in northern areas and performs as an evergreen in areas of the southeast (Batalden and Oberhauser 2015). Although common recommendations include cutting tropical milkweed to 2.5-cm-tall stems during winter to discourage year-round monarch butterfly breeding (Monarch Joint Venture), this did not reduce disease prevalence for monarch butterfly populations and, thus, may not be a viable disease prevention measure (Satterfield et al. 2016).

Given the over-reliance of propagating A. curassavica in conjunction with the potential negative effects on monarch populations, it is necessary to diversify the milkweed species used for monarch butterfly conservation efforts and restoration plantings. The market availability of North American native milkweed is currently limited, and information regarding the greenhouse propagation and production of milkweed is lacking within the literature for most species (Borders and Lee-Mäder 2014; Florida Association of Native Nurseries 2020; Landis 2014). One such potential candidate milkweed species for commercial production is sandhill milkweed. It is a nonclonal herbaceous perennial native to sandy, dry, nutrient-poor soils throughout the Southeast (Radford et al. 1964; US Department of Agriculture-Natural Resource Conservation Service 2021; Wunderlin and Hansen 2011). It is considered one of the most important southern milkweed for monarch butterflies, which use this plant as it emerges in the spring (Brower 1961, 1962; Brower et al. 2018; Cohen and Brower 1982; Daniels et al. 2018; Knight et al. 1999; Malcolm et al. 1987; Martin et al. 1992; Zalucki and Brower 1992). It also has exceptional ornamental value because of its purple leaves and striking inflorescences. Sandhill milkweed is difficult to find commercially, despite its potential widespread use (Florida Association of Native Nurseries 2020). Seed germination requirements and dormancy characteristics for sandhill milkweed are known. Seeds are nondormant when shed from the mother plant, germination is improved in dark compared with light (12-h photoperiod), and optimal germination temperatures range between constant temperatures of 24 to 28 °C and simulated summer (33/24 °C) and fall (29/19 °C) temperatures (Campbell 2016; Campbell-Martínez et al. 2017). However, information regarding greenhouse cultivation and production timing for the growth of liners or transplants for spring planting is lacking for sandhill milkweed. We conducted two independent experiments testing the effects of the fertilizer rate (Expt. 1) and substrate type and container type (Expt. 2) on the greenhouse performance (survival and growth) of sandhill milkweed.

Materials and Methods

Two experiments were conducted within a lexan-covered greenhouse at the West Florida Research and Education Center (WFREC) in Milton, FL, USA, beginning on 29 Jan 2020 (Expt. 1) and 28 Jan 2020 (Expt. 2). Initial seedling production methods were identical for both experiments before the initiation of fertilizer (Expt. 1) and substrate and container type (Expt. 2) treatments. Two brown sandhill milkweed seeds >0.5-cm-wide Campbell-Martínez et al. (2017) collected from central Florida were placed on top of substrate and lightly covered (∼2 mm) with fine vermiculite until the substrate and seeds were no longer visible. Trays containing substrate and seeds were placed on propagation benches with intermittent mist (10 s every 10 min from 10:00 AM to 9:00 PM) for 56 d. Trays were moved to production benches and hand-watered via overhead irrigation until substrate saturation using a water breaker (Dramm 400 PL; Dramm Corp., Manitowoc, WI, USA) when ∼15% of the water within the substrate was used by the plants every 2 to 4 d. Seedlings were fertigated with 50 ppm and 100 ppm N of Jack’s 20–20–20 Professional Water Soluble Fertilizer (20N–8.7P–16.6K; JR Peters Inc., Allentown, PA, USA) on 15 Mar 2020 and 24 Mar 2020, respectively, before the application of controlled-release fertilizer treatments on 29 Mar 2020, when seedlings were 8 weeks old. Disease, pests, and pathogens were controlled as described in Supplemental Table 1.

Expt. 1 (fertilizer rate).

Seeds were placed in containers (48-cell liners with a volume of 106 mL) (Table 1) with a 75% aged pine bark with fines (0.6 cm screened) and 25% MetroMix 830 (composed of peat, pine bark, and perlite; Sungro Horticulture, Agawam, MA, USA) substrate mix (volume/volume). The experimental design was a randomized complete block with a single factor (fertilizer rate) that had three levels, including low (0.34 g), medium (0.56 g), and high (0.90 g) rates of 15–9–12 controlled-release fertilizer (15N–3.9P–10.0K, 6 months at 70 °C) (Osmocote® Plus; Scotts Co., Marysville, OH, USA) top-dress application per container (3.2 g⋅L−1, 5.3 g⋅L−1, and 8.5 g⋅L−1, respectively). Five independent greenhouse benches within a single greenhouse were used as a blocking factor. Within each block (bench), there were four trays spaced 5 cm apart. One-third of each tray (four 4-cell packs) was randomly assigned one of three fertilizer rates with 16 plants per fertilizer rate. Fertilizer treatments were applied 8 weeks after sowing when seedlings had one to three true leaves. The average daily high and low temperatures in the greenhouse were 33.9 and 17.8 °C, respectively.

Table 1.

Summary of containers used to grow sandhill milkweed. Containers include industry standard liners and tree tubes with a deep soil profile conducive for use in restoration plantings.

Table 1.

Expt. 2. (substrate and container type).

The experiment was arranged in a randomized complete block design with a four (substrate type) × four (container type) incomplete factorial arrangement of treatments with all but one treatment combination tested. Substrates included a standard substrate composed of 75% aged pine bark with fines (0.6 cm screened) and 25% MetroMix 830 (Sungro Horticulture) and three perlite-free mixes including a 70% coir (FibreDust coir; FibreDust LLC., Cromwell, CT, USA) and 30% HydraFiber (Ultra 365WB fiber; Profile Products, Buffalo Grove, IL, USA) substrate, a 70% peatmoss (Berger BP-C; Berger, Quebec, Canada) and 30% HydraFiber substrate, and Sunshine Mix PF (peatmoss and naturally aged peatland aggregate; Sungro Horticulture) were tested; hereafter, they are referred to as the standard, HF coir, HF peat, and Sunshine Mix, respectively. All percentages given are by volume. A standard substrate containing perlite was used to represent a substrate with ingredients typically used by growers throughout the southeast, namely, peat, pine bark, and perlite. We investigated perlite-free substrates because perlite is viewed as pollution by land managers, may be harmful when ingested by wildlife, and becomes visible and mobile in areas with erosion (Ghale-Kandi et al. 2010). Four container types including a 48-cell liner and short tree tubes (small containers) and 32-cell liner and tall tree tubes (large containers) were tested. The small and large containers were approximately equivalent in volume (∼100 mL and ∼175 mL, respectively) but differed in their surface area and profile depth, with tree tubes having a deeper profile than standard liners (Table 1). The only substrate × container type combination not tested was short tree-tubes with Sunshine Mix substrate, hence the incomplete factorial design. Five independent greenhouse benches (blocks) within a single greenhouse were used as a blocking factor. Within each bench, a single tray (liners) or half-tray (tree tubes) containing 32 (liners) and 35 (tree tubes) subsamples, respectively, were used. For 32-cell liners, trays were filled to capacity, whereas four four-packs and two rows of tubes were removed from each tray for 48-cell liners and tree tubes, respectively. The experiment lasted a total of 16 weeks. The average daily high and low temperatures in the greenhouse were 34.0 and 17.7 °C, respectively.

Substrate characterizations.

For both experiments, physical and chemical characteristics analyses were conducted using a single sample for each substrate by an independent analytical facility (Micro Macro International Inc., Athens, GA, USA) (Supplemental Tables 2 and 3).

Data collection.

Emergence, defined as one or more seedling hypocotyls visible above substrate per container, was recorded 8 weeks after seeding and presented as the percentage calculated per tray for Expt. 2. Emergence was not recorded for Expt. 1 because the treatments were applied postemergence. The numbers of containers with plants that had aboveground photosynthetic tissue (shoots) were counted and survival (%) was calculated 16 weeks after planting for both experiments. Height was determined to be a representative measure of aboveground growth for this plant rather than plant width, leaf width, node number or stem number per plant (Campbell 2020). Height was measured for the tallest seedling per container after 12 and 16 weeks for Expt. 1, and after 16 weeks for Expt. 2. Visual quality was recorded after 16 weeks using a scale of 1 to 3, where 1 represented a highly stressed plant with yellow to white leaves (highly chlorotic), 2 represented a moderately stressed plant with yellow–green leaves (mildly chlorotic), and 3 represented a healthy plant with green to purple leaves for Expt. 2 (Fig. 1). A visual quality of 2.5 or higher was considered acceptable quality for commercial production.

Fig. 1.
Fig. 1.

Healthy (left), moderately nutrient stressed (center), and highly nutrient stressed (right) sandhill milkweed. Healthy plants had green to purple leaves, moderately stressed plants had yellow–green leaves, and highly stressed plants had yellow to white leaves; they were assigned values of 3, 2, and 1, respectively.

Citation: HortScience 58, 10; 10.21273/HORTSCI17118-23

A subset of experimental units (plant material in single cell or tube) for each treatment combination (n = 16) was randomly selected for further analysis via destructive harvesting. Plants were removed from containers and root class was determined based on the quantity of roots visible on the root ball surface, where 1 = no roots, 2 = 0.1% to 1%, 3 = >1% to 3%, 4 = >3% to 13%, and 5 = >13% root coverage of the root ball. Plants with a root class of 5 were considered of good quality because roots kept an intact root ball when removed from the container. Then, substrate was gently removed from roots, and tuberous root (hereafter referred to as tubers) widths (mm) were recorded. The aboveground (shoots) and belowground (tubers and roots) plant parts were separated, dried at 60 °C for 72 h, and weighed to determine dry weights (mg). We calculated the ratio of the shoot weight to the sum of the tuber and root weights.

Plant tissue and substrate pH and electrical conductivity characterization.

A plant tissue analysis was conducted by an independent testing facility (University of Florida, Institute of Food and Agricultural Sciences Extension, Gainesville, FL, USA). The plant tissue analysis included the determination of the concentrations (%) of macronutrients (N, P, K, Ca, Mg), micronutrients (mg⋅kg−1) (Fe, Mn, B, Cu, Zn, and Mo), and other nutrients (mg⋅kg−1) (Ni, Na, Al, Ba Si, and Pb). The aboveground (shoots) and belowground (tubers and roots) portions were analyzed separately for plants of each fertilizer rate in Expt. 1. For Expt. 2. A tissue analysis was conducted for each of the three visual quality ratings for plants grown in HC Coir substrate because this was the only substrate that produced enough plant material of each rating for analysis. Aboveground and belowground tissues were analyzed separately for healthy and mildly chlorotic plants, whereas total tissue (aboveground and belowground) was analyzed for tissue of extremely chlorotic plants because of an insufficient amount of material to provide separate samples.

The pH and electrical conductivity (EC) of substrate leachates were recorded for 16 replicates (individual tree tube or cell) of each fertilizer type (Expt. 1) and substrate and container type combination (Expt. 2) at 0, 12, and 16 weeks after seeding. Values were recorded using the pour-through method and a hand-held pH and EC meter (HI 9813-6N; Hanna Instruments, Woonsocket, RI, USA), as described by Nemali (2018). The standard substrate used in Expts. 1 and 2 was identical; therefore, the same results apply to both experiments.

Statistical analyses.

Main effects and their interactions for Expts. 1 and 2 were analyzed using generalized linear mixed models (PROC GLIMMIX in SAS 9.4). Model assumptions including normality, independence, and constant variance were checked before the analysis. A Kenward-Rogers approximation was used for computing the denominator df for the fixed effects tests. Block, number of seedlings per container, and position of the plant within the tray (on edge or inside) were coded as random effects. When necessary, repeated observations of the same experimental unit were accounted for in the model using the subject statement. Significant differences (α = 0.05) between means were computed using the ilink option of the LSMEANS statement.

Results

Substrate characterizations.

All substrates tested had moisture content ranging from 64% to 77%, and air-filled porosity, total porosity, and container capacity within or nearly within (≤5% or ≥5%) acceptable ranges defined by Yeager et al. (2007) (Supplemental Tables 2 and 3). For the standard substrates used in both experiments, the pH and EC were low (4.7 and 0.35 mS⋅cm−1).

Expt. 1 (fertilizer rate).

The fertilizer rate affected survival [F(2,23.31) = 6.45; P = 0.0059], but the date of evaluation did not. Survival was lower for plants fertilized with a high fertilizer rate (94% ± 1%) compared with medium (99% ± 1%) or low (98% ± 1%) rates, although survival was >90% for plants in all treatments. Plant height was only affected by fertilizer rate and date of evaluation [F(2,586.5) = 12.55, P < 0.0001 and F(1,583.7) = 381.40, P < 0.0001, respectively]. Plant height increased over time. At the end of the experiment, taller plants were recorded for medium and high fertilizer rates (4.7 and 5.2 cm) than for low rates (4.6 cm) (Fig. 2).

Fig. 2.
Fig. 2.

The mean (±SE) height of sandhill milkweed fertilized with low (0.34 g/plant), medium (0.56 g/plant), or high (0.90 g/plant) manufacturer recommended rates of 15N–3.9P–10.0K controlled-release fertilizer (15–9–12 Osmocote® Plus 6 mo. at 70 °C; Scotts Co.) at 12 and 16 weeks after seeding (4 and 8 weeks after fertilizer application) in 48-cell (107 mL) liners with a 75% aged pine bark with fines (0.6 cm screened) and 25% MetroMix830 substrate. Significant differences (α = 0.05) are indicated using lowercase letters.

Citation: HortScience 58, 10; 10.21273/HORTSCI17118-23

The fertilizer rate did not affect root class or tuber width, which ranged from 2.1 to 2.5 cm and 4.4 to 5.0 mm, respectively. The fertilizer rate did not affect total and shoot dry weights, which ranged from 280 to 336 mg and 146 to 193 mg, respectively. The fertilizer rate effected the root dry weight [F(2,44.02) = 3.60; P = 0.0356], which was larger for plants fertilized with a medium rate compared with those fertilized with a high rate (15 and 30 mg, respectively). The ratio of shoots to roots and tubers was unaffected by the fertilizer rate and was 1.1 to 1.5 (Table 2).

Table 2.

Dry weights and shoot to tuber and root ratio per plant of sandhill milkweed plant parts grown in 48-cell (107 mL) liners and fertilized with various rates of controlled-release fertilizer. Means (SE) are presented.

Table 2.

Aboveground (shoot) N concentrations were 2.7%, 3.0%, and 4.0%, and belowground (tuber and root) N concentrations were 2.4%, 3.0%, and 4.7% for plants fertilized with low, medium, and high fertilizer rates, respectively (Supplemental Table 4). The concentrations of N, P, Ca, and Mg were above and that of K was below recommendations described by Bennett (1993). As the fertilizer rate increased, so did tissue N concentrations. The relative concentrations of P, K, Ca, and Mg in tissues were similar across fertilizer rates.

The pH of substrates was within the recommended range (4.5–6.5) (Yeager et al. 2007) throughout the experiment (Supplemental Table 5). EC was below and within the recommended range (0.8–1.5 mS⋅cm−1) (Yeager et al. 2007) for low and medium fertilizer rates (0.46–0.55 mS⋅cm−1) and high rates, respectively (Supplemental Table 5).

Expt. 2 (substrate and container type).

Only container type influenced seedling emergence [F(3,56) = 2.20; P = 0.0012]. Emergence did not differ for 48-cell liners (59% ± 2%), 32-cell liners (53% ± 2%), and tall tree tubes (52% ± 2%), whereas emergence in short tree tubes (47% ± 2%) differed only from emergence for 48-cell liners. Only substrate and container type influenced seedling survival [F(3,60) = 9.11, P < 0.0001 and F(3,60) = 4.31, P = 0.0081, respectively]. Survival was higher for seedlings in the standard and Sunshine Mix (90% ± 2% and 91% ± 3%) compared with HF peat (68% ± 4%), whereas survival for seedlings in HC coir (81% ± 5%) was similar to that of all substrates. Survival was greater for seedlings in tall tree tubes compared with 48-cell liners (93% ± 2% and 78% ± 4%), whereas survival in short tree tubes (79% ± 5%) and 32-cell liners (78% ± 5%) was similar to that of all container types.

Substrate, container type, and their interaction affected height [F(3,1103) = 45.55, P < 0.0001; F(3,1103) = 42.65, P < 0.0001; and F(8,1103) = 13.12, P < 0.0001, respectively]. The tallest plants produced were in tree tubes with Sunshine Mix (7.4 cm) and HF Peat (6.8 cm) substrates. Plants grown in the standard mix were taller in 32-cell liners (5.5 cm) than in small containers (4.2–4.5 cm). Plants grown in the standard mix in tall tubes were of similar height (4.9 cm) compared with all standard mix containers (Fig. 3). Plants grown in all other substrates using large containers were similar in height or taller than plants grown in the standard mix. Among plants grown in small containers, only those grown in short tubes with HF Peat (6.1 cm) were taller than plants grown in the standard mix using tall tree tubes (4.9 cm).

Fig. 3.
Fig. 3.

The mean (±SE) height of sandhill milkweed grown in four different substrates and four different container types at 16 weeks after seeding. Substrate types included a standard with perlite and three perlite-free substrates. Container types included standard liners and tree tubes with a deeper soil profile of small (∼100 mL) and large (∼175 mL) sizes. Significant differences (α = 0.05) are indicated using lowercase letters.

Citation: HortScience 58, 10; 10.21273/HORTSCI17118-23

Visual quality ratings of plants were affected by substrate, container type, and their interaction [F(3,1108) = 68.14, P < 0.0001; F(3,1108) = 6.18, P = 0.0004; and F(8,1108) = 2.71, P < 0.0058, respectively]. Visual quality ratings for plants in the standard mix met or exceeded the threshold of 2.5 (Fig. 4). Similar visual quality was also recorded for plants grown in HF Peat and Sunshine Mix, whereas visual quality for plants grown in HF Coir was consistently below the threshold of 2.5. Except for plants grown with HF Coir and those grown in 48-cell liners with HF Peat, visual quality for plants grown in both large and small container types met or exceeded the threshold of 2.5. Plants with healthy visual quality had less N but similar P and K concentrations compared with mildly stressed plants (Supplemental Table 6).

Fig. 4.
Fig. 4.

The mean (±SE) visual quality of sandhill milkweed grown in four different substrates and four different container types at 16 weeks after seeding. A horizontal dotted line is provided at a visual quality of 2.5 to represent a threshold for a marketable product. Substrate types included a standard with perlite and three perlite-free substrates. Container types included standard liners and tree tubes with a deeper soil profile of small (∼100 mL) and large (∼175 mL) sizes. The visual quality scale ranged from 1 to 3, where 3 indicated a healthy plant with green to purple leaves, 2 indicated a moderately stressed plant with yellow–green leaves, and 1 indicated a highly stressed plant with yellow to white leaves. Significant differences (α = 0.05) are indicated using lowercase letters.

Citation: HortScience 58, 10; 10.21273/HORTSCI17118-23

The visual assessment of root class was only affected by the container type [F(3,147.3) = 6.62; P = 0.0003]. The root classes for seedlings grown in large containers (2.6 ± 0.2 and 2.8 ± 0.2 for 32-cell liners and tall tree-tubes, respectively) were higher than those of seedlings grown in small containers (1.8 ± 0.1 and 1.8 ± 0.2 for 48-cell liners and short tree tubes, respectively). The tuber width of seedlings was only affected by substrate and container type [F(3,143.1) = 4.49, P = 0.0048 and F(3,142.7) = 8.13, P < 0.0001, respectively]. Tuber widths were similar in standard mix, HF Coir, or HF Peat substrates (4.3 ± 0.16 mm, 3.9 ± 0.24 mm, and 4.5 ± 0.23 mm, respectively). Seedlings grown in Sunshine Mix had the widest tubers (5.2 ± 0.25 mm), but these were not different from those of plants grown in the HF Peat substrate. Tubers were wider in large containers (5.0 ± 0.20 and 5.0 ± 0.25 mm for 32-cell liners and tall tree tubes, respectively) than in small containers (4.0 ± 0.18 and 3.4 ± 0.23 for 48-cell liners and small tree tubes, respectively).

Substrate and container type affected the total [F(3,146.5) = 9.46, P = <0.0001 and F(3,146) = 10.51, P = <0.0001], shoot [F(3,146.1) = 11.40, P = <0.0001 and F(3,145.3) = 8.43, P = <0.0001], and tuber [F(3,145.4)= 5.73, P = 0.0010 and F(3,145) = 16.43, P = <0.0001] dry weights, whereas root dry weight was only influenced by the substrate type [F(3,148.5) = 2.97; P = 0.0340]. The total dry weight was greater for plants grown in Sunshine Mix (604 mg) compared with all substrates (Table 3). Dry weights were similar for standard and HF substrates (305 to 392 mg) (Table 3). Dry weights of individual tissues followed a similar pattern with one exception: tuber dry weight for plants grown in the standard (146 mg) and Sunshine Mix (176 mg) substrates were similar. Plants grown in large containers had similar total dry weights (507 mg and 555 mg for 32-cell liners and tall tree tubes, respectively) that were greater than those of plants grown in small containers (294 mg and 231 mg for 48-cell liners and short tree tubes, respectively), which also had similar dry weights (Table 3). The shoot dry weight was larger but similar among the large container types (299 mg and 291 mg for 32-cell liners and tall tree tubes, respectively) compared with the small container types (154 mg and 131 mg for 48-cell liners plug trays and short tree tubes), which were also similar. Tuber dry weight was greatest for tall tubes (201 mg). Tuber dry weights were greater for 32-cell liners (151 mg) than for small containers (94 mg and 73 mg for 48-cell liners and short tree tubes, respectively), which were similar. Root dry weights were similar among container types (27 mg to 36 mg) (Table 3). Only substrate affected the ratio of shoots to tubers and roots [F(3,146.1) = 7.19; P = 0.0002]. The ratio was lower for plants grown in the standard substrate (0.88) than for those grown in the HF Coir and Sunshine Mix substrates (1.34 and 1.57) and was similar to that of plants grown in the HF peat substrate (1.23) (Table 3).

Table 3.

Dry weights and shoot to tuber and root ratio per plant of sandhill milkweed grown in four substrates and four container types. Means (SE) are presented.

Table 3.

The initial (0 weeks after planting) pH of substrate leachates (6.0–6.5) were within the recommended ranges (4.5–6.5) (Yeager et al. 2007) for all substrates except HF Coir (7.1) (Supplemental Table 7). Substrate pH leachate was near or above (6.3–7) the recommended ranges at the end of the experiment. The ECs of substrate leachates at weeks 0 and 8 (before fertilizer application) were near or below the recommended range of 0.8 to 1.5 mS⋅cm−1 (Supplemental Table 8) (Yeager et al. 2007). Peak EC values were observed for all substrates at 12 weeks after planting. The EC was <0.8 mS⋅cm−1 for all substrates (1.9 to 2.7 mS⋅cm−1) except the standard. The substrate leachate EC was below the recommended values for all treatments at the end of the experiment.

Discussion

Survival of sandhill milkweed sown in the winter and grown in various containers and substrates was >75%, with one notable exception being for plants grown in HC Coir, indicating these production systems are economical for commercial wholesale greenhouse production. Although the fertilizer rate slightly reduced survival (94 vs. ≥98%), very high survival rates were achieved at all fertilizer rates. This contrasts with relatively low survival rates during preliminary trials and anecdotal observations of employees of Florida native plant nurseries (Bill Bissett, personal communication). The relatively low survival rate of sandhill milkweed during production is thought to be attributed to various greenhouse root pathogens, including Rhizoctonia [Rhizoctonia solani (Kuhn)], Pythium [Pythium (Pringsh.)], and black root rot [Thielaviopsis basicola (Berk. & Broome)], all of which have been documented to affect other milkweed (Borders and Lee-Mäder 2014).

An increased fertilizer rate from low (3.2 g⋅L−1) to medium (5.3 g⋅L−1) or low to high (8.5 g⋅L−1) resulted in 3% or 13% increases in the height of sandhill milkweed after 16 weeks of container production. This result was similar to, but less pronounced than, that reported by Hanson et al. (2017), who showed that two western North America native milkweed, showy milkweed [A. speciosa (Torr.)] and narrowleaf milkweed [A. fascicularis (Decne.)], grown in tree tubes (diameter, 3.8 cm; depth, 21.0 cm; volume, 164 mL) had ∼20% and ∼40% increases in height, respectively, in response to fertilizer rate increases from 2.7 to 6.5 g⋅L−1 using controlled-release fertilizer (Osmocote® 18–2.2–10 with minors, 6 mo.; applied at the time of seeding) after 6 weeks of container production. Unlike these same western milkweed, which had a root collar diameter increase of ∼20% at 6 weeks after sowing (Hanson et al. 2017), the similar measure of tuber width was unaffected by the fertilizer rate 16 weeks after sowing for sandhill milkweed. Showy milkweed exhibited a 900% increase in the proportion of plants with firm root plugs for plants with more fertilizer, whereas nearly all narrowleaf milkweed had firm root plugs regardless of the fertilizer rate at 22 weeks after seeding (Hanson et al. 2017). In the present study, the rate of fertilizer application had no effect on root class (similar to root firmness) for sandhill milkweed 16 weeks after seeding. For showy milkweed, dry weights of shoots and roots increased 38% and 44% in response to an increase in fertilizer, whereas dry weights of narrowleaf milkweed shoots increased 33% and dry rates of roots were unaffected by fertilizer rate (Hanson et al. 2017). Unlike these species, in the present experiment, the fertilizer rate did not affect sandhill milkweed shoot weights, but a high fertilizer rate decreased the dry weight of roots compared with a medium fertilizer rate. There were no sandhill milkweed with reproductive tissue (i.e., flowers) by the end of both experiments after 16 weeks; however, Hanson et al. (2017) noted floral development for a small number of narrowleaf milkweed grown in tree tubes after 15 weeks. A lack of floral development could be attributed to the time of year because wild plants do not flower until several weeks after experiment termination (Wunderlin and Hansen 2011).

We documented that an increase in the fertilizer rate from medium to high did not generally increase dry weight production and, in some cases, reduced it (e.g., root production decreased for high fertilizer rates), although some increase in aboveground growth was noted. This indicates that a medium fertilizer rate achieves critical nutrient concentrations, and that plants fertilized with high rates undergo luxury consumption of nutrients (Epstein and Bloom 2005). However, the increase of some nutrient concentrations (e.g., N) in tissues of plants fertilized with higher fertilizer rates could have effects during restoration plantings that are not apparent during production. Puértolas et al. (2012) documented no effects on the growth of two pine (Pinus L.) species with increased fertilizer rates during production in deep liner trays (volume of 150 mL and height of 12 cm); however, they documented a four-times increase in survival for both species during a forest restoration project.

The effects of fertilizer on Milkweed growth are limited; however, the effects of fertilizer on growth of closely related plants have been documented. Plants of milkwood [Tabernaemontana pachysiphon (Stapf) Apocynaceae] fertilized with 3 kg⋅m−3 or 6 kg⋅m−3 controlled-release fertilizer 18N–2.6P–9.9K (Osmocote® 18–6–12) had a similar leaf N concentration (∼3.5%–4.0%) compared with sandhill milkweed grown with high fertilizer rates, which had a 4% leaf and stem N concentration, whereas plants fertilized with low and medium rates had lower (2.7%–3%) leaf and stem N concentrations (Höft et al. 1996). Plants of dogbane (Apocynum cannabinum L. Apocynaceae) and apple of Sodom [Calotropis procera (Aiton) W.T. Aiton Apocynaceae] grown in old fields had an N concentration of ∼1% to 2% in leaves (Galal et al. 2016; Niesenbaum et al. 2006), which was less than the leaf and stem N concentrations at all fertilizer rates tested for sandhill milkweed. Leaves of dogbane also had similar P concentrations of ∼0.3 to 0.4%, approximately twice the K concentration (3.7 vs. 1.8 to–2.0%), six-times the Ca concentrations (4.2% vs. 0.7%–0.9%), and more than 10-times the Mg concentrations (4.2% vs. 0.2%–0.4%) compared with the leaves and stems of sandhill milkweed (Galal et al. 2016).

We successfully produced marketable containerized sandhill milkweed seedlings using industry standard liner plug trays and standard substrates containing perlite. We also produced marketable seedlings grown in containers with a deep soil profile and perlite-free substrates conducive for use in ecosystem restoration projects. This includes the use of tree tube containers that produce plants with a deep soil profile, which initially allows the plant more access to soil moisture during the establishment phase of restoration plantings compared with shallow rooted plants produced in standard liners (Landis et al. 2010; Luna et al. 2009). Similar to sandhill milkweed, other North American herbaceous perennials have also been grown in containers with a deep soil profile that have been successfully used in restoration planting projects (Moyes et al. 2005). Other herbaceous species within the market have been successfully grown using perlite-free, peat-based substrates and a mix of 70% peat and 30% wood fibers (Harris et al. 2020).

The majority of the belowground mass (70% to 90%) of sandhill milkweed is stored in large tuberous roots when grown in containers. These tubers are of particular importance for the sandhill milkweed in restoration projects because shoots may senescence and quickly regrow from tubers during restoration plantings (personal observation). Seedlings had the largest tubers (33%–175% larger) when grown in tall tree tubes, and seedlings grown in large containers had more tuber growth than those grown in small containers (61%–175% larger). This is similar to the observations of Hanson et al. (2017), who reported showy milkweed and narrowleaf milkweed had increased root growth in 164-mL compared with 444-mL containers. For aboveground growth, larger container sizes increased the growth of sandhill milkweed plants (30%–113% larger), similar to that reported by Hanson et al. (2017) regarding showy and narrowleaf milkweeds (Hanson et al. 2017).

All three perlite-free substrates had similar moisture contents, air-filled porosity, total porosity, and container capacity compared with the perlite-containing standard substrate. This, along with the greenhouse performance data herein, indicates that the perlite-free substrates tested could potentially replace standard substrates within liner and tree tube container production systems. However, the bulk density of perlite-free substrates was approximately half that of the standard substrate. More research including the testing of perlite-free substrate performance for growing native plants for use in ecosystem restoration projects is necessary.

One of the major drawbacks for growers when incorporating tree tubes in their production systems is the high initial cost of containers compared with that of standard liner trays. A cost analysis of container prices (and their necessary trays) used in Expt. 1 estimated 30 cents for short tree tubes and tall tree tubes when buying cases of product (i.e., a few thousand pots at one time). However, tree tubes are reusable, and growers often sell and/or ship this plant without the container. In contrast, standard liners are generally treated as single-use plastics within the horticultural industry and are rarely recycled. The initial high costs of tubes may be reduced if the tubes are reused, and if they are reused a dozen times or more, resulting in a cost similar or less than that associated with standard liners. Tree tube container sterilization is accomplished using cheap, simple, and quick methods, including warm water baths and bleach (Landis et al. 2010), allowing them to easily be reused. Additionally, tree tubes are more space-efficient than standard liners. A single tree tube tray holds 98 plants using the same bench space (26.7 cm × 53.0 cm) as the 48-cell and 32-cell liners (Table 1), resulting in increases of 104% and 206% plants per bench space for 48-cell and 32-cell liners compared with both small and large tree tubes.

Conclusion

We recommend growing plants in tall tree tubes with a peat-based substrate and applying a medium fertilizer rate when producing sandhill milkweed. However, plants can be grown in standard liners and substrates, especially when growing for ornamental use, when substrate ingredients and a deep soil profile are less crucial for project success. Sandhill milkweed sown in a heated greenhouse during winter produced transplant-ready containerized seedlings by late spring/early summer planting, with a total production time of 16 weeks from seed to transplant-ready plant. This production cycle produces a plant during locally recommended restoration planting times. In the future, we plan to test the effects of production systems on outplanting survival within a restoration context using the plants described herein.

References Cited

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Supplemental Table 1.

Insect and pathogen control products, application methods, and concentrations utilized during greenhouse container production of sandhill milkweed.

Supplemental Table 1.
Supplemental Table 2.

Physical properties for one standard substrate and three perlite-free substrates used to grow sandhill milkweed in greenhouse container production during Expt. 1 (standard substrate) and Expt. 2 (all substrates). The analysis was conducted by an independent analytical laboratory (Micro Macro International Inc.).

Supplemental Table 2.
Supplemental Table 3.

Chemical properties for one standard substrate and three perlite-free substrates used to grow sandhill milkweed in greenhouse container production during Expt. 1 (standard substrate) and Expt. 2 (all substrates). The analysis was conducted by an independent analytical laboratory (Micro Macro International Inc.).

Supplemental Table 3.
Supplemental Table 4.

Effects of fertilizer rate on the mineral nutrient concentration of sandhill milkweed shoots (aboveground) and tubers and roots (belowground). Plants were grown in 48-cell liners (107 mL) with a 75% aged pine bark with fines (0.6 cm screened) and 25% MetroMix 830 substrate (volume/volume) for 16 weeks.

Supplemental Table 4.
Supplemental Table 5.

The pH and electrical conductivity (EC) of substrate leachates at the time of seeding (0 weeks after seeding) and 12 and 16 weeks after seeding (4 and 8 weeks after fertilizer application) for sandhill milkweed fertilized with three rates of fertilizer. Means (SE) are presented.

Supplemental Table 5.
Supplemental Table 6.

Effects of visual quality on mineral nutrient concentration of sandhill milkweed aboveground (shoots) and belowground (tubers and roots) plant parts for healthy (green to purple leaves) and mildly stressed (yellow-green leaves) plants. Plants with extreme stress (yellow to white leaves) had aboveground and belowground tissues pooled and data are presented for total tissue.

Supplemental Table 6.
Supplemental Table 7.

The pH of substrate leachates at 0, 8, 12, and 16 weeks after seeding for sandhill milkweed grown in four substrates and four container types. Means (SE) are presented.

Supplemental Table 7.
Supplemental Table 8.

The electrical conductivity (EC) of substrate leachates at 0, 8, 12, and 16 weeks after seeding for sandhill milkweed grown in four substrates and four container types. Means (SE) are presented.

Supplemental Table 8.
  • Fig. 1.

    Healthy (left), moderately nutrient stressed (center), and highly nutrient stressed (right) sandhill milkweed. Healthy plants had green to purple leaves, moderately stressed plants had yellow–green leaves, and highly stressed plants had yellow to white leaves; they were assigned values of 3, 2, and 1, respectively.

  • Fig. 2.

    The mean (±SE) height of sandhill milkweed fertilized with low (0.34 g/plant), medium (0.56 g/plant), or high (0.90 g/plant) manufacturer recommended rates of 15N–3.9P–10.0K controlled-release fertilizer (15–9–12 Osmocote® Plus 6 mo. at 70 °C; Scotts Co.) at 12 and 16 weeks after seeding (4 and 8 weeks after fertilizer application) in 48-cell (107 mL) liners with a 75% aged pine bark with fines (0.6 cm screened) and 25% MetroMix830 substrate. Significant differences (α = 0.05) are indicated using lowercase letters.

  • Fig. 3.

    The mean (±SE) height of sandhill milkweed grown in four different substrates and four different container types at 16 weeks after seeding. Substrate types included a standard with perlite and three perlite-free substrates. Container types included standard liners and tree tubes with a deeper soil profile of small (∼100 mL) and large (∼175 mL) sizes. Significant differences (α = 0.05) are indicated using lowercase letters.

  • Fig. 4.

    The mean (±SE) visual quality of sandhill milkweed grown in four different substrates and four different container types at 16 weeks after seeding. A horizontal dotted line is provided at a visual quality of 2.5 to represent a threshold for a marketable product. Substrate types included a standard with perlite and three perlite-free substrates. Container types included standard liners and tree tubes with a deeper soil profile of small (∼100 mL) and large (∼175 mL) sizes. The visual quality scale ranged from 1 to 3, where 3 indicated a healthy plant with green to purple leaves, 2 indicated a moderately stressed plant with yellow–green leaves, and 1 indicated a highly stressed plant with yellow to white leaves. Significant differences (α = 0.05) are indicated using lowercase letters.

  • Agrawal A. 2017. Monarchs and milkweeds: A migrating butterfly, a poisonous plant, and their remarkable story of coevolution. Princeton Univ. Press, Princeton, NJ, USA.

  • Agrawal AA, Inamine H. 2018. Mechanisms behind the monarch’s decline. Sci. 360(6395):12941296. https://doi.org/10.1126/science.aat5066.

    • Search Google Scholar
    • Export Citation
  • Batalden RV, Oberhauser KS. 2015. Potential changes in eastern North American monarch migration in response to an introduced milkweed, Asclepias curassavica, p 215–224. In: Batalden RV, Oberhauser KS, Altizer A (eds). Monarchs in a changing world: Biology and conservation of an iconic butterfly. Cornell Univ. Press, Ithaca, NY, USA.

  • Bennett WF. 1993. Nutrient deficiencies and toxicities in crop plants. APS Press, St. Paul, MN, USA.

  • Borders B, Lee-Mäder E. 2014. Milkweeds: A conservation practitioner’s guide. The Xerces Soc. for Invertebrate Conservation, Portland, OR, USA.

  • Boyle JH, Dalgleish HJ, Puzey JR Jr. 2019. Monarch butterfly and milkweed declines substantially predate the use of genetically modified crops. Proc Natl Acad Sci USA. 116(8):30063011. https://doi.org/10.1073/pnas.1811437116.

    • Search Google Scholar
    • Export Citation
  • Brower LP. 1961. Studies on the migration of the monarch butterfly I. Breeding populations of Danaus plexippus and D. gilippus Berenice in south central Florida. Ecology. 42(1):7683.

    • Search Google Scholar
    • Export Citation
  • Brower LP. 1962. Evidence for interspecific competition in natural populations of the monarch and queen butterflies, Danaus plexippus and D. gilippus Berenice in south central Florida. Ecology. 43(3):549552.

    • Search Google Scholar
    • Export Citation
  • Brower LP, Williams EH, Dunford KS, Dunford JC, Knight AL, Daniels J, Cohen JA, Van Hook T, Saarinen E, Standridge MJ, Epstein SW. 2018. A long-term survey of spring monarch butterflies in north-central Florida. J Nat Hist. 52:20252046. https://doi.org/10.1080/00222933.2018.1510057.

    • Search Google Scholar
    • Export Citation
  • Campbell GE. 2016. Dormancy characteristics and germination requirements of Lupinus diffusus and Asclepias humistrata (MS Thesis). Univ. of Fla., Gainesville, FL, USA.

  • Campbell GE. 2020. Conservation, greenhouse production, and restoration outplantings of sandhill milkweed (Asclepias humistrata) (PhD Diss). Univ. of Fla., Gainesville, FL, USA.

  • Campbell-Martínez GE, Thetford M, Miller DL, Pérez H. 2017. Follicle maturity, seed size, temperature and photoperiod effects on seed germination of Asclepias humistrata. Seed Sci Technol. 45(3):523529. https://doi.org/10.15258/sst.2017.45.3.10.

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Gabriel Campbell-Martínez University of Florida, West Florida Research and Education Center, 5988 Highway 90, Building 4900, Milton, FL 32583, USA

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Mack Thetford University of Florida, West Florida Research and Education Center, 5988 Highway 90, Building 4900, Milton, FL 32583, USA

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Debbie Miller University of Florida, West Florida Research and Education Center, 5988 Highway 90, Building 4900, Milton, FL 32583, USA

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Sandra Wilson University of Florida/Institute of Food and Agricultural Sciences, PO Box 110670, Gainesville, FL 32611, USA

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Celina Gómez University of Florida/Institute of Food and Agricultural Sciences, 2543 Fifield Hall, PO Box 110670, Gainesville, FL 32611, USA

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Contributor Notes

A portion of this work was submitted by Gabriel Campbell-Martínez in partial fulfillment of the requirements for the PhD degree. Funding was provided in part from the US Department of Agriculture National Institute of Food and Agriculture McIntire Stennis project FLA-WFC-005653, US Fish and Wildlife Service project F16AC00519, and the Gary Henry Florida Wildflower Research Endowment. Mention of a trademark, proprietary product, or vendor does not constitute a guarantee or warranty of the product by the US Department of Agriculture and does not imply its approval to the exclusion of other products or vendors that also may be suitable.

G.C.-M. is the corresponding author. E-mail: camp5595@gmail.com.

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  • Fig. 1.

    Healthy (left), moderately nutrient stressed (center), and highly nutrient stressed (right) sandhill milkweed. Healthy plants had green to purple leaves, moderately stressed plants had yellow–green leaves, and highly stressed plants had yellow to white leaves; they were assigned values of 3, 2, and 1, respectively.

  • Fig. 2.

    The mean (±SE) height of sandhill milkweed fertilized with low (0.34 g/plant), medium (0.56 g/plant), or high (0.90 g/plant) manufacturer recommended rates of 15N–3.9P–10.0K controlled-release fertilizer (15–9–12 Osmocote® Plus 6 mo. at 70 °C; Scotts Co.) at 12 and 16 weeks after seeding (4 and 8 weeks after fertilizer application) in 48-cell (107 mL) liners with a 75% aged pine bark with fines (0.6 cm screened) and 25% MetroMix830 substrate. Significant differences (α = 0.05) are indicated using lowercase letters.

  • Fig. 3.

    The mean (±SE) height of sandhill milkweed grown in four different substrates and four different container types at 16 weeks after seeding. Substrate types included a standard with perlite and three perlite-free substrates. Container types included standard liners and tree tubes with a deeper soil profile of small (∼100 mL) and large (∼175 mL) sizes. Significant differences (α = 0.05) are indicated using lowercase letters.

  • Fig. 4.

    The mean (±SE) visual quality of sandhill milkweed grown in four different substrates and four different container types at 16 weeks after seeding. A horizontal dotted line is provided at a visual quality of 2.5 to represent a threshold for a marketable product. Substrate types included a standard with perlite and three perlite-free substrates. Container types included standard liners and tree tubes with a deeper soil profile of small (∼100 mL) and large (∼175 mL) sizes. The visual quality scale ranged from 1 to 3, where 3 indicated a healthy plant with green to purple leaves, 2 indicated a moderately stressed plant with yellow–green leaves, and 1 indicated a highly stressed plant with yellow to white leaves. Significant differences (α = 0.05) are indicated using lowercase letters.

 

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