Abstract
Prairie dropseed [Sporobolus heterolepis (A. Gray) A. Gray] is a critical North American native grass that is often not incorporated into prairie restoration seed mixes due to its low survival and growth rates. This project investigated using hydrogels, landscape plugs, and native field soil to improve the survival and growth of prairie dropseed. At three tallgrass prairie restoration sites at the Minnesota Landscape Arboretum, we planted prairie dropseed plugs in Fall 2019, Spring 2020, and Fall 2020. When grown in the field from 42 to 94 weeks, we found that potting mix–grown plugs had increased growth as measured by dry weight compared with plugs grown in native soils. Soil medium did not influence survival rates. The use of hydrogels did not demonstrate increased survival or growth compared with plugs planted with water. We recommend land managers and restorationists use plugs grown in commercial potting mix rather than grown in native soils, and we found no advantage in using hydrogels over watering at planting.
Prairie dropseed is a North American native warm-season bunchgrass. It occupies a large geographic range across the United States, occurring from Massachusetts to Colorado and from New Mexico north into Canada [U.S. Department of Agriculture (USDA), 2022] but is often locally scarce (Ladd, 1995). It is commonly found in well-drained soil and is an indicator of mesic prairies but can be found on drier sites such as hill prairies and rocky prairie slopes (Ladd, 1995). Prairie dropseed is listed as endangered in five eastern states—Connecticut, Kentucky, Maryland, North Carolina, and Pennsylvania—and is listed as threatened in two more, New York and Ohio (USDA, 2022). It is not a protected plant species in Minnesota.
Although prairie dropseed is not protected in Minnesota, it is an ecologically important species. Prairie dropseed is a critical source of food and shelter for at least five species of native Lepidoptera including the federally endangered Poweshiek skipperling, Oarisma poweshiek, and the federally threatened Dakota skipper, Hesperia dacotae (Narem and Meyer, 2017). Prairie dropseed is not only a preferred host plant for Dakota skipper (Dana, 1991), but Nordmeyer et al. (2021) found that Dakota skipper larvae on prairie dropseed plants had the highest survival rates, highest pupation weight, and shortest time to pupation compared with other native grasses. Prairie dropseed is thought to be an optimal forage for native Lepidoptera (Dana, 1991).
Despite its ecological value, prairie dropseed is not commonly included in seed mixes used for prairie restoration (Fedewa and Stewart, 2011). It is sensitive to competition as a seedling and is slow to establish, taking 4 to 5 years to reach maturity (Fedewa and Stewart, 2009). Native plant nursery and restoration specialists confirmed that prairie dropseed is not usually incorporated into restoration projects (B. Carter, personal communication, 26 Aug. 2019; G. Kastey, personal communication, 28 Aug. 2019). Instead, practitioners recommend using plugs of prairie dropseed in restored sites (B. Carter, personal communication, 26 Aug. 2019).
Using plugs in place of direct seeding was determined to be a viable way to establish species that are rare, with limited dispersal or that are difficult to establish from seed (Hedberg and Kotowski, 2010). Plug survival ranges widely, from 23% to 83%, depending on species, planting season, and competition (Page and Bork, 2005). Competition and planting density were found to influence survival, biomass, and flowering of plugs in restorations (Huddleston and Young, 2004). Although plugs are accepted for restoration use, peer-reviewed research has primarily focused on forest restoration with limited research on their application in grasslands or savannas (Palma and Laurance, 2015). This may be due to the increased cost of plugs compared with direct seeding at restoration sites (Palma and Laurance, 2015). Specific research with plugs to establish prairie dropseed could not be located.
Treating plugs with hydrogels immediately before planting may be one method to increase rates of plug survival. Hydrogels are hydrophilic gels composed of crosslinked polymers that can absorb 400 to 1500 times their dry weight (Landis and Haase, 2012). Hydrogels may function like natural polymeric mucilages produced by healthy plant roots and can protect seedlings during planting from sun and wind (Landis and Haase, 2012). For horticultural applications, synthetic hydrogels are most frequently used, but other hydrogels on the market are semisynthetic or starch based (Landis and Haase, 2012). While commonly used for a variety of applications in horticulture, peer-reviewed research on hydrogel use for restoration is limited. Thomas (2008) found that hydrogels reduced mortality and appeared to keep seedlings healthier by increasing leaf retention and accelerating root extension of two tree species commonly used in Australian forestry. Studies with the North American native grass black grama [Bouteloua eriopoda (Torr.) Torr.] found hydrogels did not affect root mass but did increase leaf mass and total leaf area of seedlings planted in a restoration (Lucero et al., 2010). However, both Thomas (2008) and Lucero et al. (2010) studied the effect of hydrogels in an arid environment. We conducted research in Minnesota, which is traditionally semiarid. We could find no research on hydrogels and prairie dropseed survival and growth in restorations.
Similar to using a hydrogel treatment before planting, inoculating plants with beneficial fungi is also a common method of increasing seedling success. Prairie dropseed is a known fungal affiliate and most commonly associates with Glomus species, specifically Glomus fasciculatum (Dhillion, 1992). Spore levels around prairie dropseed range from 7.7% to 48.6% and fluctuate seasonally (Ebbers et al., 1987). Colonization of prairie dropseed roots and spore abundance were found to be inversely related to calcium and magnesium availability (Ebbers et al., 1987).
Restoring the fungal microbiome with vegetation at a site may have major implications for the success of a restoration planting. Koziol et al. (2018) found that plugs treated with native arbuscular mycorrhizal (AM) fungi were 40% more likely to survive the first year and three times more likely to survive over 3 years compared with those left untreated. Plots treated with AM fungi had increased germination and species diversity compared with plots which were not inoculated (Koziol et al., 2018). The fungal species used are critical because the fungal strains found in commercial mixes are usually nonnative strains, and there is a risk in introducing nonnative fungi to a restoration site as their effects on native fungi are unknown (Emam, 2016).
The objective of this research was to determine whether growing plugs in native field soil or treating plugs with hydrogel would increase the survival and growth rates of prairie dropseed planted in restored prairies. We hypothesized that plugs treated with hydrogel as well as plugs grown using native field soil would have higher survival and growth rates than those grown without hydrogel or by using a commercial potting mix, respectively.
Materials and Methods
Planting sites.
We planted 1000 plugs at each of three experimental sites. All three planting sites were on the grounds of the Minnesota Landscape Arboretum in restored prairies of differing ages. The prairies varied in age and plant species composition, but each experimental site was located in an upland full sun area. The Bennett-Johnson Prairie was the oldest site and began restoration in 1965. Big bluestem (Andropogon gerardii Vitman) and yellow prairie grass [Sorghastrum nutans (L.) Nash] were the most abundant species. This soil was a Lester-Kilkenny loam with a pH of 6.6 and 4.6% organic matter. The second prairie, Spring Peeper Meadow, began restoration in 1995 with big bluestem as the predominant plant species and Lester-Kilkenny loam soil with a pH of 7.5 and 2.1% organic matter. Lake Tamarack was the youngest site with restoration beginning in 2015. Side oats grama [Bouteloua curtipendula (Michx.) Torr.] and little bluestem [Schizachyrium scoparium (Michx.) Nash] were the primary grasses, with a Lester-Kilkenny soil complex with a pH of 7.2 and 1.8% organic matter.
Plugs.
Potting mix grown plugs were purchased from Prairie Restoration Inc. (Princeton, MN) and Minnesota Native Landscapes (MNL) (Otsego, MN). We chose these nurseries as they are commonly used by Minnesota restorationists with plants available to practitioners. Plugs from Prairie Restoration Inc. were grown in 50% peatmoss and 50% sterilized field soil from the Prairie Restoration nursery property. The plugs were not treated with mycorrhizae and were fertilized once using an 8–4–4 slow-release organic fertilizer. The Prairie Restoration Inc. plugs and native soil plugs were grown in the same size/volume containers and were visually similar in size. Due to a crop failure, we sourced additional plugs from MNL for the Fall 2020 planting. The MNL plugs were grown in Plaisted Greenhouse Mix, Plaisted Companies Inc. (Elk River, MN), composed of fibrous peat, composted pine bark, perlite, and a slow-release fertilizer. The plugs were not treated with mycorrhizae. The MNL plugs were 88.9% larger by total dry weight than the Prairie Restoration Inc. plugs and the plugs grown in native field soil.
Soil was collected from each of the three sites for plugs grown in native field soil, and sifted through a 0.64-cm mesh screen to remove large plant material and rocks. The sifted soil was held at 4.4 °C until used for sowing and was not sterilized before planting.
We purchased seed from Kastey Seed Inc. (Fertile, MN). The seed was harvested in 2019 and stored in cold (4.4 °C) dry storage before sowing. Seed was tested by the Minnesota Crop Improvement Association and had 93% viability as determined by tetrazolium testing and 75% germination.
Seedlings were grown at the University of Minnesota Plant Growth Facility (St. Paul, MN). Seed was sown in native soil from each of the three experimental sites in 128 size plug trays and lightly covered with medium grade vermiculite. Three seeds were placed in each plug cell. The trays were placed in a warm (23.9 °C) greenhouse with intermittent misting (5 s mist every 8 min) for 5 weeks before being transplanted into 75 size plug trays and moved to a warm (23.9 °C) greenhouse. Six weeks after transplanting, we again transplanted the plugs from 75 size trays to 1006 trays. The plugs were watered as needed and were not treated with any fertilizer or mycorrhizae. Four days before planting at the experimental sites, plugs were moved to coldframes.
The presence of mycorrhizae in the three native soils was tested through soil and root samples submitted to the University of Florida Soil and Water Science Core Laboratory (Gainesville, FL). Soil was tested for spore presence, measured as number of spores per 1 g soil, and mycorrhizae inoculum potential. Roots were tested for colonization. Mychorrizae were found in the soil from each of the three planting sites, and colonization was identified in roots of prairie dropseed grown in soil collected from each of the three sites.
Treatments and planting.
Each of the three sites had 100 1 × 1 m experimental plots in a complete randomized block design. The day before planting, we used a 1-inch diameter masonry drill bit on a power drill to drill holes for the plugs. Holes were drilled to match the depth and diameter of the plugs. Within each plot, we planted 10 plugs. We used a 100% polyacrylamide hydrogel dip (Miracle Gro, Marysville, OH) soaked for 1 h just before planting and placed ≈4 oz of moist hydrogel into the hole immediately before planting the plugs. Plugs were grown in either potting mix or native soil and treated with moist hydrogel or left untreated without a hydrogel dip and watered directly after planting. The soil type, treatment, and planting time of each plot was randomly assigned.
Plugs were planted in Fall 2019, Fall 2020, and Spring 2020. In Fall 2019, we planted 20 plots per site (200 plugs), 10 treated with hydrogel and 10 watered following planting, using only plugs grown in potting mix purchased from Prairie Restoration Inc. In Spring 2020 and Fall 2020, we planted 40 plots (400 plugs) per experimental site: 20 plots with plugs grown in field soil, one half of which were treated with hydrogel, the other half with water, and 20 plots with nursery grown plugs, one half treated with hydrogel and one half with water. For the final Fall 2020 planting, we had to source 360 larger plugs from MNL, due to a crop failure at Prairie Restorations, Inc. To compensate for the larger plugs, we determined the percent difference in size by measuring dry weights of six of each the MNL plugs and the Prairie Restoration Inc. plugs (before planting) and averaging the weights separately from each source. The MNL plugs had an average 88.87% higher total weight, 87.84% higher shoot weight, and 88.00% higher root weight than Prairie Restoration Inc. plugs. We then adjusted the weights of the MNL plugs harvested in Aug. 2021 to reflect this difference.
After the initial planting, no further care or maintenance was used to establish the plants, in an effort to replicate conditions faced by practitioners.
Data collection and analyses.
We measured results as survival counts taken the second week of June 2020 and 2021 and the first week of Aug. 2020 and 2021. Because we had three plantings—Fall 2019, Spring 2020, and Fall 2020—and three harvests—Aug. 2020, Oct. 2020, and Aug. 2021—which affected the total number of plants remaining at a location, we used percent survival when analyzing survival by planting time. We harvested 20% of each plot on each harvest date. Percent survival was used when analyzing survival over time because we took survival counts in the same plots that we harvested from, so the total possible number surviving varied over time (Figs. 1–3). However, we used the total number of plants surviving when analyzing survival by planting site, soil medium, and hydrogel treatment, as the total possible number of plants surviving remained the same for each treatment.
Representation of the 1 m × 1 m research plot with 10 plugs. Shading indicates when plugs were harvested. Plot planted Fall 2019.
Citation: HortScience 57, 8; 10.21273/HORTSCI16560-22
Representation of the 1 m × 1 m research plot with 10 plugs. Shading indicates when plugs were harvested. Plot planted Fall 2019.
Citation: HortScience 57, 8; 10.21273/HORTSCI16560-22
Representation of the 1 m × 1 m research plot with 10 plugs. Shading indicates when plugs were harvested. Plot planted Fall 2019.
Citation: HortScience 57, 8; 10.21273/HORTSCI16560-22
Representation of the 1 m × 1 m research plot with 10 plugs. Shading indicates when plugs were harvested. Plot planted Spring 2020.
Citation: HortScience 57, 8; 10.21273/HORTSCI16560-22
Representation of the 1 m × 1 m research plot with 10 plugs. Shading indicates when plugs were harvested. Plot planted Spring 2020.
Citation: HortScience 57, 8; 10.21273/HORTSCI16560-22
Representation of the 1 m × 1 m research plot with 10 plugs. Shading indicates when plugs were harvested. Plot planted Spring 2020.
Citation: HortScience 57, 8; 10.21273/HORTSCI16560-22
Representation of the 1m × 1 m research plot with 10 plugs. Shading indicates when plugs were harvested. Plot planted Fall 2020.
Citation: HortScience 57, 8; 10.21273/HORTSCI16560-22
Representation of the 1m × 1 m research plot with 10 plugs. Shading indicates when plugs were harvested. Plot planted Fall 2020.
Citation: HortScience 57, 8; 10.21273/HORTSCI16560-22
Representation of the 1m × 1 m research plot with 10 plugs. Shading indicates when plugs were harvested. Plot planted Fall 2020.
Citation: HortScience 57, 8; 10.21273/HORTSCI16560-22
In addition to survival, we evaluated growth as a measure of dry weight. We harvested plugs in Aug. 2020, Oct. 2020, and Aug. 2021 using a 7.6-cm diameter bulb planter to remove cores of soil surrounding the root system of the plugs. We stored the plugs at 4.4 °C overnight before washing the roots and separating the roots just below the crown. Roots and shoots were dried at 35 °C for 2 to 4 days before weighing.
To determine the efficacy of planting time, hydrogel treatment, or native soil as a potting medium on plug survival and growth, we analyzed each site separately. When no significant difference was detected at each individual site, all three experimental sites were pooled and analyzed together. All analyses were done in R (R Core Team, 2021). We used analysis of variance to analyze the effects of soil type, hydrogel treatment, planting site, and planting time. When significant differences were detected (P ≤ 0.05), means were separated using Tukey’s honestly significant difference (α = 0.05).
Results
Plant growth.
We used three restored prairie sites that varied in soil type and age, but we found no significant difference in growth, as measured by total dry weight (P = 0.9811) across the three planting sites (data not shown). All three sites were similar in showing a significant difference in the size of the plants based on planting date: the older the plants, the larger dry weights. Plugs planted in Fall 2019 had significantly higher root, shoot, and total dry weights than plugs planted in Spring 2020 or Fall 2020 at all three sites (Table 1). The plugs with the fewest weeks in the ground had the least growth as measured by dry weight. Although we had to use a larger commercial plug for the final planting, these plants did not hold their larger size. At planting, the larger MNL plugs had an average of 88.9% higher total dry weight, 87.8% higher shoot dry weight and 88.0% higher root dry weight, but at harvest the averages had dropped to 76.5% higher total dry weight, 77.6% higher root dry weight, and 75.6% higher shoot dry weight.
Harvested average shoot, root, and total dry weights of prairie dropseed plugs grown at three restoration sites with three planting dates at the Minnesota Landscape Arboretum 2019–21.
Plant survival.
Overall plant survival did not vary between the three planting sites (P = 0.2800) (data not shown). There were some differences in survival rates compared with planting time and site. Plugs planted at BJP and LT in Fall 2020 (42 weeks from planting to Aug. 2021 harvest) showed a significantly higher percent survival than plugs planted in Fall 2019 (94 weeks from planting to Aug. 2021 harvest) but similar to survival for Spring 2020 plugs (Table 2). The plants with the most weeks in the ground had the lowest percentage survival at BJP and LT; however, SP site had the lowest survival from the Spring 2020 planting.
Average per plot plant survival (percentage) of prairie dropseed plugs planted in three restored prairies at the Minnesota Landscape Arboretum with three planting dates.
Hydrogel and field soil treatments.
There were no differences in growth as measured by total dry weights (P = 0.5751) or survival rates (P = 0.1297) for plugs planted with hydrogel vs. water at any of the three sites. Each site was analyzed separately (Table 3).
Harvested average dry weights of prairie dropseed plugs that were planted with hydrogel or water and grown at three restored prairies at the Minnesota Landscape Arboretum 2019–21.
Survival rates for plugs grown in the commercial potting mix vs. plugs grown in field soil at any of the three sites did not vary (P = 0.6362). However, plugs grown in potting mix did show significantly higher root, shoot, and total dry weights than plugs grown in field soil at each of the three sites (Tables 4 and 5). Each experimental site was analyzed separately.
Harvested average dry weights of prairie dropseed plugs grown in potting mix or field soil before planting in three restored prairies at the Minnesota Landscape Arboretum 2019–21.
Analysis of variance table for total plug weight for plants grown in potting mix or field soil and planted in three restored prairies at the Minnesota Landscape Arboretum 2019–21.
Discussion
Prairie dropseed exhibits a slow life cycle defined by slow growth, high root-to-shoot growth, and fitness dependent on established plants. Species that use a slow life cycle typically also have a higher dependency on mycorrhizal affiliations (Koziol and Bever, 2017). We hypothesized that plugs grown in native field soil would have higher total, root, and shoot dry weights than plugs grown in traditional potting mix because of this high dependency on mycorrhizae. However, plugs grown in commercial potting mix showed higher total, root, and shoot dry weights, even though the potting mix was not treated with mycorrhizae and mycorrhizae were present in the native field soil. Perhaps a longer experiment would show plugs with mycorrhizae to be larger or more long lived. Both the MNL plugs and the Prairie Restoration Inc. plugs were treated with a slow-release fertilizer. The use of fertilizer could be the cause of the increased growth of commercial potting mix plugs because the native soil plugs were not fertilized. Many studies have found that commercial potting mix is a superior potting medium compared with field soil because of the increased porosity and air exchange capacity, due to the high percent of organic matter (Bugbee and Frink, 1986; Meyer and Cunliffe, 2004; Whitcomb, 1984). The Bennett Johnson Prairie had the highest percent organic matter, 4.6%, of the three experimental sites. However, potting mix is often 50% to 100% organic matter (Whitcomb, 1984). We realize that sourcing plugs from two commercial greenhouses as well as our research greenhouses confounded the results; however, one of our research objectives was to compare growth using the native soil to commercial media. Sourcing a large number of identical plugs is often difficult for large-scale restorations, especially when native grasses are involved. We realize the variation in plug size could have affected our overall results; thus, we did our best to adjust for the size variation when comparing growth.
Growing plugs in commercial potting mix and inoculating with native strains of mycorrhizal fungi may be the best way to increase plug survival and growth. Research has found fertilization lowers the benefits of AM fungal inoculation (Johnson et al., 2002) and that combining nitrogen fertilizer with AM fungi is less beneficial for plant growth than using AM fungi alone (Corkidi et al., 2002). Inoculated plugs should not be fertilized.
Although we saw a difference in total, root, and shoot dry weights between plugs grown in potting mix and plugs grown in field soil, we did not find a difference in total, root, or shoot dry weight for plugs treated with hydrogel compared with plugs planted with water. We used a 100% polyacrylamide synthetic hydrogel. Whether the hydrogel used is synthetic or starch-based may affect plant growth. Lucero et al. (2010) found that seedlings of black grama treated with a starch-based hydrogel had increased biomass over seedlings treated with a synthetic hydrogel. However, Starkey et al. (2012) found no significant difference in root dry weight for seedlings of loblolly pine (Pinus taeda L.) treated with synthetic hydrogel compared with seedlings treated with a starch-based hydrogel.
We found that plugs treated with a hydrogel did not have increased rates of survival compared with untreated plugs. This confirms studies that found treating seedlings with hydrogels did not increase seedling survival (Sloan, 1994). However, the majority of this research was done using woody plants. Lucero et al. (2010) found no difference in survival rates for black grama seedlings treated with a hydrogel and untreated seedlings. More research is needed on the effectiveness of a hydrogel treatment on herbaceous plants, especially grasses.
The planting sites varied in restoration age, soil organic matter, and species composition. We anticipated lower survival rates at the Bennett Johnson Prairie, which is the oldest site with restoration beginning in 1964. Plug survival has been shown to decrease with competition at the planting site (Huddleston and Young, 2004), and we expected the Bennett Johnson Prairie, as the oldest site, to have increased competition. However, we did not see decreased survival at the Bennett Johnson Prairie compared with the other two sites, nor did we see increased survival at the youngest site, Lake Tamarack, where we expected to have the least competition.
In addition to restoration age, the three planting sites differed in soil organic matter. Lake Tamarack had the lowest soil organic matter, 1.8%, of the three sites. A lower percent soil organic matter could cause lower weights, but we did not see lower weights at Lake Tamarack than at the other sites.
Plugs planted in Fall 2019 had the highest root, shoot and total dry weight, which is expected because these plugs were the earliest planted and therefore had the most time in the ground, 94 weeks. It is unsurprising that the plugs planted in Fall 2020 had the lowest root, shoot, and total dry weight because they had the shortest number of weeks (42) in the ground. Carver County, where all three experimental sites were located, experienced an unseasonal drought in 2021 with precipitation 5.1 inches lower than average from May through August (National Oceanic and Atmospheric Association, 2021). Drought could impact the weight and survival of plugs and account for the lower weights of plants with the least amount of time in the field. Plugs planted in Fall 2020 had the lowest weights; however, they had significantly higher survival rates than plugs planted in Fall 2019 or Spring 2020. Again, this is expected because these plants had the least amount of time in the ground, and therefore less time exposed to weather conditions, predation, and other factors that could impact survival.
As is often the case in large-scale plantings, we had to use two sources and sizes of plugs for the final planting date. We found that these plugs did not hold their size in the field; the total, root, and shoot dry weights were all reduced by more than 10%, which may indicate that smaller plugs grow faster in the field than larger plugs.
Conclusion
Restorationists looking to incorporate native species such as prairie dropseed into restored prairies should use plants grown in commercial potting mix rather than plants grown in native field soils. Our research showed plugs grown in potting mix had increased growth as measured by dry weight. However, practitioners should be aware that soil medium did not influence plug survival in our project. Additionally, treating plugs with a synthetic hydrogel did not increase plug survival or growth compared with watering plugs immediately after planting. Interestingly, these results remained true at our three site locations regardless of the age of the restoration. Plugs grown in potting mix were just as likely to have increased growth rates over plugs grown in native field soil when planted at a decades old restoration or a 5-year-old restoration, and the hydrogel treatment did not increase plug survival or growth regardless of restoration age.
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