Abstract
Paeonia lactiflora is a high-value crop with a temperature-dependent growth response that requires worldwide production to satisfy year-round demand. The objective of this study was to evaluate production and timing of ‘Coral Charm’ peony as a cool-season crop in the US Intermountain West. High-tunnel and field production were trialed in North Logan, UT, USA (lat. 41.77°N, long. 111.81°W; elevation, 1382 m) with the addition of low tunnels and soil heating methods to advance growth in 2019–21. Soil and air temperatures, as well as the date and quality of harvested stems, were measured. High tunnels yielded 15.7 ± 3.3 to 19.4 ± 2.1 stems/m2 [± standard error (SE)] and the high tunnel alone advanced initial harvest 21 to 34 days earlier than natural field conditions. The field yielded 16.1 ± 1.9 to 20.8 ± 1.6 stems/m2 and staggered production, resulting in a harvest duration up to 38 days across the high tunnel and field. The use of a low tunnel with soil heating advanced the initial harvest date compared with natural (i.e., unmanipulated) high-tunnel and field conditions by 3 and 7 days in 2019, 6 days in 2020, and 16 and 6 days in 2021 in the high tunnel and field, respectively. However, the quality decreased significantly under low tunnels with soil heating within high tunnels, compared with unheated plants, as a result of superoptimal temperatures and humidity that damaged buds and led to an increase in disease and insect pressure. Overall, increasing soil temperature advanced early stages of production when the meristem was below or near the soil surface, whereas increased air temperatures accelerated stem elongation and advanced time to flowering.
Growers across the United States have developed local niche markets for specialty cut-flower crops that are in demand by consumers for their unique blooms and fresh quality (Armitage and Laushman 2003). Included in this growth are nontraditional regions for cut-flower production, such as the US Intermountain West, where more than 100 small-scale farms are established, and a Utah-based cut-flower farmer association has 145 members since launching in 2019 (Utah Cut Flower Farm Association, unpublished data). However, environmental conditions in the high-elevation, semiarid climate limit production quantity, quality, and timing. Growing seasons in northern Utah are varied and short (89–135 frost-free d), daily temperature fluctuations often exceed 20 °C, snow events occur into the growing season, and microclimates dominate over official United States Department of Agriculture (USDA) Hardiness Zone classifications (Gillies and Ramsey 2009; Utah Climate Center 2022a, 2022b). Therefore, conducting local trials is critical for optimizing cut-flower production under regional constraints (Ortiz et al. 2012; Wein 2009).
Herbaceous peonies are particularly adapted to areas with extreme winters and short growing seasons (Walton et al. 2007), although this has resulted in a narrow production window that lasts 3 d to 4 weeks by region (Halevy et al. 2002; Holloway et al. 2004). Because of the perennial life cycle and thermal periodicity (Barzilay et al. 2002), production occurs on six continents to meet the year-round demand for the primary markets of Europe and North America (Kamenetsky and Dole 2012). In the United States alone, the wholesale value of peonies was estimated at $5.9 million (USDA-National Agricultural Statistics Service 2019). Early-blooming ‘Coral Charm’ is one of the top-five most popular cultivars produced in Alaska, a top state for peony production in the United States (Holloway and Buchholz 2013). Wholesale pricing in 2012 averaged $5.82/stem when sold directly to florists (Holloway and Buchholz 2013); thus, developing production into high-elevation regions may present a premium, early-market opportunity for local growers.
In an effort to expand peony production, previous research focused primarily on manipulating chilling and growth through containerized greenhouse systems in mild climates, which helped established temperature requirements (Byrne and Halevy 1986; Evans et al. 1990; Fulton et al. 2001; Halevy et al. 2002; Hall et al. 2007; Kamenetsky et al. 2003; Mornya and Cheng 2018). For example, with the late-blooming ‘Sarah Bernhardt’, 42% of shoots reached maturity when forced at cool temperatures of 22/10 °C day/night, whereas only 15% of shoots reached maturity at 28/10 °C day/night, and nearly all buds aborted at 28/22 °C day/night (Kamenetsky et al. 2003). Moreover, the greatest number of flowers (five per plant) and second-longest stems (average length, 85 cm) were produced at 22/10 °C day/night compared with the warmer forcing cycles, indicating cooler temperatures were optimal for stem emergence and growth, as well as yield and quality (Kamenetsky et al. 2003). However, flowering at 22/10 °C day/night was delayed significantly by 25 d compared with those held at 22/16 °C day/night or warmer (Kamenetsky et al. 2003). Therefore, new regions for production should target temperatures that optimize yield and quality, as well as consider effects on timing to schedule harvest that meets local market demand.
Applying established temperature thresholds from greenhouse to field production may help advance and stagger harvest, thus benefiting small farms that do not have access to indoor growing spaces. One primary method for advancing field production is through the use of high tunnels (HTs), which allow for natural chilling in winter and passive heating in spring. In Golan Heights, Israel (lat. 32.55°N, long. 35.53°E; elevation, 600–800 m; air temperature range, 2 to 28/18 to 32 °C winter/summer), peony production was compared between plants forced in unheated HTs beginning in mid January to plants in an unprotected field, and air temperatures were monitored (Halevy et al. 2002). High tunnels (6 to 28 °C) advanced production by 4 weeks compared with unprotected field conditions (2 to 28 °C) (Halevy et al. 2002), highlighting the potential for truncating chilling periods in temperate climates to advance bloom. Moreover, the number of flowers produced per plant was up to 20% greater from peonies grown across HT and field systems compared with those in a heated greenhouse (20/17 °C day/night) (Halevy et al. 2002). This suggests that the greater temperature fluctuations in the HT and field vs. the heated greenhouse did not limit production, and emphasizes the potential of the crop quality in semiarid regions that experience large, daily temperature variations.
In the US Intermountain West, HTs can increase daytime air temperature by 10 to 25 °C, and nighttime air temperature by 5 to 10 °C, compared with ambient outdoor conditions in late winter (Drost et al. 2017). In North Logan, UT, USA (USDA Hardiness Zone 5), production of primocane raspberries, a perennial crop managed similar to peony, was advanced up to 4 weeks with HTs (Black et al. 2019). Although HTs can moderate temperatures throughout the year, diurnal temperature fluctuations can limit early-season production. The use of low tunnels (LTs) within HTs can increase air temperature by 4.9 °C during the day and by 3.1°C at night (Ward and Bomford 2013). This additional temperature lift further advanced the harvest of cool-season crops in northern Utah, such as spinach (Drost et al. 2017) and strawberries (Rowley et al. 2011). Although the effect of HTs and LTs on soil temperature was not established, earlier forcing with these systems likely increases soil temperature, allowing for earlier emergence of perennials. In a greenhouse pot study, emergence of ‘Coral Sunset’, an early-blooming peony cultivar similar to ‘Coral Charm’, was hastened by increasing soil and air temperatures from 1 to 18 °C (Hall et al. 2007). However, targeting soil heating (H) as an early-season method to advance production has not been tested on peony in HT or field environments, where soil temperature is less coupled with air temperature.
Trialing in HTs, LTs, and with H methods to advance and stagger peony production may allow growers in cool regions to target peak marketing times. ‘Coral Charm’ has exceptional market demand in the US Intermountain West because of its long stem length and double bloom, which fades from bright coral to cream. Although an early-blooming cultivar, harvest occurs from mid May to early June under unforced field conditions in northern Utah (Maughan et al. 2018). Advancing peak production to meet the demand for Mother’s Day (early to mid May), a top holiday for cut-flower sales, and for Memorial Day (late May) may create premium pricing options for growers. Therefore, the objectives of this study were 1) to evaluate the bloom timing and production duration of peony ‘Coral Charm’ with HTs, LTs, and H vs. unprotected field conditions in northern Utah; and 2) to analyze the total yield and quality of stem production from each management practice and assess market outcomes.
Materials and Methods
Site description.
Peony advancement trials were conducted at the Utah Agricultural Experiment Station Greenville Research Farm in North Logan, UT, USA (lat. 41.77°N, long. 111.81°W; elevation, 1382 m) from 2019 to 2021. The soil is a Millville silt loam with 2% organic matter. In Fall 2011, 60 ‘Coral Charm’ crowns were planted in a Quonset-style HT (Black et al. 2008), and 60 crowns were planted in an adjacent field. Each site was 4.3 m wide × 12.8 m long (area, 55 m2), oriented east–west, and divided into three 1.2-m-wide × 12.8-m-long rows. Crowns were planted 0.6 m apart within row and 0.05 m deep. Weed barrier fabric (Weed Barrier, 116 g; Dewitt, Sikeston, MO, USA) was used between rows to reduce weed pressure, and two lines of drip tape (Aqua-Traxx; Toro, El Cajon, CA, USA) were installed per row. Soil moisture was monitored using Watermark sensors (Irrometer Co., Riverside, CA, USA), and the soil was irrigated at a 60-kPa water potential. Irrigation events typically occurred every 2 to 3 weeks in the spring and fall, and every 1 to 2 weeks in summer. During establishment (2012–14), plants were 100% disbudded to divert energy to the crown. In 2015–20, lateral buds were removed upon emergence of the flower sets and 50% of the apical flower buds were disbudded after reaching a 60-mm diameter to increase the size of the remaining buds for harvest, and to improve yield quality and long-term production (Rogers 1995). Because of the mature age of the plants, no disbudding occurred in 2021 to evaluate the impact on yield and quality. Plants were pruned to the ground after fall senescence each year.
High tunnel management.
High tunnels were covered with plastic in late fall before snowfall, and doors were left open during the winter for sufficient chilling. After chilling, the doors were closed on 1 Feb each year through 2020, and on 26 Jan in 2021, and temperature was managed by venting the structure manually based on weather conditions reported from an automated weather station located 0.2 km away (Utah Climate Center 2022a). The optimal temperature range of 10 to 21 °C (Kamenetsky et al. 2003) was maintained by opening the door vents when solar radiation was greater than 400 W⋅m–2 and the outside air temperature was greater than 5 °C, the doors were opened when the air temperature was greater than 15 °C, and the sides of the HT were raised when the air temperature was greater than 25 °C. The HT plastic was removed in late May after production ceased.
Installation and management of warming practices.
In 2019–21, each bed was divided into three 1.2- × 3.6-m plots with six plants, for a total of nine plots in the HT and nine in the field. Three warming practices were then tested in triplicate with a Latin square design in each environment, with each bed as one column of the Latin square: the addition of an LT, and the addition of an LT + H, and an unaltered control. Arches for the LTs were 1.2 m tall × 0.9 m wide and were covered with frost fabric (Agribon AG-19; 18.3 g⋅m–2, 85% light transmission; San Luis Potosi, Mexico) (Rauter et al. 2021). To reduce damage to buds, the LTs were removed after the plants reached the frost fabric (late March in the HT and late April in the field). Soil was heated with two 0.30- × 3.7-m infrared heat mats (50 W⋅m–1; Angelsea LLC, Chicago, IL, USA) per plot that were installed on each side of the plant crowns and oriented along the length of the plots. Thermostatic controllers (electronic temperature control: SPDT NEMA 4X; Ranco, Plain City, OH, USA) were programmed to activate the mats when the soil temperature was less than 4 °C between the edge of the mat and plant crown at a 0.05-m depth, and the power used for the heat mats was recorded (Kill A Watt® EZ; P3 International, Sanford, NC, USA). The warming practices were implemented from 21 Mar to 15 May in the HT and 21 Mar to 14 Jun in the field in 2019, 27 Feb to 10 May in the HT and 6 Mar to 23 May in the field in 2020, and 1 Feb to 9 Apr in the HT and 23 Feb to 22 Apr in the field in 2021.
Data collection and analysis.
Site-specific environmental conditions were monitored from 19 Mar 2019 to 9 Jun 2021. Soil temperature was measured at a 0.05-m depth in each plot with thermocouples (Type T; Omega Engineering, Inc., Stamford, CT, USA), and canopy air temperature was measured in one plot per treatment (i.e., six plots) at the mature canopy height of 0.80 m (107; Campbell Scientific, Logan, UT, USA). Temperatures were measured every minute and stored as 60-min averages with two data loggers (CR1000, Campbell Scientific) connected to multiplexers (AM25T, Campbell Scientific), although in 2021, the canopy temperature of HT-LT + H was largely unrecorded because of a sensor error. Ambient air temperature in the HT was also measured at a 1.8-m height with a data logger temperature sensor (UA-001-64; Onset Computer Corp., Bourne, MA, USA), and in the field at a 2-m height by the weather station (Utah Climate Center 2022b).
Flowers were harvested up to four times per day when buds reached maturity. The optimal harvest stage was considered between a firm, tight bud showing color and a soft bud that was squeezable, per established recommendations for a similar cultivar, Coral Sunset (Eason et al. 2002). After harvest, stems were graded according to USDA standards (USDA 2016). Grade 1 stems had buds larger than 2.5 cm, relatively straight stems longer than 61 cm, and no visual damage. Grade 2 stems had buds no smaller than 2 cm, stems with moderate curvature and longer than 51 cm, and minimal visual damage. Culls were stems that could not be classified by either grade because of bud size (<2 cm) or malformation, severe stem curvature, or severe damage from insects or disease.
An economic budget was calculated for peony based on cost, yield, and sales by management practice on a 55-m2 HT or field production area. All input costs for production and transport were recorded (Lewis et al. 2021a, 2021b). Stems were sold wholesale from April to June through a local cut-flower co-op that marketed to florists from Logan to Salt Lake City, UT, USA. Local industry feedback established the price points, which were $6 per marketable stem sold before the Mother’s Day holiday, $5 per marketable stem after that holiday, and $2.00 per cull stem. Grades 1 and 2 stems were considered equally marketable by local florists, thus all commanded a single market price. Each year the demand for ‘Coral Charm’ peonies exceeded supply, and prices of marketable stems were increased steadily from $4 per stem in 2019 to $6 in 2021. Net returns were then calculated as the difference between input costs and receipts with 2021 pricing.
Yield was calculated as the number of stems harvested in each grade (i.e., grade 1, grade 2, and cull), as well as total stems harvested across grades per day and per harvest season. The timing at which 10% (T10), 50% (T50), and 90% (T90) of the total cumulative harvest occurred was also calculated. Yield, initial and final dates of harvest, peak harvest date, and the duration of harvest were analyzed with the GLIMMIX procedure in SAS/STAT 15.1 (version 9.4; SAS Institute, Cary, NC, USA). Warming practices (LT + H, LT, and control) and environment (HT and field) were fixed factors, and year was a random factor. Plot location within the rows and columns of the Latin square were treated initially as random factors, but were not significant and thus removed from the model. Year was treated as a random factor in the model; pairwise comparisons of the treatment means reflect variation by year. Data were analyzed with a negative binomial distribution, and pairwise comparisons of treatment least square means were adjusted with Tukey-Kramer’s method for multiplicity. Significance was defined at α = 0.05.
Results
Environmental conditions.
From January to May each year, the average ambient air temperature in the field ranged from –0.9 to 21.1 °C during the day and –8.6 to 7.0 °C at night, with the coolest temperatures in 2019, followed by 2020, and the warmest in 2021 (Table 1). On average across years, the monthly diurnal temperature variation in the field was 10.7 to 11.4 °C. In the HT, the average ambient air temperature ranged from 25.2 to 25.8 °C during the day and 2.8 to 5.5 °C at night for Apr and May 2019 (Table 1). From Jan to May 2020 and 2021, the average ambient air temperature in the HT ranged from 10.1 to 24.6 °C during the day and –8.1 to 6.3 °C at night (Table 1). The average diurnal temperature variation each month in the HT was 21.4 to 23.7 °C across years. The average ambient air temperature in the HT was greater than in the field by 1.9 to 4.2 °C in 2019 (across April and May), 0.7 to 6.4 °C in 2020 (January–May), and 1.5 to 8.9 °C in 2021 (January–May) (Table 1).
The average daily, nighttime, and daytime air temperatures in by month in the high tunnel and the field from Jan through May in 2019, 2020, and 2021.
With the application of the warming practices from 21 Mar to 15 May 2019 in the HT, the average soil temperature [± standard error (SE)] of HT-LT + H was 18.7 °C, which was 6.2 ± 0.1 and 6.4 ± 0.1 °C greater than HT-LT and HT-control, respectively (Fig. 1). The average canopy air temperature during this period was 14.1 °C in HT-LT + H (17.8/10.2 °C day/night), 13.9 °C in HT-LT (18.3/9.5 °C day/night), and 13.1 °C in HT-control (17.1/9.1 °C day/night). During the application of the warming practices from 27 Feb to 10 May 2020, the soil temperature averaged 18.4 °C in HT-LT + H and was 7.2 ± 0.2 °C greater than HT-LT and HT-control, which were within 0.2 °C of one another (Fig. 1). The average canopy air temperature was 11.8 °C in HT-LT + H (18.6/4.7 °C day/night), which was 1.0 ± 0.0 and 1.6 ± 0.0 °C greater than HT-LT (18.8/3.9 °C day/night) and HT-control (17.6/3.8 °C day/night), respectively. With warming practices on 1 Feb to 9 Apr 2021, the average soil temperature was 14.2 ± 0.69 °C in HT-LT + H, 11.0 ± 0.40 °C in HT-LT, and 11.0 ± 0.47 HT-control (Fig. 1). The average canopy air temperature was 10.5 °C (15.2/2.7 °C day/night) in HT-LT and 8.9 °C (14.3/1.9 °C day/night) in HT-control.
Soil temperature (Tsoil) at a 0.05-m depth by date (m/dd) from Jan through May 2019 (top), 2020 (center), and 2021 (bottom). Management practices in the high tunnel (HT, purple), and field (F, green) include the use of low tunnels and soil heating (H, solid lines), low tunnels and no soil heating (LT, dashed lines), and no low tunnels or soil heating controls (C, dotted lines). Low tunnels with no soil heating and H were applied in the high tunnel and field, as indicated by the purple (HT) and green (F) vertical lines, with solid lines indicating the initiation and dashed lines indicating the termination of the warming practices.
Citation: HortScience 58, 4; 10.21273/HORTSCI16942-22
Soil temperature (Tsoil) at a 0.05-m depth by date (m/dd) from Jan through May 2019 (top), 2020 (center), and 2021 (bottom). Management practices in the high tunnel (HT, purple), and field (F, green) include the use of low tunnels and soil heating (H, solid lines), low tunnels and no soil heating (LT, dashed lines), and no low tunnels or soil heating controls (C, dotted lines). Low tunnels with no soil heating and H were applied in the high tunnel and field, as indicated by the purple (HT) and green (F) vertical lines, with solid lines indicating the initiation and dashed lines indicating the termination of the warming practices.
Citation: HortScience 58, 4; 10.21273/HORTSCI16942-22
Soil temperature (Tsoil) at a 0.05-m depth by date (m/dd) from Jan through May 2019 (top), 2020 (center), and 2021 (bottom). Management practices in the high tunnel (HT, purple), and field (F, green) include the use of low tunnels and soil heating (H, solid lines), low tunnels and no soil heating (LT, dashed lines), and no low tunnels or soil heating controls (C, dotted lines). Low tunnels with no soil heating and H were applied in the high tunnel and field, as indicated by the purple (HT) and green (F) vertical lines, with solid lines indicating the initiation and dashed lines indicating the termination of the warming practices.
Citation: HortScience 58, 4; 10.21273/HORTSCI16942-22
In the field from 21 Mar to 14 Jun 2019, the average soil temperature of field-LT + H was 19.0 °C, which was 7.0 ± 0.3 and 7.7 ± 0.3 °C greater than field-LT and field-control, respectively (Fig. 1). The average canopy air temperature was 13.2 °C (16.2/10.1 °C day/night), which was 0.8 ± 0.1 and 2.4 ± 0.3 °C greater than field-LT (15.8/9.4 °C day/night) and field-control (14.3/9.8 °C day/night), respectively. From 6 Mar to 23 May 2020, the average soil temperature was 17.4 °C for field-LT + H, which was 6.6 ± 0.2 and 7.4 ± 0.3 °C greater than field-LT and field-control, respectively (Fig. 1). The average canopy air temperature was 10.3 °C (15.3/4.5 °C day/night) in field-LT + H, which was 1.1 ± 0.1 °C greater than field-LT (14.2/3.9 °C day/night) and 2.4 ± 0.2 °C greater than field-control (12.0/4.2 °C day/night). In 2021, the average soil temperature was 14.3 ± 0.4 °C with field-LT + H, 11.7 ± 0.5 °C with field-LT, and 10.4 ± 0.4 °C with field-control from 23 Feb to 22 Apr (Fig. 1). The average canopy air temperature of the field-LT + H was 9.3 °C (13.0/3.6 °C day/night), whereas field-LT was 8.0 °C (13.5/2.4 °C day/night), and field-control was 7.6 °C (13.1/1.9 °C day/night).
Yield.
The average annual yields ± SE in stems per square meter in the HT across years were 15.7 ± 3.3 in HT-control, 19.4 ± 2.1 in HT-LT, and 16.4 ± 0.8 in HT-LT + H, with no significant differences among these management practices (Fig. 2). In terms of quality, the grade 1 yield of HT-LT + H was 28.4 ± 5.0% of the total yield, which was significantly less than both the percentage of grade 1 stems from HT-control at 70.3 ± 5.1% (P = 0.0002) and HT-LT at 60.3 ± 5.4% (P < 0.0001) (Fig. 3). Grade 2 stems ranged from 20.6 ± 4.1% of the total yield in HT-control to 32.6 ± 4.8% in HT-LT + H, with no significant differences among treatments (P > 0.05) (Fig. 3). The percentage of culls per total yield was 9.1 ± 5.6% in HT-control, 15.4 ± 7% in HT-LT, and 39 ± 9.4% in HT-LT + H, with only culls in HT-LT + H significantly different from HT-control (P = 0.0484) (Fig. 3).
Cumulative peony yield by harvest date (m/dd) in 2019 (top), 2020 (center), and 2021 (bottom) in northern Utah (lat. 41.77°N, long. 111.81°W; elevation, 1382 m). Management systems include high tunnel (HT, purple) and field (F, green), with the use of low tunnels and soil heating (LT + H, solid lines), low tunnels and no soil heating (LT, dashed lines), and unmanipulated controls (C, dotted lines). Black symbols denote the timings at which 10% (T10, circle), 50% (T50, triangle), and 90% (T90, square) of the total cumulative harvest occurred.
Citation: HortScience 58, 4; 10.21273/HORTSCI16942-22
Cumulative peony yield by harvest date (m/dd) in 2019 (top), 2020 (center), and 2021 (bottom) in northern Utah (lat. 41.77°N, long. 111.81°W; elevation, 1382 m). Management systems include high tunnel (HT, purple) and field (F, green), with the use of low tunnels and soil heating (LT + H, solid lines), low tunnels and no soil heating (LT, dashed lines), and unmanipulated controls (C, dotted lines). Black symbols denote the timings at which 10% (T10, circle), 50% (T50, triangle), and 90% (T90, square) of the total cumulative harvest occurred.
Citation: HortScience 58, 4; 10.21273/HORTSCI16942-22
Cumulative peony yield by harvest date (m/dd) in 2019 (top), 2020 (center), and 2021 (bottom) in northern Utah (lat. 41.77°N, long. 111.81°W; elevation, 1382 m). Management systems include high tunnel (HT, purple) and field (F, green), with the use of low tunnels and soil heating (LT + H, solid lines), low tunnels and no soil heating (LT, dashed lines), and unmanipulated controls (C, dotted lines). Black symbols denote the timings at which 10% (T10, circle), 50% (T50, triangle), and 90% (T90, square) of the total cumulative harvest occurred.
Citation: HortScience 58, 4; 10.21273/HORTSCI16942-22
The percentage of grade 1 (green), grade 2 (purple), and cull (blue) stems per average total yield across years with standard error in (A) the high tunnel and (B) the field by management practice, including an unmanipulated control (C), use of a low tunnel (LT), and use of a low tunnel and soil heating (LT + H). Lowercase letters indicate significance for grade 1, grade 2, cull, and total yield compared between management practices within the high tunnel and field, respectively, and not across production systems (α = 0.05).
Citation: HortScience 58, 4; 10.21273/HORTSCI16942-22
The percentage of grade 1 (green), grade 2 (purple), and cull (blue) stems per average total yield across years with standard error in (A) the high tunnel and (B) the field by management practice, including an unmanipulated control (C), use of a low tunnel (LT), and use of a low tunnel and soil heating (LT + H). Lowercase letters indicate significance for grade 1, grade 2, cull, and total yield compared between management practices within the high tunnel and field, respectively, and not across production systems (α = 0.05).
Citation: HortScience 58, 4; 10.21273/HORTSCI16942-22
The percentage of grade 1 (green), grade 2 (purple), and cull (blue) stems per average total yield across years with standard error in (A) the high tunnel and (B) the field by management practice, including an unmanipulated control (C), use of a low tunnel (LT), and use of a low tunnel and soil heating (LT + H). Lowercase letters indicate significance for grade 1, grade 2, cull, and total yield compared between management practices within the high tunnel and field, respectively, and not across production systems (α = 0.05).
Citation: HortScience 58, 4; 10.21273/HORTSCI16942-22
Average annual yields across years by field management practice were 16.1 ± 1.9, 16.2 ± 1.0, and 20.8 ± 1.6 stems/m2 for field-control, field-LT, and field-LT + H, respectively (Fig. 2), with total yield in field-LT + H significantly greater than field-control (P = 0.0102) and field-LT (P = 0.0310). The percentages of grade 1 stems were 70.1 ± 5.0%, 63.3 ± 5.3%, and 61.9 ± 5.4% for field-control, field-LT, and field-LT + H, respectively, whereas grade 2 percentages were 19.6 ± 4.1%, 26.7 ± 4.5%, and 27.8 ± 4.6% for field-control, field-LT, and field-LT + H, respectively (Fig. 3). Culls per total yield ranged from 10.0% to 10.4% across field treatments, and no differences in percentages of grade 1, grade 2, or cull were significant among any of the field management practices (P > 0.05) (Fig. 3).
Harvest timing.
Across the HT practices, harvest occurred from 18 to 28 Apr until 5 to 13 May, with the spread of harvest (T10–T90) lasting as little as 6 d in HT-control to 16 d with HT-LT + H (Fig. 2). Initial harvest ranged from 3 to 16 d earlier in HT-LT + H compared with HT-control, and 4 to 9 d earlier in HT-LT compared with HT-control. Across years, the initial harvest of HT-control was significantly later than HT-LT + H (P = 0.0061) and HT-LT (P = 0.0163), but differences were not significant between HT-LT + H and HT-LT (P = 0.6575). The timing at which 50% of the total cumulative harvest occurred was 4 to 12 d earlier in HT-LT + H compared with HT-control, with greater differences in timing occurring in years with an earlier onset of heating (Fig. 2). Differences in peak and final harvest dates, however, were not significant across the HT management practices (P > 0.05). In the field, the average timing of harvest ± SE in days occurred from 19 May ± 2 d to 4 Jun 2019 ± 1 d, 16 May ± 1 d to 30 May 2020 ± 1 d, and 19 May ± 1 d to 3 Jun 2021 ± 1 d, with the spread (T10–T90) lasting as little as 4 d in field-control to a maximum of 10 and 15 d in field-LT + H and field-LT, respectively (Fig. 2). Initial harvest ranged from 6 to 7 d earlier in field-LT + H compared with field-control, and up to 3 d earlier in HT-LT compared with HT-control. The timing at which 50% of the total cumulative harvest occurred was 10 to 14 d earlier in field-LT + H and field-control, which was the only significant difference in timing among management practices in the field (P = 0.0321).
Economic assessment.
For one HT, the average yield was 552, 636, and 385 marketable stems, and 56, 116, and 251 cull stems with HT-control, HT-LT, and HT-LT + H, respectively. With HT stems harvested before Mother’s Day, and 100% of marketable stems sold at $6 per stem and 70% of cull stems at $2 per stem, the average receipts were $3390, $3978, and $2661 for HT-control, HT-LT, and HT-LT + H, respectively, whereas net returns were $1433, $768, and –$559 for HT-control, HT-LT, and HT-LT + H, respectively. In the field, the average yield was 567, 567, and 735 marketable stems, and 60, 64, and 74 cull stems with field-control, field-LT, and field-LT + H, respectively. With harvest after Mother’s Day and 100% of marketable stems sold at $5 apiece and 70% of cull stems at $2 apiece, the average receipts were $2919, $2926, and $3779 for field-control, field-LT, and field-LT + H, respectively, whereas net returns were $1441, $679, and $604 for field-control, field-LT, and field-LT + H, respectively.
Discussion
The HT alone advanced the initial harvest of ‘Coral Charm’ by 20 to 25 d in northern Utah, which allowed for production in time for the Mother’s Day holiday, compared with the natural timing of unprotected field production in late May. The use of LTs and H further advanced initial and peak harvest compared with control practices by 3 to 16 d in the HT and 6 to 7 d in the field, allowing staggered harvest for a total production season length of 34 to 38 d. Although demand remained high throughout the production season (i.e., all stems were sold-out via preorder to florists), the market only commanded premium pricing before the Mother’s Day holiday. Through earlier onset of H practices, HT production began as early as the first week of April in 2021, indicating the potential in reaching other early holiday markets, such as Easter, in some years. Advancing and staggering early-season production of ‘Coral Charm’ peony demonstrates peony as a promising, early-season crop for high-elevation growers without greenhouse access, as well as indicates the ability to advance and schedule production ahead of targeted markets.
Initiating soil warming practices earlier in the high tunnel, beginning in mid-March in 2019 to early February in 2021, advanced emergence, and hence production. This indicated chilling requirements were satisfied by January or earlier in Northern Utah because of the cool winter temperatures of the region. Initially increasing soil temperature effectively targeted emergence, after which focusing on air temperature helped advance flowering, which is supported by previous studies (Fulton et al. 2001; Hall et al. 2007). The duration of applying H and LTs was optimized from early February through early April in the HT, which ensured stem harvest before Mother’s Day, maintained greater stem quality, and reduced input costs, such as electrical use and labor to vent low tunnels. Using surface heat mats to warm the soil protected peony roots from disturbance, and the greater heat capacity of the soil allowed for greater efficiency in warming compared with air. Although maintaining a recommended 22/10 °C day/night temperature (Hall et al. 2007) was difficult to regulate in a passively heated HT, constraining the average day/night canopy air temperatures from 14 to 19 °C (day) and 2 to 10 °C (night) was possible from early February onward with the warming practices. Surface heating also likely warmed the air under LTs at night, when the heat mats were activated, which led to a greater nighttime canopy temperature under the LT. Additional research on this potential benefit is needed.
The HT warming practices did not affect total yield but did reduce stem quality because of increased insect and disease pressure. As canopy air temperature and humidity increased under LT + H, increased incidence of botrytis blight (Botrytis paeoniae) killed stems, and strawberry root weevil (Otiorhynchus ovatus) caused visible damage to the leaves. Physical damage also resulted from the LTs, as stems quickly curved in response to contact with tunnel fabric. Although the average day/night temperatures were cool to optimal (Table 1), hourly temperature data highlighted challenges with superoptimal afternoon temperatures later in the season. Earlier and additional venting of LTs may reduce the bud and leaf burn that occurred when daytime canopy air temperatures exceeded 32 °C, although this must balance practical limits to added management costs in HT systems. However, plants may be able to tolerate greater daytime temperatures for short periods of time when nighttime tunnel conditions are cool and well ventilated. In our study, flower bud maturity accelerated, and damage did not occur below 32/17 °C day/night in the vented HT or field. Previous research found bud abortion rates were high when greenhouse temperatures were held at 28/22 °C day/night (Halevy et al. 2005), or with nighttime temperatures greater than 20 °C (Kamenetsky et al. 2003). Therefore, the combined effects of managing airflow, humidity, and canopy air temperature should be considered in establishing responses to superoptimal conditions in the field, and to inform cultural practices when managing HTs, LTs, or H techniques.
In the field, the use of heating and LTs did not stagger harvest significantly despite earlier treatment application dates in 2020 and 2021. However, by 2021, the plants with H produced more stems, leading to a statistically greater yield across the 3-year study, whereas plants in the other management practices remained consistent. Although not previously studied with peony, the influence of soil temperature has been evaluated for cane growth and berry yield of primocane raspberries (Prive et al. 1993). Adequate irrigation and soil temperatures greater than 16 °C in April and May led to increased cane growth, length between nodes, flowers, berry weight, and overall yield during initial harvests (Prive et al. 1993). Primocane raspberries and peony are both maintained as herbaceous multistem plants. These results from raspberry may indicate that, for peonies, increased soil temperatures early in the year may have an impact on stem yields, especially over several years. Soil heating may optimize production temperature in the field for cool environments, such as the high-elevation US Intermountain West.
Differences in flower quality were not significant in the field, likely because lower temperatures and humidity were maintained across all management practices as a result of greater air circulation among plants. The maximum daily temperatures were also typically less in the field than the HT during April and May, which resulted in less heat damage. Moreover, LTs were also removed earlier during plant growth, when ambient field air temperature exceeded the optimum day/night temperatures, which reduced insect and disease pressure. The critical temperatures for bud abortion can be as high as 17 °C at night (Byrne and Halevy 1986) to 28 °C during the day (Kamenetsky et al. 2003) under greenhouse conditions. For HT and field systems, which are exposed to greater environmental fluctuations, increased venting or removal of heating treatments when air temperatures exceed 16 °C is recommended.
Overall, HTs advanced the production of early-season peony cut flowers compared with unforced field production in the US Intermountain West, with added LT and H practices staggering harvest up to 38 d. Chilling requirements were met by January, and earlier application of H and LTs in 2020 and 2021 further advanced and staggered field production, although the premium early-market pricing did not outweigh the greater input costs for the tested markets. Increasing stem quality through additional venting to limit bud abortion and disease may justify economically the increased cost of the H practices, in addition to limiting the H duration further and testing stem pricing in larger, higher end markets (i.e., urban hubs or critical market dates such as holidays). Other early-season benefits include earlier seasonal association of local flowers with consumers, farm name recognition from innovative production timing, and additional opportunity to support and use farm labor. Although this work is aimed toward high-elevation growers in USDA Hardiness Zone 5 regions, the findings add to the collective body of research that furthers peony field production methods and maximizes HT systems for small-scale growers.
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