Abstract
Dahlias (Dahlia ×hybrida) are a popular cut flower for local production in the northeastern United States. However, there are more than 20,000 cultivars to choose from, and the suitability of these cultivars as cut flowers varies regionally. Fourteen dahlia cultivars were grown in Orono, ME, USA: Blizzard, Burlesca, Café au Lait, Café au Lait Rose, Clearview Daniel, Cornel, Cornel Bronze, Ivanetti, Lollipop, Neon Splendor, Rock Run Ashley, Sunspot, Tanjoh, and Tempest. These cultivars were selected after interviews with local dahlia growers. These cultivars all produced similar numbers of flowers, but they differed in the time to form flowers, stem length, and stem diameter. ‘Rock Run Ashley’ was the earliest to begin flowering, at 35 days earlier than ‘Tempest’ and ‘Café au Lait’, which flowered last. ‘Blizzard’ and ‘Tempest’ had the longest stems and ‘Lollipop’ had the shortest stems. Growers may want to choose ‘Rock Run Ashley’ if they need flowers earlier in the season, or ‘Blizzard’ or ‘Tempest’ if a longer stem length is desired. During a second study, we harvested field-grown flowers of ‘Burlesca’, ‘Cornel’, and ‘Ivanetti’ and treated them with deionized water or one of two commercial holding solutions. Holding solutions did not extend the vase life of ‘Burlesca’ or ‘Ivanetti’, but they increased the vase life of ‘Cornel’ by 4 or 5 days.
Dahlias (Dahlia ×hybrida) are a popular specialty cut flower for local production in the United States. Local production is favored, in part, because dahlias do not have a sufficient postharvest lifespan to survive shipping long distances (Armitage and Laushman 2003). Loyola et al. (2019) reported that 73% of cut flower growers produce these tender perennials for their late summer and fall blooms. Wide diversity exists among the ∼20,000 cultivars of dahlias, and most are hybrids (Armitage 2008). Flower colors include yellow, orange, red, pink, lavender, and bicolors. There are at least 29 recognized flower forms, and flower sizes range from less than 2 inches to more than 10 inches in diameter (American Dahlia Society 2023). Of these cultivars, some are grown for landscape use and would not be appropriate for cut flower production because of small plant size or short stem length. Even considering this, growers may choose from a wide variety of dahlias for cut flower production.
Differences in regional weather patterns and climate make it important to trial cut flowers to determine which cultivars are most appropriate in each region (Lewis et al. 2021). This is particularly true for dahlias grown in the northeastern United States because dahlias begin to produce flowers during late summer to early fall, and frost typically ends the production season. Some cultivars, which begin to produce flowers later than others, may have a limited harvest season. Moreover, certain cultivars may produce more flowers or produce flowers of better quality (e.g., flowers with longer stems).
Another challenge with using dahlias as cut flowers is that the vase life is not particularly long; this is a concern identified by specialty cut flower growers (Loyola et al. 2019). In general, it is desirable for cut flowers to have a vase life of ∼1 week; dahlias typically have a shorter vase life of 4 to 5 d when they are held in water only (Bergmann et al. 2018; Dole et al. 2017). This short vase life makes dahlias more suitable for local production and sales, and for use at special events, such as weddings. However, a longer vase life would provide more flexibility for growers and florists.
Few commercially available postharvest floral treatments extend the vase life of dahlias. Commercial hydrating solutions are used for short periods of time of 1 d or less to improve water uptake in stems. They improve the vase life of some cut flower species, but they do not have an impact on dahlia vase life (Clark et al. 2010). Holding solutions, which contain carbohydrates, acidifying agents, and germicides, reduce microbial growth and extend the life of dahlias in some cases, but not others (Clark et al. 2010; Dole et al. 2009). Benzyladenine and gibberellic acid are applied to dahlias as a pulse, which, typically, is a concentrated solution in which flowers are soaked for a short period of time (e.g., approximately 24 h). Although these plant growth regulators can extend the vase life of dahlias, they are not as versatile or commonly used as commercially available floral preservatives (Bergmann et al. 2018). Other preservatives or approaches to extending vase life may be desirable.
Our primary objective was to grow and evaluate a variety of dahlia cultivars in the northeastern United States (United States Department of Agriculture Hardiness Zone 5a). We compared cultivars to determine whether the flower timing, yield, and quality differ. Our second objective was to determine whether two commonly available commercial holding solutions extend the vase life of dahlia. We compared the vase life of three cultivars stored in holding solutions intended for flower storage and transport.
Materials and methods
Comparison of cultivars for field-grown flower production
The following dahlia cultivars were grown during our trial: Café au Lait, Café au Lait Rose, Burlesca (Fleur Farm, East Dorset, VT, USA); Cornel, Clearview Daniel, Lollipop, Neon Splendor, Tanjoh (Swan Island Dahlias, Canby, OR, USA); Blizzard, Tempest (Endless Summer Flower Farm, Camden, ME, USA); Cornel Bronze and Rock Run Ashley (Ridgeview Dahlias, Hardinsburg, IN, USA). All of the dahlia cultivars were planted as one large single or two small tuberous roots, with the exception of Café au Lait, Café au Lait Rose, and Burlesca, which were planted as clumps consisting of more than two tuberous roots. The classifications of flower type, diameter, and color of cultivars that growers recommended are listed in Table 1 (American Dahlia Society 2023).
Dahlia cultivars recommended by at least one of four growers interviewed in Maine, USA, and the number of growers who recommended each cultivar.
We selected these cultivars based on in-person, phone, or e-mail interviews with four dahlia growers in Maine, USA, during Winter and Spring 2022. With the exception of Lollipop, all of the cultivars included in this trial were recommended by at least one grower, with most cultivars recommended by two or three growers. Although we were unable to locate or plant all of the suggested cultivars, all cultivars that growers suggested are listed in Table 1. We included ‘Lollipop’ because a grower included them in a shipment of other cultivars.
The field trial was conducted at the Lyle E. Littlefield Garden in Orono, ME, USA. The soil at the Littlefield Garden is loam (47% sand, 41% silt, 12% clay) with an unadjusted pH of 6.1. The ground was tilled 1 week before planting, and dolomitic lime was added at a rate of 5 lb/100 feet2 based on recommendations from the University of Maine Soil Testing Service (Orono, ME, USA). Five tuberous roots of each cultivar were planted on 18 May 2022, when the soil was 60 °F. One tablespoon (∼10 g) of 5N–4.3P–8.3K fertilizer (Morcrop Tomato and Vegetable Food; Lilly Miller Brands, Atlanta, GA, USA) was applied as a sidedress next to each dahlia at planting. Each tuberous root was planted 4 to 6 inches deep, with the eye facing up. Dahlias were planted within a 10-foot × 60-foot area in two rows spaced 45 inches apart, with 24 inches between plants within rows. Plants were mulched with ∼1 to 2 inches of bark mulch after planting. At planting, drip irrigation was installed using drip tape with emitters spaced 8 inches apart. A mesh trellising system (Hortonova; Tenax Corp., Baltimore, MD, USA) was installed to support the plants on 25 May 2022. The trellising system had 6-inch openings and was attached to 5-foot wooden stakes spaced ∼7 feet apart. The trellis was suspended horizontally and ∼2 feet above the ground. On 12 Jul, an additional 10 g of the same fertilizer was applied as a sidedress when plants were ∼1 foot tall. Irrigation was applied as needed beginning on 13 Jul 2022, after plants had emerged. The soil moisture was checked daily at a depth of ∼1 inch, and plants were irrigated if the soil was dry. Plants were pinched on 13 Jul; two to three nodes were removed from each lateral shoot on each plant. On 28 Jul 2022, the staking system was changed to a corralling system with two stakes and sisal twine, rather than the trellis, to provide more support and easier access to flowers during harvest.
From 3 Jun to 13 Jul, plants were checked every weekday to determine when shoots emerged from the ground. From 5 Jul to 12 Sep, plants were checked each weekday to determine when the first visible bud appeared; from 5 Aug to 26 Sep, they were checked to determine when the first flower emerged. Flowers were considered open when the two outermost rows of ray florets were fully open (Dole et al. 2017). Beginning with the first open flowers on 8 Aug, flowers were harvested twice weekly (on Monday and Friday) until 3 Oct, which was the date of the first frost. All flowers included in the postharvest study were harvested before 10:00 AM. Other flowers were harvested in the morning, but not necessarily before 10:00 AM. Flower stems were cut to a length of at least 30 cm. They were cut above the node nearest 30 cm; in some cases, they were cut longer than this length for plants with long internodes. Any stems shorter than 30 cm with open flowers were removed from plants, but they were not included in the results. The length of all harvested stems and diameter of a representative flower from each plant were measured on each harvest date. The dates of the first and last harvest for each cultivar were recorded, as was the total number of flowers harvested from each plant throughout the season.
Soil temperature, soil moisture, air temperature, and relative humidity were monitored throughout the study using three soil moisture and temperature sensors (Echo TM; METER Group, Inc., Pullman, WA, USA) and a relative humidity and temperature sensor (VP-4; METER Group, Inc.) connected to a datalogger (EM50, METER Group, Inc.). All data were collected every 10 minutes. The average volumetric water content was 0.19 m3⋅m−3. The average soil temperature was 19 °C. The average daily air temperature was 17.8 °C. The average relative humidity was 73.5%.
The plants were arranged in a randomized complete block design with five blocks. One experimental unit was a plant representative of each cultivar. Measurements of the flower diameter and stem length were averaged for each cultivar in each block before analysis. Differences among cultivars were analyzed by an analysis of variance (ANOVA) and Tukey’s means separation using statistical analysis software (JMP Pro version 16; SAS Institute Inc., Cary, NC, USA), with an alpha of 0.05.
Postharvest evaluation of cut flowers
Flowers of ‘Cornel’, ‘Burlesca’, and ‘Ivanetti’ were harvested as described on 19 Sep (‘Cornel’), 26 Sep (‘Burlesca’), or 30 Sep (‘Ivanetti’). Stems were re-cut to a length of 30 cm, with the bottoms of stems cut horizontally. Cut stems were placed in pint-size glass ball jars containing 300 mL of one of the following three solutions: deionized water with citric acid added to achieve a solution pH of 3.1 to 4.6 (control); deionized water with 2 teaspoons/L concentrated transport and display solution (Chrysal Professional 2; Chrysal International, Naarden, the Netherlands); or deionized water with 2 teaspoons/L concentrated storage and transport solution (Floralife 200; Smithers-Oasis, Kent, OH, USA). Flowers were placed in individual vases on benches in a room in which the average temperature and relative humidity recorded by a portable data logger (Omega OM-92; Omega Engineering, Inc., Norwalk, CT, USA) were 24.7 °C and 25.5%, respectively. Fluorescent lights in the room were set to a 12-h photoperiod. To determine the vase life, flowers were evaluated daily until they would no longer be acceptable for home consumers. Flowers were considered no longer acceptable when most of the ray florets in the outer two rows were brown, at which point, browning was noticeable when looking directly at the flower head, which is the standard used by Dole et al. (2009).
This experiment was arranged as a randomized complete block design with four blocks. Each experimental unit consisted of a single flower in a jar with water or holding solution, and each treatment was replicated four times. Data were analyzed by an ANOVA and Tukey’s means separation using statistical analysis software (JMP Pro version 16).
Results and discussion
Comparison of cultivars for field-grown flower production
The number of flowers produced throughout the season did not differ among cultivars (Table 2). Cultivars differed in the cut flower characteristics of stem length and flower diameter, as well as in the number of days until shoots emerged from the ground, until visible buds formed, and until flowering (Table 2). Although seasonal yield did not differ, growers may prefer cultivars based on their morphological differences or timing of plant and flower development.
Traits of 14 cultivars of field-grown dahlias, including the number of flowers harvested during the growing season, average stem length and flower diameter, and days from planting until plant emergence, until formation of visible buds, and until first flower.i
In general, longer stems are more desirable for cut flowers because they provide more flexibility for use in taller vases or in a variety of arrangements. A stem length more than 41 cm is ideal, but dahlias are marketable with stem lengths of 30 cm or more (Dole et al. 2009; Starman et al. 1995). We harvested stems that were at least 30 cm long, but some cultivars had very large internodes. Because we cut stems above the node closest to 30 cm, stems with longer internodes were generally cut longer. We removed stems that were shorter than 30 cm at the time of flower formation, but we did not count them as marketable stems or include them in our data collection. Some cultivars, such as Blizzard and Tempest, seemed to produce naturally longer stems than others, such as Lollipop (Table 2). ‘Blizzard’ and ‘Tempest’ stems were 19% and 20% longer, respectively, than ‘Lollipop’ stems. Among all other cultivars, the stems had similar lengths.
The flower diameter of dahlias varies widely depending on the cultivar. The American Dahlia Society (2023) classifies dahlias based on flower diameter as follows: giant (AA or >10 inches); large (A or 8–10 inches); medium (B or 6–8 inches); small (BB or 4–6 inches); miniature (M or 2–4 inches); or micro (MC or up to 2 inches). Because there are so many dahlia cultivars, not all cultivars available on the market or used for this study have been classified according to their flower diameter. Based on our flower diameter measurements, those of all cultivars that have been classified based on size by the American Dahlia Society agree with our measured flower sizes (Tables 1 and 2). The exception is ‘Tempest’, which would have been categorized as M rather than BB based on our results. Some natural variation in flower diameter based on the growing environment or tuberous root quality likely caused this difference; in general, ‘Tempest’ did not perform well during our study because not all plants emerged from the ground.
Among the cultivars that the American Dahlia Society has not classified according to flower diameter, our measurements suggest that Burlesca, Clearview Daniel, Cornel, Cornel Bronze, and Ivanetti should be classified as M, whereas Café au Lait Rose should be classified as B (Table 2). It is notable that most of the cultivars we included in this study (9 of 14) were classified as M. Growers mostly recommended plants with B (8 cultivars), BB (16 cultivars), or M (11 cultivars) diameter flowers (Table 1). Very few AA (2 cultivars) and A (2 cultivars) cultivars were recommended. One of the growers we interviewed reported not growing many large dahlias because the flowers seem to take longer to open. None of the growers recommended cultivars with the smallest diameter (MC).
The timing of plant emergence, formation of the first visible bud, and formation of the first flower varied among cultivars (Table 2). The shoots from the earliest cultivars emerged from the ground ∼2 weeks after tuberous roots were planted; the last date for the emergence of shoots was 26 Jul (Table 2). The earliest cultivar to emerge from the ground was Rock Run Ashley, whereas Tanjoh was the last to emerge. There were no significant differences in time to the emergence of shoots among other cultivars. Dahlia plants formed visible buds starting at the end of July (27 Jul for the earliest plants), and the latest plants formed their first buds during the last week of August (∼31 Aug). ‘Cornel Bronze’ and ‘Burlesca’ flowers were the first to form buds, whereas ‘Tempest’ was the last to form buds (Table 2).
None of our plants flowered before the second week of August (13 Aug); however, growers in warmer coastal or more southern areas of the northeastern United States reported that they begin to see flowers in late July. ‘Rock Run Ashley’ flowered before any other plants, and ‘Café au Lait’ and ‘Tempest’ flowered last (Table 2). These plants flowered so late in the season that some ‘Café au Lait’ plants never formed flowers before the frost on 3 Oct. The frost occurred early and allowed for a harvest season of only 14 d for ‘Café au Lait’, which is the cultivar that flowered the latest. In comparison, Rock Run Ashley, which was the earliest cultivar to flower, had a harvest season of 51 d. Season extension would be helpful, particularly for plants such as ‘Café au Lait’ and ‘Tempest’, that started flowering last. High tunnels are useful for extending the season for a variety of cut flowers (Wien 2009), and some dahlia growers we interviewed successfully use high tunnels for season extension. Oritz et al. (2012) found that dahlia ‘Karma Thalia Dark Fuchsia’ produced flowers ∼4 d earlier in high tunnels than in the field; however, the seasonal yield and flower quality were not increased in high tunnels.
Postharvest evaluation of cut flowers
Placing flower stems in a holding solution may improve the vase life of cut flowers. For example, the vase life of ‘Karma Thalia’ dahlia flower stems placed in holding solutions was 3 to 4 d longer than the vase life of stems held in deionized water (Dole et al. 2009). In our study, the impact of floral preservatives on vase life depended on the cultivar. ‘Ivanetti’ and ‘Burlesca’ had the same vase life regardless of whether they were held in deionized water or in holding solutions (Table 3). In contrast, the vase life of ‘Cornel’ flower stems was 4 or 5 d longer when they were held in commercial holding solutions (Floralife 200 and Chrysal Professional 2, respectively) (Table 3). Floral preservatives provide vase life extension for dahlias, but the impact is not consistent across all cultivars. We did not compare cultivars directly, but Ivanetti and Burlesca had a vase life that was 3 d longer than that of Cornel when they were held in water. Some dahlia cultivars evidently have a naturally shorter vase life, such as Naomi, which has a vase life of 4 d, and Thalia, which has a vase life of 3 d (Dole et al. 2009). If other dahlia cultivars have a naturally longer vase life, then they may not need floral preservatives if they are sold locally. In the future, it would be interesting to compare the vase life of a variety of cultivars without the use of floral preservatives.
Vase life of dahlia cultivars Cornel, Ivanetti, and Burlesca when flowers were held in pure water (control) or one of two commercial holding solutions.
Conclusions
The dahlia cultivars that we grew during this trial produced similar numbers of flowers during the season. However, they differed with respect to stem length, flower diameter, and timing of flower formation. Stem length was greatest for ‘Blizzard’ and ‘Tempest’ (40.5 cm and 39.8 cm, respectively) and the least for ‘Lollipop’ (32.3 cm). Six of the cultivars we grew during this study have not been classified according to flower diameter. We recommend classifying the cultivars Burlesca, Clearview Daniel, Cornel, Cornel Bronze, and Ivanetti as M and Café au Lait Rose as B for flower diameter. ‘Rock Run Ashley’ formed flowers 87 d after planting, which was earlier than any other cultivars. In contrast, ‘Café au Lait’ and ‘Tempest’ formed flowers 122 to 124 d after planting and were the last cultivars to begin to flower. Although some cultivars, such as Café au Lait, flowered so late that they produced very few flowers, growers may want to cultivate them for their large bloom size and unusual flower form. In the case of ‘Tempest’, growers may choose to plant this cultivar for long stems, even though the flowers formed late. When the harvest season is short, growers may choose to use season extension techniques. Alternatively, some growers may want to grow additional early-flowering cultivars, such as Rock Run Ashley, so that they can begin marketing dahlias several weeks earlier.
Although dahlia is an excellent flower for local production in the northeastern United States, the vase life of ‘Ivanetti’, ‘Burlesca’, and ‘Cornel’ was never long enough for flowers to be held during long-distance shipping, even with the use of floral preservatives. Commercial holding solutions (Floralife 200 and Chrysal Professional 2) increased the vase life of ‘Cornel’ dahlia flowers by 4 to 5 d, but they had no impact on the vase life of ‘Ivanetti’ or ‘Burlesca’, indicating that these preservatives may be useful for certain cultivars.
Units
References cited
American Dahlia Society. 2023. ADS dahlia classification and number guide. https://dahlia.org/wp-content/uploads/2018/01/4digit_conversion.pdf. [accessed 12 Jan 2023].
Armitage AM. 2008. Herbaceous perennial plants: A treatise on their identification, culture, and garden attributes (3rd ed). Stipes Publishing LLC, Champaign, IL, USA.
Armitage AM, Laushman JM. 2003. Specialty cut flowers: The production of annuals, perennials, bulbs, and woody plants for fresh and dried cut flowers (2nd ed). Timber Press, Portland, OR, USA.
Bergmann BA, Ahmad I, Dole JM. 2018. Benzyladenine and gibberellic acid pulses improve flower quality and extend vase life of cut dahlias. Can J Plant Sci. 99:97–101. https://doi.org/10.1139/cjps-2018-0126.
Clark EMR, Dole JM, Carlson AS, McCall IF, Fanelli FL, Fonteno WC. 2010. Vase life of new cut flower cultivars. HortTechnology. 20:1016–1025. https://doi.org/10.21273/HORTSCI.20.6.1016.
Dole JM, Viloria Z, Fanelli FL, Fonteno W. 2009. Postharvest evaluation of cut dahlia, linaria, lupine, poppy, trachelium, and zinnia. HortTechnology. 19:593–600. https://doi.org/10.21273/HORTSCI.19.3.593.
Dole J, Stamps R, Carlson A, Ahmad I, Greer L, Laushman J. 2017. Postharvest handling of cut flowers and greens: A practical guide for commercial growers, wholesalers, and retailers. Association of Specialty Cut Flower Growers, Inc., Oberlin, OH, USA.
Lewis M, Stock M, Black B, Drost D, Dai X. 2021. Improving snapdragon cut flower production through high tunnel season extension, transplant timing, and cultivar selection. HortScience. 56:1206–1212. https://doi.org/10.21273/HORTSCI15910-21.
Loyola CE, Dole JM, Dunning R. 2019. North American specialty cut flower production and postharvest survey. HortTechnology. 29:338–359. https://doi.org/10.21273/HORTTECH04270-19.
Oritz MA, Hyrczyk K, Lopez RG. 2012. Comparison of high tunnel and field production of specialty cut flowers in the Midwest. HortScience. 47:1265–1269. https://doi.org/10.21273/HORTSCI.47.9.1265.
Starman TW, Cerny TA, MacKenzie AJ. 1995. Productivity and profitability of some field-grown specialty cut flowers. HortScience. 30:1217–1220. https://doi.org/10.21273/HORTSCI.30.6.1217.
Wien C. 2009. Floral crop production in high tunnels. HortTechnology. 19:56–60. https://doi.org/10.21273/HORTSCI.19.1.56.