Forcing Herbaceous Perennials to Flower after Storage Outdoors under a Thermoblanket

Jeffery K. Iles1 and Nancy H. Agnew2

Additional index words. flowering potted , inflorescence, low temperature, glasshouse, photoperiod

Summary. Nine herbaceous perennial were evaluated for use as flowering potted plants for late winter and early spring sales. Plugs of ‘King Edward’ Achillea × Lewisii Ingw. (yarrow), Arabis sturii Mottet. (rock- cress), ‘Alba’ Armeria maritima (Mill.) Willd. (common thrift), ‘New Hybrid’ Bergenia cordifolia (Haw.) Sternb. (bergenia), Chrysogonum virgianum L. (goldenstar), ‘War Bonnet’ Dianthus × Allwoodii Hort. Allw. (Allwood pinks), Phlox × chattahoochee L. (Chattahoochee phlox), ‘Sentimental Blue’ Platycodon grandiflorus (Jacq.) A. DC. (balloonflower), and L. × ‘Sunny Border Blue’ (veronica) were established in 14-cm (0.8-liter) round plastic containers, grown for one season and covered with a thermoblanket for winter. Five plants of each species were transferred to a 21 ± 3C glasshouse for forcing under natural daylengths at six 10-day intervals beginning 1 Dec. 1993. Arabis sturii, Phlox × chattahoochee, Platycodon grandiflorus ‘Sentimental Blue’, and Veronica × ‘Sunny Border Blue’ flowered out of season without supplemental lighting. ‘Alba’ Armeria maritima and Chrysogonum virginianum also flowered; however,

1Assistant professor, Department of Horticulture, Iowa State University, Ames, IA 50011. 2Associate professor, Department of Horticulture, Iowa State Unversity, Ames, IA 50011. Journal paper no. J-16092 of the Iowa Agriculture and Home Economics Experiment Station, Ames. Project no. 0121. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regu - lations, this paper therefore must be hereby marked adver- tisement solely to indicate this fact.

HortTechnology . July/Sept. 1995 5(3) 239 their floral displays were less effective. under natural daylength after winter polyethylene–microfoam 0.6-cm ‘New Hybrid’ Bergenia cordifolia did storage outdoors beneath an insulat- thermoblanket (Ametek, Wurtland, not flower and ‘King Edward’ ing thermoblanket. Ky. ), which was pulled tightly over the Achillea x Lewisii and ‘War Bonnet’ plants and secured with landscape tim- Dianthus x Allwoodii only flowered Materials and methods bers. Every 10 days beginning 1 Dec. sporadically, therefore, these perenni- Plugs (3.2 cm wide x 5.1 cm 1993 and ending 20 Jan. 1994, five als are not recommended for forcing plants of each species were chosen ran- out of season using our vernalization deep) of ‘King Edward’ Achillea × method. Lewisii, Arabis sturii, ‘Alba’ Armeria domly from beneath the overwintering maritima, ‘New Hybrid’ Bergenia cover and transferred to a 21 ± 3C glass- cordifolia, Chrysogonum virginianum, glazed greenhouse for forcing under ‘War Bonnet’ Dianthus × Allwoodii, natural daylength (lat. 42°3’N ). Plants etail greenhouse and gar- Phlox × chattahoochee, ‘Sentimental in each treatment (forcing date) were den center managers, par- Blue’ Platycodon grandiflorus, and randomized in separate blocks on Rticularly those located in cold Veronica × ‘Sunny Border Blue’ were benches and fertilized weekly with 200 climates, are continually searching for planted on 10 Apr. 1993 in 0.8-liter ppm N from a 21N–2.2P–16.6K Excel ways to expand the selling season. (14-cm) round plastic containers us- al-purpose water-soluble fertilizer (21- Toward this end, researchers have ing a medium of 2 Canadian sphag- 5–20) (The Scotts Co.). Days to first evaluated many hardy herbaceous pe- num peat: 2 perlite :1 sandy-loam soil flower, days of flowering (from first rennials as out-of-season flowering (by volume) amended with ground open flower to wilting of last flower), potted plants for late winter and early calcitic limestone (pH 6.5). On 20 flower number (number of racemes spring sales (Armitage, 1993). May, plants were transferred to hoop- for Veronica and individual flower In the landscape, seasonal changes houses covered with woven black heads for Armeria), and height in temperature and photoperiod pro- polypropylene shade fabric (20% ra- at first open flower were recorded for vide herbaceous perennials with the diation exclusion). Shade fabric was each species. Spent flowers were re- necessary cues to break dormancy and removed 1 Oct. 1993. Each plant re- moved from plants as data were re- to flower. Forcing container-grown ceived 5 g of 17N–2.6P–10K Sierra corded. Data were subjected to analy- perennials to flower out of season re- fertilizer plus micronutrients (17-6- quires that growers imitate specific 12) (The Scotts Co., Marietta, Ga.) Fig. 1. Maximum and minimum environmental conditions required by and was irrigated as needed. daily ambient air temperatures each species to complete its natural from 30 Sept. 1993 to 20 Jan. 1994 On 20 Nov. 1993, plants were recorded at the Iowa State Univ. growth and development (Iversen and randomized, placed pot to pot, and Horticulture Research Station, Weiler, 1994). Some species, such as covered with a bonded 4-mil white Gilbert. Paeonia officinalis (Byrne, 1989), ‘Gloriosa’ Liatris spicata (Waithaka and Wanjao, 1983), Astilbe × arendsii (Beattie and Holcomb, 1983), Aqui- legia × hybrida (Shedron and Weiler, 1982a), and Aurinia saxatilis (Shedron and Weiler, 1982 b), must be exposed to a critical range of low temperatures for a specific time to induce growth and flowering. However, other plants, such as ‘Moonbeam’ Coreopsis verticillata, clethroides, ‘Fairy’s Petticoat’ Phlox paniculata, and ‘Taplow Blue’ Echinopsritro, require long days (at least 16 h) for rapid and uniform inflores- cence development (Iversen and Weiler, 1994). Based on recommendations from existing data, growers and retailers wishing to increase sales by forcing herbaceous perennials to flower out of season must have access to a facility that permits precise control of tem- perature and photoperiod. Unfortu- nately, many small-scale perennial growers and retailers do not have the capacity for such stringent environ- mental control. Therefore, the pur- pose of this study was to investigate forcing container-grown herbaceous perennials to flower in a glasshouse

240 2. Relationship between days to first flower and forcing date. Fig. 4. Relationship between days of flowering and forcing date. For each data point, n = 5. Regression equations and r values are For each data point, n = 5. Regression equations and r values are as follows: as follows: Arabis: y = 28.3 + 0.06x r = 0.78* Arabis: y = 77.9+ 1.13x r = 0.94* Chrysogonum: y = 26.4 + 0.24x r = 0.94* Chrysogonum: y = 65.9+ 0.90x r = 0. 99* Phlox: y = 28.9+ 0.22x r = 0.59* Phlox: y = 65+ 0.70x r = 0.89’ P 0.05. *P < 0.05.

Fig. 5. Relationship between days of flowering and forcing date. Fig. 3. Relationship between days to first flower and forcing date. For each data point, n. 5. Regression equations and r values are For each data point, n= 5. Regression equations and r values are as follows: as follows: Armeria: y = 97.5 + 0.84x r = 0.70* Armeria: y = 62.2 + 0.40x r = 0.71* Platycodon: y = 47 + 0.44x r . 0.93* Platycodon: y = 95.3+ 0.54x r= 0.95* Veronica: y = 51.6+ 0.10X r =0. 51NS Veronica: y = 81 + 0.26x r= 0.94* NS,*Nonsiflnificant or significant at P 0.05, respectively. *P 0.05.

241 even under thermo- flower number increased for Arabis and blankets (Iles et al., Platycodon resulting in a more impres- 1993). sive floral display over a shorter period Relationships of time. Because Arabis suffered low- between days to first temperature injury by the 20 Jan. forc- flower and forcing ing date, flower number data ii-em that date were negative treatment were dropped from the analy- and linear for Arabis sis. (Fig, 2), Armeria, Changes in plant height relative Platycodon, and Ver- to forcing date were significant only onica (Fig, 3). This for Platycodon (data not shown). In- trend was especially creasing exposure to low temperature noteworthy for Plat- or longer photoperiods increased stem ycodon, as days to first length for Platycodon forced in the flower decreased. by later treatments. Increased stem length 23 days between the is not always desirable when forcing 1 Dec. and 10 Jan. herbaceous perennials, but, in this forcing date. Low study, longer erect stems improved temperatures occur- the visual quality of this species. ring beneath the Achillea and Dianthus survived overwintering cover the outdoor vernalization treatment may have injured and regrew vegetatively inside the glass- Chrysogonum and house, but flowers developed sporadi- Fig. 6. Relationship between floweR number and forcing date. For Phlox and may have cally. Bergenia displayed handsome each data point, n. 5. Regression equations and r values are as been responsible for foliage, but failed to flower regardless follows: the significant in- of forcing date. Sporadic, sparse, or Arabis: y = 394 + 2.99x r= 0.65* crease in days to first the complete absence of flowering sug- Phlox: y= 284+ 4.0xr = 0. 96* flower over the course of the study gests a requirement by these species of (Fig. 2). longer photoperiods for flower initia- *P 0.05. The relationship between days of tion and development. flowering and forc- sis of variance procedures and regres- ing date were nega- Fig. 7. Relationship between flower number and forcing date. For sion analysis. tive and linear for all each data point, n = 5. Regression equations and r values are as species except Ver follows: NS Results and discussion onica (Figs. 4 and 5). Armeria:y = 17.9 + 0.11x r = 0.29 Growers that force herbaceous Days of flowering for Cbrysogonum: y = 15+ 0.17x r= 0.60* perennials to initiate and develop flow- Arabis, Armeria, Platycodon: y = 20.3 + 0.34x r = 0.83* ers for early spring sales must be cog- NS and Phlox decreased Veronica: y= 7.43+ 0.02x r = 0.35 nizant of the natural flowering ten- 60, 27, and 33 days, NS ,*Nonsignificant or significant at P 0.05, respectively. dencies of their crop and then provide respectively, from the requisite environmental cues. Low the 1 Dec. to the 20 temperature, photoperiod, and light Jan. forcing date. quality play important roles in flower Days of flowering for initiation. Results from this study are Chrysogonum and largely attributed to the effects of low Platycodon decreased temperature; however, longer photo- 36 and 19 days, re- periods present at the later forcing spectively, from 1 dates must be considered when inter- Dec. to 10 Jan. No preting these data. significant change in By the first forcing treatment (1 flower number was Dec. ), perennials beneath the thermo- observed with re- blanket had experienced about 4 weeks spect to forcing date of temperatures below 5C (Fig. 1). for Armeria and Ver Very low ambient temperatures (be- onica but a signifi- low–25C) occurring near the comple- cant decrease was tion of the study were probably re- noted for Chryso - sponsible for the death of Chry - gonum and Phlox sogonum and Platycodon and for the (Figs. 6 and 7). For poor performance of Arabis and Phlox Arabis and Plat - in the last forcing date treatment (20 ycodon, fewer days of Jan.). Ambient temperatures below – flowering did not re- 25C can result in potentially injurious sult in lower flower root-zone temperatures (below –10C), numbers. In fact,

242 Herbaceous perennials are a di- Chrysogonum virginianum is best verse and variable group of landscape forced in early winter (after 4 to 6 plants, and providing general guide- weeks of cold) to capitalize on its lines for inducing out-of-season flow- ability to flower after a modest cold ering is difficult. Iversen and Weiler treatment and to avoid injurious mid- (1994) stated that early spring-flower- winter low temperatures. Results from ing plants that require cold but lack a this study suggest C. virgnianum may daylength requirement should be have limited use as a flowering potted forced in the glasshouse after receiving plant because only four to five flowers 12 weeks of cold. In this study, several are open and effective at a time, and perennials were forced to flower out of many of those are hidden by foliage season under natural daylengths with frequently disfigured by powdery mil- < 12 weeks of cold. dew, which is encouraged by the forc- Perennials recommended for ing environment. forcing. Compact and uniform habit, Perennials not recommended attractive foliage, and abundant white for forcing. ‘New Hybrid’ Bergenia flowers qualifies Arabis sturii for serious cordifolia did not flower and ‘King consideration as a flowering potted plant Edward’ Achilles × Lewisii and ‘War for late winter and early spring sales. Bonnet’ Dianthus × Allwoodii flow- Providing at least 7 weeks of cold (tem- ered only sporadically. Therefore, these peratures below 5C) improved plant perennials are not recommended for quality by synchronizing fiord develop- forcing out of season using our vernal- ment over a condensed period. ization method. Phlox × chattahoochee produced an Thermoblankets are useful insu- abundance of fragrant pale purple flow- lating covers for protecting hardy pe- ers when forced in the glasshouse. rennial species from low-temperature And, like Armeria and Chrysogonum, injury (Iles et al., 1993). Chrysogonum Phlox × chattahoochee is capable of flow- and Platycodon, and to a lesser degree ering after only 4 weeks ‘of cold. Arabis and Phlox, were injured by low Results from this study indicate temperatures in late January beneath that ‘Sentimental Blue’ Platycodon these covers. These cold-sensitive spe- grandiflorus can produce its blue, ba- cies destined for forcing using ambient lloon-like flowers under natural air for vernalization will require added daylengths, but requires at least 8 weeks protection from cold in regions where of cold for the development. of accept- minimum temperatures routinely fall able plant height and flower number. below -25C. For Veronica × ‘Sunny Border Blue’, increasing exposure to cold re- duced days to first flower but had no Literature Cited significant effect on days of flowering, Armitage, A.M. 1993. Forcing perennials for number of racemes produced, or plant early spring sales. PPGA News 6:2-3. height. By providing about 4 to 6 Beattie, D.J. and E.J. Holcomb. 1983. Effects of weeks of cold, growers can force sev- chilling and photoperiod on forcing Astilbe. HortScience 18:449-450. eral crops of Veronica × ‘Sunny Border Blue’ to flower at intervals throughout Byrne, T.G. 1989. Peonies as a potential forcing crop. Perennial Plants 17:3-6. the winter without reducing plant qual- ity. The violet-blue racemes are a de- Iles, J.K., N.H. Agnew, H.G. Taber, and N.E, Christians. 1993. Evaluation of structureless terminate inflorescence and are most overwintering systems for container-grown her- ornamental as flowering proceeds past baceous perennials. J. Environ. Hort. 11:48-55. the midpoint of the raceme. Unfortu- Iversen, R.R. and T.C. Weiler. 1994. Strategies nately, as the most distal portion of the to force flowering of six herbaceous garden in florescence is flowering, flowers at perennials. HortTechnology 4:61-65. the proximal end wither and die, de- Shedron, K. G. and T. C. Weiler. 1982a. regula- tracting from the plant’s appearance. tion of growth and flowering in Aquilegia × hy- Perennials recommended, with brida Sims. J. Amer. Soc. Hort. Sci. 107:878- reservation, for forcing. ‘Alba’ 882. Armeria maritima was success filly Shedron, K. G. and T. C. Weiler. 1982b. Regula- induced to flower over a wide range of tion of growth and flowering in Basket of Gold, Aurinia saxatilis (L.) Desv. HortScience 17:338- forcing dates without reducing flower 340. number or plant quality. Unfortu- Waithaka, K. and L. Wanjao. 1983. The effect nately, ‘Alba’ flowers discolor quickly of duration of cold treatments on growth and and detract from the overall appear- flowering of Liatris Scientia Hort. 18:153- ance of the plant. 158.