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Potted Delphiniums 



E. Jay Holcomb and David J. Beattie 







ne of the advantages of perennials 

 in pots is that they be dual-use plants: 

 they can be enjoyed as a flowering 

 plant in the home, then planted in the 

 garden and enjoyed for years to come. 



A tall plant like a delphinium doesn't 

 seem like the type to grow in a pot. In 

 gardens, delphiniums range from 3 to 

 8 feet tall, but the group of varieties 

 that are naturally 2 1/2 to 3 feet in 

 height should work well. "Blue 

 Springs," "Blue Fountains," and "Con- 

 necticut Yankee" are recommended. 

 Propagation. The most practical 

 method of propagating delphiniums 

 is from seed. The time of sowing de- 

 pends on when you want the del- 

 phinium to flower. For example, if 

 you want the plants large enough to 

 force in the fall, you would sew the 

 seed in June. Delphinium seed germi- 

 nates very well when fresh, but store 

 it cold after collection. 



Sow seed in plug trays in a well- 

 drained medium, covering lightly. 

 Most peatlite mixes have adequate 

 drainage. It's best not to use a plug 

 tray with more than 200 cells so that 

 the seedlings have some space in 

 which to grow before transplanting. 



Temperature is critical. Keep medium 

 temperature 70 F with daytime air 

 temperature between 70 and 80 F. 

 Germination should occur within 10- 

 14 days. When germinating in sum- 

 mer, be careful that temperatures 

 don't get too high. 



Growing On. Once the seed has ger- 

 minated, grow seedlings on at lower 

 temperatures, as low as 55-60 F at 

 night and up to 75 F during the day. In 

 summer, this may require a fan and 

 pad cooled greenhouse. You can also 

 control high day temperatures with 

 shading, but this reduces the light in- 

 tensity. 



Fertilize lightly. A general-purpose 

 fertilizer (20-20-20) is suitable, al- 

 though a fertilizer that is lower in 

 phosphorus and ammonium nitrogen 

 (15-15-15, for example) produces 

 more compact seedlings. It takes 7-9 

 weeks for the seedlings to grow to a 

 transplantable size of 1-2 inches. 



> o o o o c 



It's possible to purchase plugs from a 

 commercial grower. Some bare-root 

 field-grown transplants are also avail- 

 able, but often don't do well because 

 of root-borne diseases. 

 Forcing. For forcing, grow plants in 

 5-inch or 6-inch pots. A 4-inch pot is 

 too small to hold the plant upright. 

 Media and fertilizer. Use a porous, 

 well-drained growing media with a 

 natural pH. Tlie coarse peatlite mixes 

 work very well. The fertility of the 

 mix should initially be low, but you 

 should begin fertilizing soon after 

 transplanting. A complete fertilizer 

 like 20-10-20 at 100 to 200 parts per 

 million at each irrigation is very effec- 

 tive. 



Light. Light is critical for delphinium. 

 High iiTadiance appears to be more 

 important than pholoperiod. l-"or ex- 

 ample, in November, December, and 

 January, the naturally low light levels 

 in New England won't stimulate rapid 

 flowering. If you want to flower del- 

 phiniums in winter, use supplemen- 

 tary light. 



High-intensity discharge lamps at 

 650 foot-candles used from 5 p.m. to 

 8 a.m. successfully stimulated flower- 

 ing in the work at Penn State, but 

 there are probably other light re- 

 gimes that could do the same thing 

 equally well. 



In the brighter months of March and 

 April, delphinium grow and flower 

 rapidly under natural greenhouse 

 conditions. (The researchers also 

 flowered delphinium as a cut flower 



under totally artificial light in about 

 60 days using large plugs as starting 

 material.) 



Temperature. Plants tolerate night 

 temperatures as low as 40 to 50 F, but 

 their growth rate is much slower. 

 Sensible temperatures are a night 

 temperature of 60 F and a day temper- 

 ature of 70. 



Keeping day and night temperatures 

 fairly close to each other tends to 

 shorten stem length. However, a lower 

 day temperature may increase the time 

 needed to produce the crop. 



Height control. Use growing tem- 

 peratures to control height. Tliere are 

 no giowth retardants specifically la- 

 belled for delphiniums. Research has 

 shown, however, that some of the re- 

 tardants (A- Rest and Su magic, for 

 example) do reduce plant height. 



Pests. Delphinium will be affected by 

 the same insects that aUecl other 

 greenhouse crops. Scouting and yel- 

 low sticky cards are the first line of 

 defense. Delphinium are also suscep- 

 tible to some disease, but using a 

 pathogen-free media should elimi- 

 nate most problems. 



E. Jay Holcomb and David J. Beattie 

 are professors of floriculture at Penn 

 State University, University Park, 

 Pennsylvania. This research was 

 funded by Bedding Plants Foundation, 

 iMnsing, Michigan. This article is 

 excerpted from "Culture Notes" in the 

 August 1990 GrowerTalks. ^^ 



1992 AARS Winners 



Reprinted from GrowerTalks, 

 April 1991 



Brigadoon, a hybrid tea rose with 

 coral pink blooms, Pride 'n' Joy, a 

 miniature rose with orange blooms, 

 and All That Jazz, a shrub with large 

 coral and salmon petals, are 1992 

 .All America Rose Selections, Inc., 

 winners. Brigadoon and Pride 'n' 

 Joy were introduced by Jackson & 

 Perkins Co., Medford, Oregon, and 

 All That Jazz, by DeVor Nurseries, 

 Inc., Watsonvillc, California. All are 

 available in spring, 1992. For infor- 

 mation, call AARS, Chicago, Illinois, 

 (312)372-7090. 



June/July 1991 25 



