September 2005

Perennial Solutions: Delphinium elatum Guardian Series By Paul Pilon

The Guardian series provides growers producing delphiniums with predictability and uniformity, which historically has been difficult to achieve. This series of F1 hybrids is considered a breeding breakthrough, providing early bloom that is programmable throughout the year. The Guardian series provides uniform flowering across a range of color lines including blue, lavender and white.

Guardian blooms a full six weeks before the open-pollinated cultivars, producing an abundance of flowers on strong stems. Plants reach 24-30 inches tall when blooming and are excellent for containers, cut flowers and perennial gardens. This series, like other delphinium cultivars, prefers full sun and grows well in USDA Hardiness Zones 4-7. In Southern portions of the country, Zones 8 and higher, Guardian is best when used as an annual and should be grown under partial shade. This cultivar is classified by American Horticultural Society (AHS) as a Heat Zone 6-1 plant, meaning it does not tolerate extreme summer temperatures. Delphinium, also commonly referred to as larkspur, is rather short lived in the garden, often only surviving a couple of years. Regardless of where they are grown or how many years they live, gardeners and landscapers embrace the incomparable color and value delphiniums bring to the landscape.

Propagation

Guardian can easily be started from seed and is supplied by PanAmerican Seed with an 80-percent minimum germination standard. Most greenhouses sow the seed in 200-cell or larger plug trays filled with a growing medium that provides both good aeration and water-holding capacity. At sowing, it is recommended to cover the seed lightly with fine- to medium-grade vermiculite to help preserve moisture around the seed.

Starting delphiniums inside of a germination chamber will increase both the germination rate and percent germination, while decreasing the time necessary for all of the seeds to sprout. Delphiniums do not germinate uniformly, and germination occurs over several days. It is recommended to remove them from the chamber 2-3 days following the first signs of germination to ensure a high germination percentage. It is critical to maintain high humidity (95 percent) until the cotyledons have emerged. Germination will ç occur in 7-10 days at temperatures between 65 and 70¼ F. Once germinated, they can be grown with temperatures from 60 to 68¡ F. At these temperatures, Guardian will finish the plug stage in approximately 6-8 weeks.

During the plug stage it is important to keep the media uniformly moist but not wet. Fungus gnat larvae may become problematic during plug production and may need to be prevented using products such as Adept (Chemtura), Azatin (OHP), Duraguard (Whitmire Micro-Gen), Gnatrol (Valent USA) or Ornazin (SePRO Corp.). Production

Guardian performs best in 1-gal. or larger-sized containers. For fullness, plant one or two plugs into a 1-gal. pot or three plugs into 2-gal. containers. It is important to avoid planting the plugs too deeply, or crown rots may result. Delphiniums perform best when grown in a moist, well-drained medium with a slightly acidic pH of 5.8-6.2. They are light to moderate feeders and do not tolerate high salts. Deliver nutrients to delphiniums using either a constant liquid fertilization program, feeding at rates of 75-100 ppm nitrates, or a controlled-release fertilizer incorporated at a rate equivalent to 1 lb. of nitrogen per yard of growing medium. They are susceptible to root rots when over-irrigated and should be grown on the dry side. When irrigation is necessary, I recommend watering thoroughly then allowing the soil to dry slightly between waterings.

Generally, delphiniums are relatively free of insect and disease problems. Aphids, whiteflies and thrips may occasionally be observed feeding on delphiniums, but they rarely become problematic. As mentioned above, when the root zone remains wet for extended periods of time or is exposed to high salt levels root rots, namely Pythium and Phytopthora, are likely to occur. I recommend growers apply a fungicide drench, such as Banrot (Scotts Company) or a combination of Subdue Maxx (Syngenta Professional Products) with a thiophanate methyl product (Cleary's 3336, Cleary Chemical Corp.; FungoFlo, Scotts Company; or OHP 6672, OHP), at the time of planting, then monthly thereafter.

Botrytis is occasionally a problem on the lower foliage where air movement is limited and the foliage often stays wet after irrigation for extended periods of time. To control this disease, it is best to manage the environment by providing proper plant spacing and adequate air movement, and controlling the humidity. If desired, you can also follow a preventative spray program using the appropriate chemicals. Under certain growing conditions, particularly at high plant densities, it may be necessary to use chemical plant growth regulators. In the Northern parts of the country, I would recommend applying Bonzi (Syngenta Professional Products) at 30 ppm or Sumagic (Valent USA) at 5 ppm. The first application should occur as the flower stem is just beginning to rise above the basal foliage. If an additional application is necessary, apply it 7-10 days following the first. Applications in this manner usually provide adequate height control.

Forcing

Guardian is easy to force into bloom any time of the year. Although there is not a juvenility phase, it is recommended to transplant plugs that have at least seven leaves into the final container. Large starting materials, such as 72-cell plugs, will finish quicker than smaller plug sizes. This will be demonstrated below.

Guardian does not have a cold requirement, nor is cold beneficial to flowering. In fact, delphiniums exposed to a cold treatment are often somewhat delayed and exhibit sporadic flowering. Whenever possible, avoid putting them through a cold period and use fresh plug materials. If cold is provided, shorter durations of time, such as six weeks, are better than providing longer periods of cold exposure.

Guardian will flower under any photoperiod and can be forced into bloom under natural daylengths. As the daylength increases growers can expect the plant height to increase as well as the flower number. For the best plant quality, it is recommended to produce this variety under photoperiods of 12 hours or longer.

The time to bloom after transplanting is a function of temperature. It is recommended to force delphiniums with temperatures ranging from 60 to 70¼ F. Guardian grown at 70¼ F will take 10 weeks to reach flowering, while plants grown at 60¼ F will flower in 14 weeks. Growers using large starting materials, such as 72-cell plugs, can finish the crop 3-4 weeks earlier than when smaller plug sizes are used. To optimize plant development and produce high-quality plants, I recommend growers force delphiniums using 72-cell plugs as the starting materials and produce them at 68¼ F.

Availability

Guardian is available to the industry as seed, plug or finished container. It was bred by PanAmerican Seed and is available through standard seed brokers. Plugs can be acquired from many perennial plug producers or plant brokers. Finished containers may be purchased from many reputable companies across the country.