Delphinium grandiflorum ‘Summer Nights’ By Paul Pilon

In Greek the name delphinium translates to "dolphin flower," describing the shape of the spur on the upper part of the flower. With an abundance of deep blue flowers, combined with its compact habit of only 8-14 inches in height, Summer Nights is an excellent choice for containers, mixed containers and perennial borders.

This cultivar, like other delphinium varieties, prefers full sun and grows well in USDA Hardiness Zones 3-7. In Southern portions of the country, Zones 8 and higher, Summer Nights is best when used as an annual and should be grown under partial shade. This cultivar is classified by the American Horticultural Society 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.


Summer Nights can easily be started from seed. Most greenhouses sow the seed in 200- or 288-cell 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 a fine-grade vermiculite to help preserve moisture around the seed. Germination will occur in 14-18 days at temperatures between 68 and 72¼ F. Starting delphiniums in a germination chamber will increase both the germination rate and percent, while decreasing the time necessary for all of the seeds to sprout. Once germinated, plants can be grown with temperatures from 60-68¡ F. At these temperatures Summer Nights 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.


Summer Nights performs well when grown in either small or large containers. A single 200- or 288-cell plug can be planted into a 1-quart pot or three plugs planted into a 1-gal. container. 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 basic pH of 6.5-7.0. They are light feeders and do not tolerate high salts. Deliver nutrients to delphiniums using either a constant liquid fertilization program, feeding at rates of 50-75 ppm nitrates or a controlled-release fertilizer incorporated at a rate equivalent to 1 lb. of nitrogen per yard of growing medium. Delphiniums 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.

Since Summer Nights is a compact variety, it is usually not necessary to control the plant height. Under certain growing conditions or under high plant densities it may be necessary, although not common, to use chemical plant growth regulators. In the northern parts of the country, I would recommend applying Sumagic (Valent USA) at 5 ppm. Applying 1-2 applications seven days apart should provide adequate height control.


Generally, Summer Nights is relatively free of insect and disease problems. Aphids, whiteflies and thrips may occasionally be observed feeding on delphiniums but rarely become problematic. As mentioned earlier, 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 preventative fungicide drench, such as Subdue MAXX (Syngenta Professional Products) or OHP 6672 (Olympic Horticultural Products), at the time of planting, then monthly thereafter.

Botrytis is occasionally a problem on the lower foliage where air movement is limited, causing the foliage to stay wet after irrigation for extended periods of time. To control this disease, it is best to manage the environment by providing proper plant spacing, adequate air movement, controlling the humidity, or if desired, follow a preventative spray program using the appropriate chemicals.


Summer Nights is easy to force into bloom any time of the year. Although there is not a juvenility phase, it is recommended to transplant plugs, which have at least seven leaves, into the final container. Delphiniums do 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.

Summer Nights will flower under any photoperiod and can be forced into bloom under natural day lengths. As the day length 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. Summer Nights grown at 70¼ F will take six weeks to reach flowering, while plants grown at 60¼ F will flower in 10 weeks.

Researchers have shown that temperatures above 79¼ F will reduce the overall quality of the plant by reducing its height and the number of flowers produced. To optimize plant development and produce high-quality plants, I recommend growers force delphiniums at 68¼ F.


Summer Nights is available to the industry as seed, plug or finished container. The seed is supplied by Ernst Benary of America and is available through many seed distributors. Plugs can be acquired from many perennial plug producers or plant brokers. Finished containers may be purchased from many reputable companies across the country.

Paul Pilon

Paul Pilon is head grower at Sawyer Nursery, Hudsonville, Mich. He can be reached by E-mail at If you have any suggestions for future article topics, please contact Paul at the E-mail address listed above.

Latest Photos see all »

GPN recognizes 40 industry professionals under the age of 40 who are helping to determine the future of the horticulture industry. These individuals are today’s movers and shakers who are already setting the pace for tomorrow.

75 Applewood Drive, Suite A
P.O. Box 128
Sparta, MI 49345

Interested in reading the print edition of Greenhouse Product News? Preview our digital edition »

Get one year of Greenhouse Product News in both print and digital editions for free.

Subscribe Today »

Be sure to check
out our sister site.
website development by deyo designs