The Right Place
Natives of Siberia, delphiniums are happy in colder regions. Their range stretches well into Canada (zone 2). They are hardy in areas where winter temperatures go as low as minus 35°F. They prefer a cool, moist climate such as the Pacific Northwest or southern New England close to the coast. For that reason, delphiniums are not grown in the South.
Plant delphinium seedlings from the nursery or garden center in late spring, May or June. They love sun and require at least a half a day of it. Do not site them where wind will be a problem.
Planting Nursery Stock
When setting the plants in their holes, be sure to keep their crowns, the thick place where the roots join the stem, level with the soil surface. Space plants 2 to 3 feet apart. Delphiniums do best in deep, rich soil enriched with lots of organic matter. Drainage is very important because delphiniums do not like wet, clayey soils. Build their beds higher than the surrounding soil surface, and add lots of organic matter (such as sand, chopped leaves, leaf mold, peat moss) to the soil to improve its drainage. If possible, loosen the soil 1-foot deep when preparing the bed for planting. Ideally, the soil should be slightly alkaline (pH 7.0 to 7.5).
Amendments In Planting or Transplanting
There are a number of products at the garden center that will help your newly planted or transplanted plants deal better with the stress inherent in the planting process. All healthy plants have beneficial fungi, called mycorrhizal fungi, living on their roots. You can buy these valuable additions to your plant’s ecosystem. See the file describing Using Micorrhizae When Planting.
In addition, there are a number of products such as seaweed, compost tea, and beneficial soil microbes that when added to the planting process will help your newly established plants get going faster. See the file New Technology In Plant Growth Activators