Growing Zones

• Delphiniums are best adapted to regions with cool summers, where they thrive as biennials or annuals. The plants are short lived as perennials and are hardy in U.S. Department of Agriculture zones 3 to 7, as listed by Barbara Ellis in "Taylor's Guide to Growing North America's Favorite Plants." The areas where the plants grow optimally include the Pacific Northwest and northern and coastal areas of New England. In other regions of the country, more attention is required to help plants deal with the summer heat.
  
  

Site

• The flowers grow best in well-drained, fertile soil with neutral to alkaline conditions. However, delphiniums also adapt to slightly acidic soils. Plant on raised beds if the site drains poorly. Select an area of full to partial sun. In regions with hot summers, select a site that receives morning sun and is shaded from midday heat. Look for north- or east-facing areas with good air ventilation and shelter from high winds.
  
  

Propagation

• It is easy to start delphiniums either with seed or from nursery transplants. Recommended planting time is during spring. Amend the planting site with well-rotted manure, compost or leaf mold before sowing seed or transplanting. Place transplants carefully, as the delicate roots are prone to breaking. Leave about two feet between plants and plant in groups of threes for best effect.
  
  

Problems

• Common diseases in delphiniums include black leaf spot, powdery mildew, botrytis blight, crown rot and fungal leaf spots. Pest infestations include leaf miners, aphids and mites. The plants are also prone to a bacterial disease called "blacks," which causes tarlike spots on the foliage. Making sure plants are growing under optimal conditions and providing good cultural care is the best way to prevent disease. Remove diseased plants and avoid planting healthy delphiniums in infected sites. Do not propagate new plants from infected ones.