Summary: Deciding when to plant delphinium, a classic English cottage garden flower, depends on whether you are growing the plants from seeds, nursery plants, or root divisions from an established clump. Delphinium is a moderately difficult plant to grow, but choosing the right location improves the prospect.
Starting from Seed
Seed for delphinium is often started indoors in perlite or a mixture of perlite and compost during the winter months of January through March. Be sure to use new seed, as delphinium seeds do not stay viable for more than a year or two. Do not plant the seeds too deeply. One-eighth inch of light soil over them is plenty. They can take up to two weeks to germinate. The plants may be planted in the garden after all danger of frost is past in May or June.
Delphinium seed can also be direct-seeded outdoors after all danger of frost is past. If started outdoors, they are unlikely to bloom until the next spring. You should select a site that gets at least 5 hours of sunshine per day. The soil needs to be light, sandy, and rich with compost. A raised bed is appropriate because delphinium needs good drainage.
When to Plant Delphinium Plants from the Nursery
Nursery plants do best when set outdoors in the spring after all chance of frost is past. They can also be set out in the fall. Apply a good layer of mulch to help protect the young plants through the winter if you choose to plant them in the fall. Make sure the soil is first loosened to a depth of 12 to 18 inches so that the plants will have good drainage.
Dividing Roots of Established Plants
Established delphinium plants should be divided every three years. Carefully dig up the root clump in the spring after leaves begin to appear. Use a knife to divide into smaller clumps of three or more roots. Prune off any extra foliage and replant in loose, light soil. Regular division of delphinium will facilitate better blooming.