The English have referred to delphiniums as larkspurs since before the 16th century, but you can use the two aforementioned common names interchangeably. Breeders bred and selected varieties of branching larkspurs (Delphinium consolida) in the early 20th century and created the “Giant Imperial” strain of larkspurs (Consolida ambigua “Giant Imperial”), which have other botanical names associated with them, including Consolida ajacis and Consolida orientalis. It sounds confusing, but you only have to remember that if you see “Giant Imperial” on your package of larkspur seeds, you have some easy-to-grow annuals that reach about 4 feet tall, and explode with red, white, yellow or blue flowers in late spring or early summer when planted in winter.
Combine equal amounts of moist sphagnum peat moss, perlite and sterilized loam in a bushel basket by hand six to eight weeks before the last expected frost date in your area. Fill 4-inch-deep, draining containers 1/2 inch from the top with the seed-starting medium.
Irrigate the medium until water drains from the containers. Press two or three larkspur seeds about 1/8 inch deep in the growing medium, spaced about 1 inch apart. Cover the seeds with about 1/8 inch of growing medium.
Cover the top of each container with plastic food film. Set them in an east-facing window sill in a room that is around 55 degrees Fahrenheit. Leave the curtain open on the window.
Mist the medium a few times a day with a spray bottle to keep it moist at all times during the germination period, which lasts between 20 and 30 days. Remove the plastic from the pots as soon as you see any green growth emerge.
Set the pots outside for five or six hours each day after they spend six weeks inside to harden them off. Place the pots in an area protected from wind and precipitation, and take them back inside when it starts to get cold in the evening. You can keep the pots in a warmer room when they’re hardening off.
Pull out the weakest seedlings from the pots after the first set of true leaves emerge, leaving only the healthiest one intact and undisturbed.
Work the soil 12 to 15 inches deep in an area that gets partial shade about eight weeks after starting the seeds inside, using a spade fork or shovel. Larkspurs grow to about 1 foot wide, and need between 1 and 3 feet of space around them, so you need to work at least 2 to 4 square feet of soil for each plant.
Spread 2 to 4 inches of compost over the planting area, and work it in 12 to 15 inches deep, using a spade fork or shovel. Dig one 4-inch-deep hole for each larkspur seedling in the composted soil. Make each hole twice as wide as the containers you planted the seeds in.
Water the seedlings enough to moisten the top inch or two of growing medium. Squeeze the sides of the containers if they’re made of flexible plastic, or hit them against the side of a table if not, to loosen the growing medium. Let the medium and the seedling slide from the container intact into your hand.
Position the mass of growing medium in the hole so the top of it sits even with the surrounding soil. Add or remove soil as needed. Fill in the hole with the excavated soil.
Firm the soil around the stem with your hands, and water until it saturates the root system, about 4 inches deep. Spread a 2-inch layer of bark chips, pine needles or shredded bark over the soil starting about 1 inch from the stem and extending out about 4 inches on all sides. Mulch the larkspurs each spring with 2 inches of mulch.