Grow larkspur in a sunny bed near the coast or under partial shade inland. Choose a bed with rich, fertile soil and excellent drainage. Avoid heavy shade or areas where water pools after rain.
Prepare the soil
Weed the planting site thoroughly to eliminate competition with the larkspur seedling. Break up the soil to a depth of 8 inches using a rotary tiller. Work a 3-inch-thick layer of compost and a 1/4-inch-thick layer of wood ash into the bed.
Plant the larkspur seeds
Create a series of 1/4-inch-deep furrows across the surface of the bed using a leaf rake. Sprinkle the larkspur seeds into the furrows. Lightly rake the bed to cover the seeds.
Water the bed thoroughly immediately after planting the larkspur seeds. Spray the bed lightly using a garden hose. Water until the soil feels moderately moist in the top inch. Maintain constant moisture in the top inch of soil while the seeds germinate. Allow the surface to dry slightly, but make sure the soil does not dry out in the top 1/4 inch since the larkspur seeds will die.
Thin the seedlings
Watch for germination in 20 to 30 days. Larkspurs germinate best at around 55 degrees Fahrenheit, and will cease germination once temperatures top 65 degrees. Thin the seedlings to one every 8 to 10 inches once they produce a mature set of leaves. Remove the weakest seedlings and leave the healthiest.
Mulch and fertilize
Spread a 1-inch-thick layer of neutral mulch between the larkspur seedlings to help keep the soil cool and moist. Feed the seedlings with general purpose, 12-12-12 ratio fertilizer four weeks after germination. Apply the fertilizer at quarter-strength.
Perform regular care
Typical bloom time is early to late summer. After the flowers fade, deadhead them to prolong flowering, recommends fine gardening. This plant will self-seed, so you will enjoy thicker and more luxurious plantings in future years. You may need to control for pests and disease, including slugs, nails, powdery mildew and crown rot.
Wash your hands thoroughly after handling the seeds, leaves or flowers of larkspur since they are mildly toxic if ingested.
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