Larkspur is a delicate, vertical flower prized for its tall, colorful spikes that add dramatic height to the garden. This annual plant (Consolida ajacis) is closely related to the perennial delphinium. Precise planting time is important because the seeds require a period of cold exposure, known as cold stratification, which mimics winter and breaks dormancy.
Primary Planting Window: Direct Sowing Annual Larkspur
The most effective method for growing annual larkspur is to sow the seeds directly into the garden in late winter or very early spring. This timing is designed to ensure the seeds experience the necessary cold period naturally in the soil. The optimal window is typically four to six weeks before the last expected spring frost date in your region.
Seeds will not germinate well if the soil temperature rises above 55°F (13°C). Planting early while the ground is still cool, even if snow is present nearby, allows the seeds to receive the necessary chill to trigger germination. Sowing the seeds about a quarter-inch deep in a dark environment is also necessary for germination.
Early spring sowing provides the required cold stratification period, leading to robust, earlier-blooming plants. Larkspur develops a long taproot, and direct sowing minimizes the root disturbance that can stunt growth. The cold exposure also promotes vernalization, which is required for young plants to develop long, straight flower stems later in the season.
Alternative Timing: Fall Sowing and Indoor Starts
Fall sowing is an excellent alternative strategy for achieving earlier blooms the following spring, especially in regions with mild winters. Seeds should be sown late in the fall, right before the ground is expected to freeze solid. This ensures the seeds remain dormant throughout the winter, undergoing natural, prolonged cold stratification.
These seeds germinate as soon as the soil warms slightly in the spring, often resulting in plants that flower weeks earlier than spring-sown varieties. Fall planting is common for cut flower growers aiming to extend their harvest window.
Starting larkspur indoors is generally considered a backup method due to the plants’ high sensitivity to root disturbance during transplanting. If necessary, sow seeds approximately eight to ten weeks before the last frost. The seeds should be pre-chilled in a refrigerator for one to two weeks before sowing to artificially provide cold stratification.
After germination, seedlings must be grown in a consistently cool environment, ideally around 55°F (13°C), to mimic the cool spring conditions they prefer. Carefully transplant the seedlings into the garden after hardening them off, taking care not to disturb the delicate taproots.
Planting Perennial Delphinium
Many gardeners confuse annual larkspur (Consolida) with its perennial relative, the delphinium (Delphinium), which requires different planting timing. Perennial delphiniums are typically planted as established starts or bare-root divisions, rather than from seed. The timing for planting established plants is flexible, but it is best to plant them in early spring after the danger of a hard frost has passed.
Planting in early spring allows the roots to establish fully before the summer heat arrives. Another ideal window for planting perennial starts is in the late summer or early fall. This timing allows the plants to develop a strong root system in the cooling soil before winter dormancy, preparing them for vigorous growth the following spring.
While perennial delphiniums can be grown from seed, it is a longer process, and seedlings often will not bloom until their second year. When planting bare-root divisions, ensure the crown is set just at or slightly below the soil surface for best establishment.