Delphiniums are perennial flowers known for their striking, towering spikes of blue, purple, pink, or white blooms, often considered a staple in cottage gardens. These hardy plants are typically grown from seed. While many gardeners default to spring planting, September presents an optimal time for sowing specific perennial varieties. This autumn timing leverages natural environmental conditions difficult to replicate indoors, ultimately leading to more robust plants the following spring. Successfully sowing delphinium seeds in September requires a calculated approach, relying on the predictable temperature fluctuations of the colder months.
The Autumn Sowing Strategy
Sowing delphinium seeds in September is advantageous because it harnesses the biological process of cold stratification, a natural mechanism many perennial seeds require to break dormancy. This process, also known as vernalization, involves exposing the seeds to a prolonged period of cold and moist conditions. Delphinium seeds possess an internal mechanism that prevents germination until they have experienced this specific chilling requirement.
The autumn planting mimics the natural cycle of seeds dropped in the wild, allowing them to remain dormant but biologically active as temperatures drop. Delphinium seeds germinate best at relatively low temperatures, ideally between 15°C and 20°C (60°F and 68°F), but only after they have undergone a cold period. The winter months provide the necessary sustained cold of roughly 4°C (40°F) or below while the surrounding soil remains moist from rain or snow.
This natural temperature cycling encourages the release of germination-inhibiting hormones. Seeds sown in September take full advantage of the first warming soil temperatures, often resulting in quicker and stronger germination than those artificially stratified in a refrigerator. The resulting seedlings develop a more extensive root system over the spring and summer, which can lead to flowering in the plant’s second year.
Step-by-Step September Sowing Method
When sowing in September, gardeners can choose between direct sowing into the garden bed or using containers for more controlled winter protection. For direct sowing, select a site with full sun or light afternoon shade and ensure the soil is well-draining. Incorporate grit or compost if necessary to prevent waterlogging. Prepare a finely raked seedbed, distribute the seeds thinly across the surface, and cover them with only about 3 millimeters (1/8 inch) of fine soil or vermiculite. The seeds require darkness to germinate but should not be buried deeply.
Container sowing offers greater control over the environment and is recommended for maximizing germination rates. Fill seed trays or small pots with a sterile, well-draining seed-starting mix. Pre-moisten the compost thoroughly before sowing to prevent displacement of the small seeds.
Sow the seeds lightly on the surface of the pre-moistened medium, then cover them with a thin layer of fine compost or vermiculite. Label the containers clearly with the variety and date, which is especially important when dealing with multiple seed types. After sowing, water the containers gently from the bottom by standing them in a tray of water, allowing the moisture to wick up, ensuring the seeds remain undisturbed. These containers are then ready to be placed outdoors to begin the cold stratification process.
Essential Overwintering Care
The immediate concern after September sowing is maintaining consistent moisture without allowing the seeds to become waterlogged, which is the most common cause of failure. For container-sown seeds, place the trays in a sheltered location that receives natural cold temperatures but is protected from excessive rain and prolonged periods of saturation. A cold frame, a plastic-covered mini-greenhouse, or even an unheated garage provides an ideal environment.
The primary threat to dormant seeds and emerging seedlings is moisture that can lead to rot and fungal diseases. Ensure that all containers have excellent drainage holes and elevate them slightly to prevent sitting in standing water. Water only sparingly during dry spells in the winter, especially once air temperatures consistently dip below 5°C (40°F).
For seeds that were direct-sown into the garden, supplemental protection can be applied once the ground has frozen slightly and the plants are securely dormant. Applying a 5-10 centimeter (2-4 inch) layer of organic mulch, such as straw or shredded leaves, will insulate the soil and prevent repeated freezing and thawing cycles. This mulch helps prevent the seeds or any newly sprouted, tiny seedlings from being heaved out of the soil by frost action. Placing containers on raised benches or using mesh covers can help protect the seeds from rodents.