Alliums, including ornamental onions and edible varieties such as chives and leeks, can be successfully grown from seed. While planting allium bulbs in the fall is a common practice, starting these plants from seed requires a different approach focused on precise timing for germination. Successful growth depends on meeting specific environmental needs, which dictate whether to sow indoors or directly into the garden. Understanding the biological requirements of the seed itself is the first step toward determining the correct planting schedule.
Understanding Allium Seed Requirements
Most perennial and many hardy annual allium seeds possess dormancy, which prevents them from germinating during unfavorable conditions. To break this dormancy, the seeds require a period of cold and moist conditions, a process known as cold stratification. This mimics the seeds naturally overwintering in the soil.
This chilled, damp environment helps to soften the hard outer seed coat and signals to the embryo inside that winter has passed. Without this period of stratification, germination rates are typically poor and inconsistent. The necessary cold period is generally between six to eight weeks, whether achieved naturally outdoors or artificially in a refrigerator.
Gardeners can fulfill this requirement either by sowing seeds directly outside in the late fall for natural stratification or by simulating the conditions indoors. Artificial stratification involves placing the seeds with a moist medium, such as a damp paper towel or sand, into a sealed bag and refrigerating them for the required duration.
Planting Seeds Outdoors
Direct sowing allium seeds into the garden is best done in the fall, typically from October to November, just after the first light frost. This timing allows the seeds to undergo natural cold stratification over the winter months. Planting in the fall ensures that once the ground warms in the spring, the seeds are primed and ready to germinate.
To prepare the soil, select a location with full sun and well-draining soil, as alliums are intolerant of soggy conditions that can lead to rot. Sow the seeds very shallowly, covering them with only about one-eighth of an inch of fine soil or grit. Mark the rows clearly, as allium seedlings can be slow to emerge and may take several weeks to become noticeable in the spring.
If you miss the fall planting window, you can still sow pre-stratified seeds outdoors in early spring, as soon as the soil is workable. These seeds should have already completed their six to eight weeks of artificial chilling indoors. While some varieties may be sown in spring without prior stratification, the germination rate will generally be lower and much more erratic.
Starting Seeds Indoors
Starting allium seeds indoors offers a controlled environment for better germination rates and gives plants a significant head start on the growing season. The ideal time to begin this process is in late winter, approximately 8 to 12 weeks before your region’s average last expected frost date. This indoor timing is often preceded by the six to eight weeks of artificial cold stratification, meaning planning must begin well in advance.
Sow seeds in sterile, well-draining seed-starting mix using shallow trays or small containers. The seeds should be covered with only a thin layer of mix, about one-eighth to one-quarter of an inch deep. Alliums germinate best at room temperature, around 68°F.
Once sown, place the trays under bright grow lights for 14 to 16 hours daily, which prevents seedlings from becoming long and weak. Maintaining good air circulation with a small fan helps prevent fungal issues like damping off, a common problem in moist, crowded seedling trays.
Post-Germination Care and Hardening Off
Once the allium seedlings emerge, they require close attention to develop into sturdy plants ready for the garden. Thinning is the first important step, which involves removing the weakest seedlings to ensure the remaining plants have enough space and resources. Aim to leave the strongest seedlings spaced about one inch apart, or follow the “clump” method by thinning to three to five plants per cell or plug.
Potting On
As the seedlings grow, they will need to be transitioned to larger containers if they begin to outgrow their starting trays. This process, sometimes called potting on, supports continued root development indoors until the weather is suitable for planting outside. Allium seedlings, particularly edible varieties like onions and leeks, benefit from being grown until they are about one-eighth to one-quarter of an inch thick before transplanting.
Hardening Off
Before moving the indoor-grown alliums to their final garden location, they must undergo hardening off, a gradual acclimatization process to outdoor conditions. Over seven to ten days, expose the seedlings to increasing amounts of direct sunlight, wind, and fluctuating temperatures. Start with just a few hours in a shady, sheltered spot and progressively increase the exposure until they can withstand a full day outdoors, which prevents transplant shock and ensures their survival.