Starting onion seeds indoors allows gardeners in temperate climates to get a significant head start on the growing season. This ensures the development of large, mature bulbs before the summer heat arrives. Beginning this phase indoors, typically 8 to 10 weeks before the final expected frost date, provides the necessary time for the slow-growing onion to establish itself. Successfully cultivating robust, pencil-thin seedlings requires a careful balance of materials, temperature control, moisture, and light, followed by the transition period known as hardening off.
Preparation and Necessary Materials
The first step in a successful start is selecting the appropriate onion variety, which is determined by your geographic location’s day length. Northern regions should choose long-day onions, while southern regions require short-day types, as bulbing is triggered by a specific number of daylight hours. Next, gather a quality, sterile seed-starting mix, which provides a fine texture and is free of pathogens that could harm delicate sprouts. Avoid using garden soil, which is too dense and may harbor disease.
You will need planting containers, such as standard seed trays or deeper four-inch pots, as onions have a longer indoor growth period than many other vegetables. Deeper containers are often preferred because they allow the seedlings to grow without needing to be transplanted before moving them outdoors. Ensure your chosen containers have adequate drainage holes to prevent the planting medium from becoming waterlogged.
Sowing the Seeds for Initial Sprouting
Begin by moistening your seed-starting mix until it is evenly damp but not saturated, resembling a wrung-out sponge. Fill your chosen containers with the mix, lightly tamping the surface to create a level, firm bed for the tiny seeds. Create shallow depressions or furrows approximately one-quarter to one-half inch deep across the surface of the soil.
Onion seeds are often sown densely. Sprinkle the seeds thickly along the furrows, aiming for a seed every quarter-inch, or simply scatter them across the container’s surface. Lightly cover the seeds with a thin layer of the starting mix, about one-eighth of an inch, or a dusting of vermiculite. Gently water the surface using a misting bottle or by bottom-watering the tray to avoid dislodging the newly planted seeds.
Maintaining Optimal Conditions
To promote fast germination, the containers should be placed in a warm environment. Onion seeds germinate best when the soil temperature is consistently maintained between 68 and 77°F, with an optimum near 75°F. A seedling heat mat placed beneath the tray is the most effective way to provide this necessary bottom warmth. Covering the tray with a humidity dome or plastic wrap will help retain moisture and warmth until the first sprouts emerge.
Once the seedlings have broken the soil surface, immediately remove the heat mat and humidity dome, as the temperature requirement changes. Onion seedlings prefer cooler air temperatures for growth, and excess humidity can encourage fungal diseases like damping off. Move the tray under a bright light source, ideally a dedicated grow light, positioning it about one to three inches above the young plants. The seedlings need 12 to 16 hours of light daily to develop strong, stocky growth.
Maintain even moisture in the planting medium throughout the growth period, allowing the surface to dry slightly between waterings to prevent root rot. If the slender, grass-like foliage begins to grow too long and floppy, it can be trimmed. Use clean scissors to cut the tops down to about three to five inches tall. This periodic “haircut” encourages the plant to focus energy on root development and keeps the seedlings manageable.
Preparing Seedlings for Transplant
When the onion seedlings have reached the desired thickness—roughly the diameter of a pencil—they are ready to be moved to the garden. This usually takes 8 to 10 weeks of indoor growth. If you sowed the seeds very thickly, you may need to thin the seedlings by gently separating clusters, aiming for a final spacing of four to six inches between individual plants in the garden for mature bulb production.
Before planting outdoors, the seedlings must undergo a process called “hardening off,” which gradually acclimates them to outdoor conditions like wind, direct sun, and fluctuating temperatures. Start by placing the trays outside in a protected, shaded area for just one or two hours a day, ensuring temperatures are above 45°F. Over a period of one to two weeks, gradually increase the duration of their time outside and their exposure to direct sunlight and wind.
This slow introduction to the elements drastically reduces the risk of shock when they are permanently transplanted. The seedlings can be moved to the garden bed around the time of your last expected frost, or even a few weeks before, as mature onion seedlings are tolerant of cold temperatures. Once planted, water them thoroughly to help the roots establish quickly in their new environment.