The onion set, a small, immature bulb grown from seed the previous season, offers a head start for home gardeners. The precise timing of planting these sets is the most important factor for a successful harvest of full-sized bulbs. Planting time depends heavily on your local climate and your specific goal, whether you intend to harvest large storage onions or simply gather green onions. Onions are highly sensitive to temperature fluctuations, which dictate the best moment to place them in the ground.
Understanding Onion Sets and Planting Goals
Timing is critical because onions are biennials, though gardeners treat them as annuals. If an onion set is exposed to cold followed by warmth, the plant can be tricked into thinking it survived winter, triggering it to “bolt” or prematurely produce a seed stalk. Bolting diverts energy away from bulb formation, resulting in a smaller, woody onion that does not store well. Planting must be timed to avoid the temperature stress that causes this flowering response.
Gardeners typically have two distinct planting goals. The first is to grow large, dry bulbs for long-term storage, requiring careful timing to ensure maximum leaf growth before bulbing is triggered by day length. The second goal is to harvest the plants as scallions or green onions, which allows for much more flexible planting throughout the season. For storage bulbs, the timing window is narrow and must balance early root establishment with the risk of temperature-induced bolting.
Optimal Spring Planting Schedule
The most common method is planting in early spring to produce a harvestable bulb by mid-to-late summer. The primary planting window is as soon as the soil becomes workable, often four to six weeks before the last expected hard frost. “Workable soil” means the ground is not frozen and is dry enough to be tilled without compacting. This early timing is necessary to give the onion a long period for vegetative growth before increasing day length signals it to form a bulb.
Planting sets when daytime temperatures are consistently above 40°F (4°C) is generally safe, but avoid sustained periods of cold after planting. Exposure to temperatures below 50°F (10°C) for two weeks or more after growth starts increases the risk of bolting. Monitoring local microclimate and hardiness zone data helps pinpoint the precise moment to plant. A slight delay is safer than planting too early and risking the loss of the crop to premature flowering.
Fall Planting for Overwintering
Fall planting is an alternative strategy successful in areas with milder winters, typically USDA Hardiness Zones 6 and warmer. The goal is to establish a robust root system before the ground freezes, allowing the plant to enter dormancy over the winter. This gives the onion a significant head start, often resulting in a harvest weeks earlier than spring-planted sets.
The ideal window for fall planting is four to six weeks before the first expected hard frost. This timeframe allows the sets to establish strong roots without developing top growth vulnerable to winter damage. Select specific overwintering varieties, which are bred to withstand the cold and are less likely to bolt. In colder zones, heavy mulching is necessary for survival, and the resulting onions are often best for fresh use, as overwintering can compromise their storage life.
Preparing the Ground for Timely Planting
While planting time is important, soil readiness dictates the precise moment to plant the sets. The soil must be above 35°F (2°C) for roots to begin growth, though temperatures closer to 50°F (10°C) are better for vigorous development. Using a soil thermometer provides a more accurate measure than relying solely on calendar dates or air temperature.
Proper drainage is non-negotiable for onion sets, as they will rot in waterlogged conditions. Incorporating well-aged compost or other organic matter in the fall or very early spring improves soil structure and nutrient availability. When planting the sets, press them gently into the loose soil about one to two inches deep, ensuring the pointed tip is just visible above the surface.