Onion sets are small, immature bulbs grown from seed the previous year and allowed to go dormant. They offer the simplest method for home gardeners to cultivate a bulb harvest. These dormant bulbs provide a significant head start over planting seeds, bypassing lengthy germination and fragile seedling stages. For a successful crop in Ohio, planting timing and proper soil preparation are the most important factors.
Timing the Planting Window in Ohio
Onion sets are a cool-season crop and must be planted early in the spring as soon as the soil is workable. In Ohio, the planting window typically opens in late March and extends through April. The goal is to establish the sets during the cool, moist spring weather before summer days trigger bulbing.
Sets tolerate light frost, making them one of the first vegetables planted. The soil should not be frozen or overly saturated. Gardeners should aim to plant four to six weeks before the area’s average last expected spring frost. The ideal soil temperature for rapid root growth is consistently 50°F.
Preparing the Soil and Onion Sets
Successful bulb development requires a location that receives full, direct sunlight throughout the day. Onions are heavy feeders with shallow root systems, making soil structure and fertility important. The planting site must have loose, well-draining soil, ideally a sandy-loam or silt-loam composition.
Heavy clay soils should be amended with aged compost or other organic matter to improve aeration and drainage. Onions thrive best in a soil pH range between 6.0 and 6.8, as they are sensitive to highly acidic conditions. Before planting, inspect the onion sets and discard any that feel soft, show signs of mold, or appear damaged.
Planting Depth and Spacing Techniques
The depth of the set directly influences the final bulb size. Onion sets should be planted shallowly, no more than one to two inches deep. The goal is to cover the bulb just enough to anchor it, often leaving the pointed tip slightly exposed above the soil line. Planting too deep can restrict the swelling and formation of the mature bulb.
The spacing between sets depends on the desired harvest. For full-sized dry bulbs, space the sets four to six inches apart. If the intent is to harvest green onions, a closer spacing of two to three inches is acceptable, thinning out every other plant as needed for an early harvest. When planting multiple rows, leave twelve to eighteen inches of space between rows to allow for air circulation and cultivation.
Early Season Maintenance
After planting, initial care focuses on providing a stable environment for the sets to establish their root systems. Because onions have a limited root zone, they require consistent soil moisture, especially during dry spells. A minimum of one inch of water per week, either from rain or irrigation, is necessary.
Weeding is important because onions compete poorly with fast-growing weeds for water and nutrients. Shallow cultivation or the application of an organic mulch layer will help suppress weeds while avoiding damage to the shallow roots. A first application of a balanced or slightly low-nitrogen fertilizer can be applied once the shoots have emerged and the sets begin growth.