Planting and Spacing
Onion seeds need firm, finely textured soil in the seed bed for good germination and stand establishment. Onions grow best in a soil with good organic matter and a pH between 6.0 and 7.8. Seed-bed preparation begins in the fall of the previous year. First, the field is plowed to improve soil condition, and then the soil is shaped into beds that are allowed to settle through freezing and thawing action during the winter months. In the spring just before planting, the beds are smoothed with a bed shaper harrow or roller, and planted. Because some onion diseases will carry-over in the soil, most fields should be planted to onions only once every five to seven years.
Seeding Rates and Spacing
Fields should be seeded from early March through mid-April when onion beds are dry enough to avoid compaction or germination problems during planting. Onion seeds germinate at temperatures above 40°F; the optimum soil temperature is 75°F. If seeds are planted too early, cooler air and soil temperatures will delay germination and emergence (15- to 25-day requirement), and cause seedling growth to slow. Irrigation water is generally not available until after mid-April, so planting in March and April allows onion seeds to germinate following spring rain storms. Research has shown that most plantings made before April 15 will allow for the best onion crop yields. If seeded later, hot summer temperatures induce bulb development before sufficient leaf growth has occurred, leading to reduced final bulb size.
Onions may be grown from sets, seed, or transplants. An onion set is a small, dormant bulb that will produce a larger bulb once it is planted. Onion transplants are started from seed in a greenhouse or are field grown (in the southwest U.S.) then shipped to growers prior to planting in the field. Growers who are looking to produce onions for niche markets like farmers markets, roadside stands and community supported agriculture (CSAs), may want to use transplants, which allow for earlier harvest.
Onions are seeded on beds of varying width, depending on the cropping system and the equipment of the individual grower. Use a bed width of 26 to 44 inches (from center to center) with 2-4 seed rows per bed. Uniform seed placement and in-row plant spacing has a major influence on bulb size and is critical to a good stand establishment. Seeds should be planted 0.5 to 1 inch deep. Avoid wide spacings which promote large bulbs with thick necks. Generally, an in-row spacing of 3 to 4 inches ensures both high total yield and a higher percentage of onions in the jumbo (3.0-3.5”), large jumbo (3.5-4.0”), and colossal (4+”) market classes.
Many types of planters are used to seed onions and all must be carefully set to maintain proper seeding depth and rate. A “small seed” type planter with short seed drop is recommended. Vacuum and other types of precision planters can be very effective at controlling plant spacing and reducing the amount of seed used.
How to Plant Onion Sets