Planting and Spacing
Seedbed Preparation
Onion seeds need firm, finely textured soil in the seedbed for good germination and stand establishment. Onions grow best in soil with good organic matter and a pH between 6.0 and 7.8. Seedbed 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, the beds are smoothed with a bed-shaped harrow or roller and planted. Because some onion diseases will carry over in the soil, most fields should be planted with onions only once every 5-7 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 (15-25 day requirement) and cause seedling growth to slow. Irrigation water is generally unavailable 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, leading to reduced final bulb size.
Onions may be grown from sets, seeds, or transplants. An onion set is a small, dormant bulb that will produce a larger bulb once planted. Onion transplants are started from seed in a greenhouse or are field grown (in the southwest U.S.), and then shipped to growers prior to planting in the field. Growers looking to produce onions for niche markets like farmers markets, roadside stands, and community-supported agriculture (CSA) shares may want to use transplants, which allow for earlier harvest.
Planting
Onions are seeded on beds of varying widths, depending on the cropping system and the individual grower’s equipment. Use a bed width of 26 to 44 inches (from center to center) with two to four seed rows per bed. Uniform seed placement and in-row plant spacing have a major influence on bulb size and are critical to a good stand establishment. Plant seeds 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 inches), large jumbo (3.5-4.0 inches), and colossal (4+ inches) 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.
More Information
How to Plant Onion Sets