Successful onion cultivation depends on timing the planting to match the plant’s biological requirements. Onion bulb formation is highly sensitive to day length, a phenomenon known as photoperiodism. The plant must reach a certain size before increasing daylight hours trigger the switch from leaf growth to bulb enlargement. Understanding this relationship is fundamental to achieving a successful harvest in North Carolina.
Selecting the Proper Onion Variety for North Carolina
Onion varieties are categorized into three groups based on the daylight hours required to initiate bulbing. Short-day onions form bulbs with 10 to 12 hours of daylight, intermediate-day varieties require 12 to 14 hours, and long-day types need 14 to 16 hours. Due to North Carolina’s latitude, the state primarily falls within the short-day and intermediate-day zones. Planting long-day onions here typically results only in small, undeveloped bulbs or green tops, as the required photoperiod is never reached.
Gardeners should select short-day varieties like Texas Supersweet, Grano, or Granex for the most reliable bulb production. Intermediate-day varieties, such as Candy, can also perform well, especially in the central Piedmont region. Choosing the correct variety ensures the plant shifts its energy from producing foliage to swelling the underground bulb at the appropriate time in the spring. This selection is the first step toward a bountiful late-spring or early-summer harvest.
Determining the Best Planting Time
The state’s mild winters allow for two distinct planting windows, with the timing depending on the variety chosen. The most common method involves a late-fall planting for an overwintering crop, which yields the largest bulbs. Short-day onions should be planted between late September and late October, allowing them to establish a robust root system before the onset of colder weather. These plants remain mostly dormant through the winter and begin active growth and bulbing as the days lengthen in the following spring.
Fall Planting Window
This fall planting window is variable across North Carolina’s geographic regions. Gardeners in the warmer Eastern Coastal Plain can wait until the end of October or early November to plant. Those in the cooler Western Mountain regions should aim for mid-to-late September. Planting too early in the fall risks premature bolting, where the plant sends up a seed stalk, which ruins the bulb. The resulting main crop is ready for harvest in late May or June.
Spring Planting Window
A secondary planting window occurs in late winter and early spring, suited for intermediate-day varieties or as a backup crop. Transplants or sets can be put in the ground from early February through mid-March in the Piedmont and Coastal regions. Mountain gardeners generally delay this spring planting until later in March or early April due to the colder conditions. This spring-planted crop bypasses the overwintering stage, offering a quicker harvest of slightly smaller bulbs later in the summer.
Practical Planting Techniques
Onions can be started from seeds, sets, or transplants, with transplants being the easiest and most reliable method for most home gardeners in North Carolina. Transplants are small, pre-grown seedlings that give the plant a six-week head start over direct seeding, leading to more uniform stands and bulb sizes. When planting transplants, they should be set into the prepared soil so the base of the roots is just covered, usually about one to one and a half inches deep.
Planting from sets, which are small, dormant bulbs, is generally not recommended for the South because they are often long-day varieties and are prone to bolting when planted in short-day regions. Proper spacing is necessary for the bulb to swell correctly. Plants should be spaced two to four inches apart in the row. This spacing provides enough room for the bulb to expand to its full size without competing for nutrients or soil space.