Growing onions successfully depends on correct timing and selecting the appropriate variety for the local climate. Oklahoma sits in a transitional zone where environmental cues, like temperature and daylight hours, directly govern the plant’s growth cycle. Planting at the wrong time or using an unsuitable variety prevents the onion bulb from forming properly, yielding only green tops.
Selecting the Correct Onion Type for Oklahoma
Onion plants determine when to form a bulb based on daylight hours, a process known as photoperiodism. Oklahoma requires intermediate to short-day varieties for optimal bulb development. Short-day onions begin to bulb when daylight reaches 10 to 12 hours, while intermediate varieties need 12 to 14 hours of light. Planting a long-day variety, which requires 14 to 16 hours of light, will only result in green tops, as Oklahoma does not experience the necessary day length.
Recommended varieties include Granex types, such as Texas Early Grano and Vidalia-style onions, which are popular for their mild flavor. The Red Creole is another excellent choice, offering a spicier flavor and good storage qualities.
Primary Planting Window: Spring Timing
The main planting window for a bulb onion harvest is early spring, allowing plants to establish a strong root system before bulbing begins. Transplants and sets should be placed in the ground two to four weeks before the last expected hard frost. This early timing allows the onion to develop large top growth, which directly correlates with the final size of the mature bulb. In Southern Oklahoma, planting can begin in mid-February, while Northern Oklahoma gardeners should aim for late February through mid-March. Bulbing onions are typically ready for harvest in late June to July, once the tops begin to yellow and fall over.
Secondary Planting Window: Fall Timing
A secondary planting window exists in the fall, primarily for an early spring harvest of green onions or scallions, rather than mature bulbs. The ideal time for planting is late August through September. This timing allows the onions to establish roots and top growth before winter dormancy. Gardeners attempting to overwinter small bulbs for an early harvest should apply a layer of mulch, such as straw or leaves, to protect them from harsh temperature fluctuations.
Planting Methods and Initial Setup
Onions are commonly started from sets (small dormant bulbs) or bare-root transplants. Transplants should be planted with roots buried, covering the lower portion of the plant with about an inch of soil. Sets are planted slightly deeper, one to two inches deep, with the pointed end facing upward.
For large, dry bulbs, plants should be spaced four to six inches apart. The soil must be well-drained and loose, as onions have shallow roots that need constant moisture but cannot tolerate standing water. After planting, a thorough initial watering helps settle the soil, followed by a consistent application of about one inch of water per week.