The success of growing onions depends heavily on matching the plant’s needs to the local climate. USDA Hardiness Zone 8b is characterized by mild winters, where the average minimum temperature falls between 15°F and 20°F, allowing for an extended growing season. Since onions are sensitive to both temperature and daylight duration, strategically selecting the correct variety and timing the planting is essential for a bountiful harvest.
Selecting the Right Onion Variety for Zone 8b
Onions are photoperiodic, meaning the formation of the underground bulb is triggered by the number of daylight hours, not just the plant’s age. Zone 8b requires short-day onion varieties for proper development, as these need approximately 10 to 12 hours of daylight to initiate the bulbing process.
Planting an inappropriate variety, such as a long-day onion, will lead to poor results because they require 14 to 16 hours of daylight and will only produce green tops in Zone 8b. Short-day varieties develop substantial foliage during the mild winter before the increasing spring day length signals them to form a bulb. Suitable short-day types include popular varieties like Granex, Texas Grano, and Vidalia types.
Optimal Timing for Planting in Zone 8b
The mild climate of Zone 8b allows for two primary planting windows, both aimed at maximizing the vegetative growth period before bulbing begins. The most common and successful approach for large bulbs is fall planting for overwintering. Planting onion sets or transplants between late October and early December allows the plants to establish a robust root system during the cooler months.
This initial establishment is crucial because the size of the mature bulb is directly related to the number and size of the leaves the plant has developed before bulbing starts. The plants will remain dormant or grow slowly through the coldest part of winter, then resume active growth in late winter and early spring. These overwintered onions are ready for harvest in the late spring to early summer.
Starting from seed requires an indoor start in late summer or early fall to produce transplants that are six to eight weeks old by the time the fall planting window arrives. Direct sowing of seeds can also be done in late winter, around January or February, as soon as the soil is workable.
Essential Steps for Successful Planting
Onions thrive in loose, well-draining soil that is rich in organic matter. They prefer a slightly acidic to neutral soil pH, ideally ranging between 6.0 and 6.8. Incorporating compost or well-rotted manure into the planting bed a few weeks before planting will improve drainage and fertility.
When planting sets or transplants, depth must be controlled to ensure proper bulb development. Onion sets should be planted shallowly, about one to two inches deep, barely covering the bulb. Transplants should be set with the roots covered and only about one inch of the lower portion of the plant beneath the soil surface.
Spacing is important for producing full-sized bulbs; plants should be spaced approximately four to six inches apart in rows. Onions are heavy feeders, especially needing nitrogen early in their life cycle to support leaf growth. Apply a balanced fertilizer at planting and then a high-nitrogen granular fertilizer every two weeks until bulbing begins. Consistent watering is also necessary immediately after planting and throughout the growing season to maintain soil moisture.