Virginia’s diverse climate, which spans multiple USDA hardiness zones, means a single planting date will not work for every region. Onions are sensitive to day length and temperature, requiring gardeners to synchronize planting with specific seasonal markers. This ensures the plant focuses its energy on developing a large bulb rather than simply growing foliage. Coordinating the biological needs of the chosen onion variety with local frost dates is the first step to a successful summer harvest.
Understanding Onion Types and Day Length Needs
Onions are photoperiodic; their bulb formation is triggered by a specific number of daylight hours. The plant first uses cooler weather to grow a robust top. Once the required day length is reached, it shifts its energy into bulbing. Planting the incorrect type for a specific latitude can result in plants that only grow large tops or prematurely bolt to seed.
Onion types are categorized into three groups based on their photoperiod requirement. Short-day varieties bulb when daylight reaches 10 to 12 hours and are best suited for the southernmost parts of Virginia, particularly the warmest areas in Zone 8. Long-day onions require 14 to 16 hours of daylight and are less reliable for most of Virginia.
Intermediate-day, or day-neutral, onions are the most successful choice for the majority of Virginia gardeners, as they bulb with 12 to 14 hours of daylight. Since Virginia’s longest summer day is approximately 14 hours and 46 minutes, intermediate-day varieties have the best chance to grow large tops before the bulbing trigger is met. Gardeners in the Piedmont and mountainous regions should select intermediate-day types for the most consistent results.
Optimal Spring Planting Schedules
The primary planting window for a summer onion harvest is early spring, when sets or transplants are placed into the ground. Onions are cold-tolerant and should be planted as soon as the soil is workable, often four to six weeks before the last expected hard frost. This early timing allows the plants to establish a substantial root system and grow maximum foliage before lengthening days initiate bulbing.
For most of the state, planting typically occurs from late February through March, varying by hardiness zone. Warmer zones can begin earlier in February, while cooler mountainous regions may wait until late March. Transplants are often preferred over sets because sets are more prone to bolting later in the season.
Transplants should be planted one inch deep, burying the roots and a small portion of the lower stem. Sets are placed one to two inches deep. Avoid burying them too deeply, which can hinder proper bulb development.
Overwintering and Fall Planting
A secondary strategy involves planting in late summer or early fall for overwintering, often yielding an earlier spring harvest. This technique is more reliable in the milder parts of Virginia (Zone 7 and Zone 8), where the risk of winterkill is lower. Seeds or small sets of hardy varieties, such as certain short-day types, are sown in August or September.
The benefit of fall planting is that established onions resume growth immediately when temperatures warm in early spring, providing a harvest several weeks ahead of spring-planted crops. However, a severe or prolonged cold snap can kill the young plants.
If fall-planted seedlings are too large, or if they experience alternating warm and cold spells, they may be prone to premature bolting. Seedlings must not reach a diameter thicker than a pencil before cold temperatures arrive. Overwintering onions require a well-drained site to prevent rot during winter moisture.
Site Preparation and Initial Care
Onions are shallow-rooted and heavy feeders. The planting site must offer full sun and loose, well-drained soil, ideally a sandy-loam texture. Before planting, the soil should be amended with organic matter to improve drainage and fertility. The pH should be tested and adjusted to a slightly acidic range between 6.0 and 6.8.
Onions require an initial application of fertilizer before planting. Virginia Cooperative Extension often recommends four to five pounds of a balanced fertilizer, such as 10-10-10, per 100 square feet. This provides the necessary phosphorus and potassium for root and bulb development. Since the plant’s initial focus is on leaf growth, a high-nitrogen side dressing is applied one to two weeks after bulb enlargement begins.
Proper spacing is a factor in achieving bulb size, with plants typically spaced three to four inches apart within the row. After planting sets or transplants, consistent moisture is necessary, as is aggressive weed control. Cultivation must be frequent but shallow to avoid damaging the root system.