Growing onions successfully in Tennessee requires careful consideration of both planting time and the specific variety chosen. Understanding the onion’s requirement for day length to form a bulb is the first step toward a bountiful harvest. By selecting the correct type and timing the planting precisely, home gardeners can produce large, flavorful dry bulbs.
Selecting the Right Onion Variety for Tennessee
Successful onion bulbing depends on the plant’s response to daylight duration, which is determined by geographic latitude. Onions are classified into three types: short-day (10–12 hours of light), long-day (14–16 hours), and intermediate-day. Tennessee sits in a transitional zone, making intermediate-day varieties the most reliable choice for a full-sized crop. These types initiate bulbing when day length reaches approximately 12 to 14 hours, aligning with the state’s spring and early summer light cycle. Intermediate-day varieties offer the best assurance of a large, mature bulb.
Optimal Planting Times and Methods
Timing the planting is important to ensure plants develop large tops before bulbing begins. For the home gardener, planting sets or transplants is the most common and successful method. The ideal window for planting these immature bulbs is late winter or early spring, typically from late February through March, as soon as the soil is workable. Planting four to six weeks before the last expected spring freeze allows the plant to establish a robust root system during the cool season.
Onion sets are small, dormant bulbs, while transplants are young plants started from seed. Sets are convenient, generally more tolerant of light frost, and establish quickly. Plant sets with the point facing upward, pressing them about one to two inches deep into the prepared soil.
Growing onions from seed offers a wider choice of varieties but requires an earlier start indoors. Seeds should be sown eight to ten weeks before the planned outdoor transplant date, generally in January or early February. Once seedlings reach sufficient size, they are transplanted into the garden during the same late February to March window.
Preparing the Site and Initial Care
Onions thrive in loose, friable soil that allows the bulb to expand easily. Good drainage is imperative, as onions have shallow root systems and will rot quickly in waterlogged conditions, making raised beds an excellent choice. The soil pH should be slightly acidic to neutral, ideally ranging between 6.0 and 7.0.
Prior to planting, incorporate a balanced fertilizer with a higher middle number, such as a 10-20-10 blend, into the soil to promote early root development. Onions are heavy feeders, especially of nitrogen, and benefit from subsequent side-dressings. Apply a nitrogen-rich fertilizer every two to three weeks after planting until the bulbs begin to swell.
Proper spacing is necessary for maximum bulb size, requiring plants to be four to six inches apart in the row. If planting densely, thinning seedlings to this final spacing is necessary. Consistent moisture is required during the vegetative growth phase and bulb development, but watering must stop once the plants show signs of maturity.
Harvesting and Curing Onions
Knowing when to harvest is determined by the plant’s natural life cycle. Onions are ready when the tops turn yellow or brown and approximately 70% to 80% of the foliage has naturally fallen over. This bending of the neck signals that the plant has finished sending energy to the bulb and is entering dormancy.
Once the tops have fallen, stop all watering to allow the soil to dry out, which prepares the bulbs for storage. Carefully lift the onions from the soil on a dry, clear day, taking care not to bruise the bulbs, as damage can lead to premature decay. The curing process is required to seal the neck and dry the outer skin for long-term storage.
Curing involves laying the harvested onions in a warm, dry, and well-ventilated area, such as a covered porch or shed, for two to four weeks. The onions are fully cured when the neck is completely dry and tight, and the outer layer is papery. After curing, trim the dry tops to about one to two inches above the bulb. Store the onions in a cool, moderately dry location, typically between 32°F and 40°F, in mesh bags for optimal air circulation.