Growing onions starts with sourcing the correct planting material. Gardeners must understand the various forms available: sets, transplants, or seeds. This initial decision influences the time to harvest, the range of varieties, and the overall effort required for a successful season.
Choosing Between Onion Sets Transplants and Seeds
Gardeners have three options for starting onions: sets, transplants, and seeds. Onion sets are small, immature bulbs grown the previous year and forced into dormancy for sale. They offer the easiest and fastest path to harvest since they are partially grown. However, variety selection is limited, and sets are prone to bolting (prematurely flowering), making them better suited for green onions than large storage bulbs.
Onion transplants, also called starts, are young, green seedlings professionally started from seed, often indoors, and sold in bunches. Transplants offer a good balance of convenience and varietal choice, giving the gardener a six-week head start compared to direct seeding. They generally produce larger, higher-quality bulbs than sets. Transplants are also less likely to bolt because they have not gone through the storage cycle that triggers the plant’s flowering response.
Growing from seed is the most economical method and provides access to the largest range of unique and heirloom varieties. This method requires the most effort, as seeds must be sown indoors 10 to 15 weeks before the last frost date for proper bulb development. While seeds rarely bolt, their viability decreases significantly after the first year, requiring the purchase of fresh seed annually.
Where to Find Onion Planting Material
The sources for onion planting material depend on the type of material sought. For onion sets, local garden centers, hardware stores, and large national retailers are the most common places to purchase them. These locations stock sets in bulk mesh bags or containers starting in late winter and early spring, offering immediate availability for the early planting window.
Local nurseries and farmers’ co-operatives are excellent sources for fresh onion transplants, often selling them in bunches tailored to regional growing conditions. These regional suppliers frequently offer advice specific to the local climate and may carry varieties harder to find in big-box stores. Purchasing locally also reduces the stress of shipping on the delicate seedlings.
For the widest selection of varieties, especially transplants and seeds, online and catalog retailers are the best option. Specialized seed companies often grow and ship transplants directly to the customer at the appropriate planting time for their climate zone. This method is beneficial for growers seeking specific long-day, short-day, or intermediate-day varieties that may not be available locally.
Selecting the Right Variety and Quality
A successful onion harvest requires selecting a variety that matches the geographical location, determined by the plant’s photoperiodic response to daylight hours. Onions are categorized into three main groups based on the minimum daylight needed to trigger bulb formation. Northern latitudes, generally above 37° North, require long-day varieties, which need 14 to 16 hours of daylight to bulb.
Southern regions, typically below 35° North, must use short-day varieties, which begin bulbing when daylight reaches 10 to 12 hours. Intermediate-day onions, or day-neutral types, are suitable for central latitudes between 32° and 42° and require 12 to 14 hours of daylight. Planting the incorrect day-length variety will result in the onion plant only growing leaves without forming a substantial bulb.
When purchasing onion sets, check the quality of the bulbs before buying. The ideal size for a set is between one-half and three-quarters of an inch in diameter; larger sets are more likely to bolt prematurely. Sets should feel firm when gently squeezed and have a dry, papery outer skin, free of soft spots, mold, or green sprouts.
For onion transplants, quality is judged by the health of the live plant. Look for seedlings with white, fibrous roots and avoid any that appear mushy or brown at the base. While some paleness or wilting is normal after shipping, the plants should be mostly dry and not slimy, which indicates rot. Ensuring the material is healthy and the variety is appropriate for the local day length provides the best chance for a substantial harvest.