The time it takes to grow red onions (Allium cepa) is variable, influenced primarily by the method used to start the crop. Red onions, a biennial bulb often grown as an annual, require a complete growth cycle to develop a mature bulb. Understanding this timeline is fundamental for gardeners to time their planting correctly and achieve a successful harvest. The overall duration ranges widely, depending on whether the gardener begins with seeds, small bulbs called sets, or pre-grown transplants.
Starting Methods and Initial Time Investment
The initial time investment is determined by the choice between three primary starting materials. Starting red onions from seed requires the longest commitment, as seeds are typically sown indoors eight to ten weeks before the last expected frost date. This indoor period allows seedlings to establish themselves before being transplanted into the garden soil in early spring. This method offers the widest selection of varieties, but requires managing the initial two to two-and-a-half months of growth indoors.
Onion sets, which are small, immature bulbs grown the previous year, provide a significant shortcut. These miniature bulbs are planted directly into the garden soil as soon as the ground is workable in early spring, bypassing the lengthy indoor germination and seedling phase. Sets are popular because they offer nearly immediate planting with a high success rate.
A third option involves purchasing transplants, which are seedlings started by a commercial grower. These young plants are ready to be planted outdoors in early spring, similar to sets. Transplants are programmed to develop into a full-sized bulb without the risk of bolting that can sometimes occur with sets. They offer a balance, eliminating the indoor seed-starting period while retaining access to a good range of varieties.
The Red Onion Growth Timeline by Method
The total time from initial planting to a mature, ready-to-harvest bulb varies considerably based on the starting material. Onions grown from sets have the shortest field time, often reaching maturity in just 70 to 90 days after planting. This rapid growth makes sets an attractive choice for regions with shorter growing seasons.
Transplants require more time to finish bulbing, needing between 90 and 120 days from the date they are placed in the garden. This longer duration allows the plant to build a larger foliage mass, which translates directly into a larger, more substantial bulb. Choosing a variety suited to your climate is important for achieving a mature harvest within these ranges.
Growing red onions from seed requires the longest total commitment, with field time extending to 150 to 180 days from the initial indoor sowing. The total duration is heavily influenced by the variety’s day-length classification, which dictates the sunlight required to trigger bulbing. This photoperiodic response is the most important factor in determining the final harvest date.
Day-Length Classifications
Short-day onions require 10 to 12 hours of light and mature faster in southern latitudes. Long-day varieties need 14 to 16 hours of daylight and are best suited for northern regions. Intermediate-day onions need 12 to 14 hours and thrive in middle latitudes.
Recognizing Readiness for Harvest
The calendar timeline serves as a rough guide, but the onion plant provides clear visual cues indicating its readiness for harvest. The first major sign is the softening of the neck, where the green foliage meets the top of the bulb. Once the neck loses its rigidity, it signals that the transfer of carbohydrates from the leaves to the bulb has concluded.
This softening is soon followed by the natural collapse, or “lodging,” of the green tops as they start to yellow and wither. For optimal maturity and storage potential, gardeners should wait until 85 to 90 percent of the tops have fallen over before pulling the bulbs. Harvesting before this point can result in smaller bulbs that do not store well, while harvesting too late risks the outer skins splitting.
In the final week or two before the anticipated harvest, stop watering the crop completely. This intentional drying period forces the plant to begin forming its protective outer skin, which is essential for curing and long-term storage. Reducing moisture at this stage helps prevent the development of rot and fungal diseases in the maturing bulb.
The Post-Harvest Curing Process
The growth process does not end at harvest, as a necessary final stage called curing must take place before the red onions are ready for long-term storage. Curing is a drying procedure that hardens the neck and the papery outer layers of the bulb. This protective skin acts as a barrier, preventing moisture loss and blocking the entry of decay-causing microorganisms.
The ideal environment for curing is a warm, dry, and well-ventilated space, with temperatures ranging between 75 and 80 degrees Fahrenheit. The harvested bulbs should be spread out in a single layer, out of direct sunlight, to allow for maximum air circulation. Good ventilation is necessary to wick away moisture and complete the drying process efficiently.
The curing process takes two to four weeks, depending on humidity and temperature conditions. The onions are fully cured when the necks are completely dry and brittle, and the outer skins rustle when touched. Once cured, the roots and the dried tops are trimmed, and the onions are ready to be moved to a cool, dark place for extended storage.