Red onions (Allium cepa) are a popular garden vegetable, valued for their distinctive color and sharp flavor. The time required to cultivate a full-sized bulb varies significantly based on the initial planting method and the local environment. The overall duration is a range influenced by whether the plant is started from a seed, a set (small bulb), or a transplant (seedling). Selecting the appropriate starting material and managing growing conditions are important for a successful harvest.
Starting the Growth Process
Growers typically cultivate red onions using one of three methods: seeds, sets, or transplants. Seeds offer the widest selection of varieties but require the longest time commitment. They are generally started indoors eight to ten weeks before the last anticipated spring frost date, sown about one-half inch deep.
Sets, which are small, immature bulbs from the previous year, provide the fastest path to harvest. They are planted pointed end upward, one to two inches deep, as soon as the ground is workable in early spring. Transplants, or commercially purchased seedlings, offer an intermediate approach, giving the plant a head start without indoor preparation. These are planted approximately one inch deep to establish the root system before bulb formation begins.
Red onions require loose, well-draining soil, ideally a sandy loam with a pH between 6.0 and 6.8. Proper spacing is necessary to allow the bulb to expand to its full size, requiring three to four inches between mature plants. Planting too deeply can impede bulb development, which is why the sets and transplants are often just barely covered with soil. The initial focus of growth is root and foliage development to maximize the plant’s capacity for photosynthesis before bulbing begins.
The Standard Timeline for Red Onion Maturity
The duration of the red onion growth cycle depends on the maturity level of the material used at planting. Onion sets, being dormant bulbs, offer the quickest turnaround, typically reaching full maturity in approximately 60 to 80 days after planting. This accelerated timeline is because they bypass the lengthy germination and initial seedling establishment phases. They quickly resume growth, focusing energy on foliage production before the final bulbing stage.
Transplants, which are established seedlings, take an intermediate period, generally maturing within 90 to 120 days from planting. This method is preferred for its balance of speed and reliable varietal selection. The plant utilizes its existing foliage to start the process of storing carbohydrates in the developing bulb.
Starting red onions from seeds requires the longest commitment, with maturity taking between 120 and 175 days. The cycle begins with germination, followed by a prolonged period of vegetative growth where the plant develops a large fan of leaves. This foliage generates the carbohydrates necessary to fuel the final stage of bulb expansion. A larger top growth before bulbing correlates directly with a larger final bulb size.
The entire process involves distinct stages: root establishment, extended leaf growth, and bulb formation. Bulb initiation begins once the plant has developed a sufficient number of leaves and the required day length is achieved. The final period involves a shift in energy from the leaves to the bulb, leading to visual cues that indicate harvest readiness.
Environmental Factors That Affect Growth Speed
External variables significantly influence whether red onions mature on the shorter or longer end of their timeline. The most impactful factor is the plant’s response to day length, which determines when bulbing is initiated. Onions are classified as short-day, intermediate-day, or long-day types. Planting the incorrect type for the local latitude can halt bulb formation entirely.
Day Length Requirements
Short-day varieties require 10 to 12 hours of daylight to begin forming a bulb, making them suitable for southern regions. Long-day varieties need 14 to 16 hours of light and are adapted for northern latitudes. Intermediate-day types require 12 to 14 hours of light and perform well in middle regions. If the wrong type is planted, it may never receive the necessary daylight hours to trigger bulbing.
Temperature and Moisture
Temperature and moisture management are important throughout the growing season. Onions are a cool-season crop, preferring moderate temperatures for vegetative growth. Extreme heat or drought can cause the plant to stop growing or prematurely send up a flower stalk (bolting), which diverts energy from the bulb. Consistent moisture is necessary during the bulb expansion phase, but waterlogged soil must be avoided to prevent disease.
Soil Nutrition
Soil nutrition plays a role in managing the pace of growth. Excess nitrogen applied late in the season can delay maturity and lead to a softer bulb that stores poorly. Phosphorus is important for vigorous root development early on, and potassium supports overall plant health. The final stages of bulb development benefit from a reduced nitrogen supply. Nitrogen application should cease at least four weeks before the anticipated harvest to encourage timely maturation.
Knowing When to Harvest
The end of the red onion’s growth cycle is marked by a series of physical changes that signal the bulb is fully mature and ready to be lifted from the soil. The clearest indicator is the natural softening and collapse of the green tops, or foliage. When 50 to 85 percent of the plant tops have bent over, the process of transferring energy from the leaves to the underground bulb is complete.
This falling over of the tops is accompanied by a softening of the neck, which is the area where the leaves meet the top of the bulb. A stiff, green neck indicates the plant is still actively growing and should be left undisturbed. Once the neck becomes pliable and the foliage starts to yellow, the final cell division in the bulb rings has occurred, and the onion should be harvested within a week or two.
After gently pulling the bulbs from the ground, a necessary process called curing must take place to prepare the red onions for long-term storage. Curing involves allowing the outer skin and the neck to dry completely, which is typically done in a warm, dry, and well-ventilated location out of direct sunlight for two to four weeks. This process seals the bulb, creating the papery outer layers that extend the onion’s shelf life.