A new onion can generally be grown from an existing one, but success and crop quality depend entirely on the method used. Onion cultivation is governed by specific biological requirements and horticultural timing. Producing a large, storable bulb requires a different approach than simply growing fresh green tops. Understanding the initial starting material is the first step toward a successful harvest.
Choosing the Starting Material
Gardeners typically choose from three main options to begin an onion crop: seeds, sets, or transplants. Starting onions from seed offers the greatest range of variety, allowing access to specialized cultivars not available through other methods. Seeds require the longest growing period, often needing four to five months to mature into full-sized bulbs. They must also be started indoors six to ten weeks before the last frost, requiring dedicated light and precise temperature control.
Onion sets are small, immature bulbs grown the previous year and sold dormant for quick spring planting. They are the easiest material for beginners to handle and establish quickly, often providing a harvest 60 to 80 days sooner than those started from seed. A drawback to using sets is their tendency toward premature flowering, known as bolting, which diverts the plant’s energy into seed production and halts the development of the main bulb.
Transplants are young seedlings started professionally in greenhouses and shipped when they are pencil-thin. They offer a favorable balance, providing a wider choice of varieties than sets while significantly reducing the long growing time required for seeds. Transplants are generally more reliable than sets for producing large, uniform bulbs because the risk of bolting is minimized.
Understanding Onion Types and Timing
The most significant biological factor governing successful onion growth is photoperiodism, the plant’s response to day length, which triggers bulb formation. Onions are classified into three types based on the minimum number of daylight hours required to signal the plant to stop growing leaves and start swelling the bulb. Planting the wrong type for a given latitude will result only in green tops and no marketable bulb.
Short-Day onions initiate bulbing when daylight reaches 10 to 12 hours, making them suitable for cultivation in Southern regions. These varieties are often planted in the fall and mature in the early spring before the long summer days arrive. They do not store as well as the other types due to their higher water content and thinner protective outer layers.
Long-Day varieties require 14 to 16 hours of daylight to begin the bulbing process, making them the standard choice for Northern growers. They are typically sown or planted in the early spring to ensure the longest possible day length coincides with the peak growth phase. These types develop a thicker outer skin and higher solid content, which contributes to superior storage quality lasting several months.
Intermediate-Day, or Day-Neutral, onions require an intermediate photoperiod of 12 to 14 hours of sunlight for successful bulb development. These varieties perform best in the middle latitudes, acting as a flexible option where seasonal day lengths fall between the extremes. This classification system ensures the onion’s life cycle aligns with the local climate’s solar schedule.
The Physical Planting Process
Preparing the planting site requires creating a loose, well-draining soil structure, as onions have shallow roots and are highly sensitive to standing water. The soil should be amended with organic matter to promote friability and prevent compaction, which can impede the lateral expansion of the developing bulb. A soil pH slightly acidic to neutral, ideally between 6.0 and 6.8, provides the most favorable environment for nutrient uptake.
When planting sets or transplants, proper depth determines success. They should be placed shallowly, with the base of the bulb or root mass approximately one inch beneath the soil surface. Planting too deep encourages neck rot and can restrict the bulb’s ability to swell outwards, resulting in elongated, undersized onions.
Spacing should be four to six inches between individual plants to ensure each onion has enough room to reach its full potential size. Rows should be spaced about twelve to eighteen inches apart to allow for cultivation and air circulation, minimizing the risk of fungal diseases. Crowding plants leads to competition for resources and limits the final size of the mature bulbs.
Onions require a steady supply of nutrients but initially benefit from lower nitrogen levels compared to leafy vegetables. Excessive nitrogen fertilizer applied late in the growing season promotes continued leaf growth at the expense of bulbing, potentially delaying maturity and reducing the final yield. High levels of phosphorus and potassium are important for strong root development and disease resistance. Sulfur contributes significantly to the characteristic flavor and pungency of the mature onion bulb.
Replanting Grocery Store Onions
Planting a mature onion bulb purchased from a grocery store will quickly produce green shoots, which can be harvested as scallions or green onions for immediate consumption. The mature bulb is already programmed to complete its life cycle and will not form a single, large new bulb suitable for long-term storage. Instead, the original bulb will internally divide.
This division results in the formation of several small, segmented offsets or cloves around the base of the original planted onion. While this provides a source of fresh green tops, it is not a viable method for regenerating a crop of large, uniform onions. This practice is best utilized for a continuous, small harvest of fresh green tops rather than a main crop yield.