It is possible to regrow an onion from an existing bulb, utilizing the plant’s natural structure for revival. The onion bulb itself is a modified subterranean stem with layers of fleshy leaves that serve as a food storage unit for the biennial plant. Regrowth is possible because the bottom portion, known as the basal plate, contains the dormant growing point and root primordia. Keeping the basal plate intact provides the foundation for a new plant to emerge, drawing on stored energy. This capacity for regeneration offers two distinct paths for the home gardener: a rapid harvest of fresh greens or the long-term cultivation of a new, full bulb.
Growing Onion Greens for Immediate Harvest
The fastest way to utilize an old onion for new growth is by cultivating its greens, commonly known as scallions. This method focuses on the rapid production of leaves, providing a continuous, mild flavor for immediate use. To start, simply slice off the bottom section of the bulb, leaving about a half-inch to one inch of the white base where the roots are attached.
One popular technique is to place this root end, cut-side up, into a small glass or jar filled with shallow water. The water should only cover the root hairs and the basal plate, preventing the rest of the bulb from rotting. Positioned on a sunny windowsill, new green shoots will often begin to emerge from the center within a few days. Alternatively, you can plant the root section directly into a shallow container filled with potting mix, with the top of the cut bulb barely covered by the soil.
Once the greens reach a usable height of a few inches, you can begin harvesting them using the “cut-and-come-again” technique. Use scissors to snip the green stalks about one inch above the white base. The plant will continue to regenerate new leaves from the central growing point, allowing for multiple harvests over several weeks. While growing in water is convenient, the plant will eventually exhaust the nutrients stored in the base, and planting in soil offers a more sustained yield of thicker, healthier greens.
Propagating a New Bulb from the Base
Growing a new, full-sized onion bulb from a cut base requires a full growing season. The goal is the development of a mature, layered bulb ready for storage, not a quick harvest of greens. Start by cutting a larger section of the onion, approximately one to two inches thick, ensuring the basal plate and a substantial portion of the fleshy layers are retained.
This section, which may be dried for a few hours to allow the cut surface to callus, should be planted directly into the soil outdoors or in a large container. Unlike the shallow planting for greens, the entire section should be buried about one to two inches deep. The planted base will first grow roots and then sprout multiple green shoots, as the original bulb contains several dormant growth points. These shoots will eventually separate and form individual new bulbs, which is a common occurrence when growing from a bulb section.
Onions are biennial plants, meaning they naturally produce a bulb in the first year and a flower stalk in the second year to produce seeds. Since the planted base is technically in its second year of growth, it has a high tendency to “bolt,” or send up a flower stalk. If a flower stalk appears, remove it immediately to redirect energy back into bulb development. Note that a bolted onion will not store as well and should be used shortly after harvest. This process requires patience, as a full bulb takes several months to mature.
Essential Care and Maintenance Tips
Successful growth, whether for greens or a full bulb, depends on providing the right environmental conditions once the new plant is established. Onions are sun-loving plants that require a minimum of six to eight hours of direct sunlight each day for robust growth. Insufficient light will result in weak, thin greens and a failure to form a large, firm bulb.
Consistent moisture is also necessary, as onions are shallow-rooted and do not efficiently seek out water deep in the soil. The soil should be kept consistently moist but never waterlogged, which can quickly lead to the basal plate rotting. If you are aiming for a full bulb, a light application of a nitrogen-rich fertilizer every few weeks during the early, vegetative growth phase will support the development of strong foliage, which directly correlates to the final size of the bulb.