The question of whether a mature onion bulb can be planted to regrow is common for home gardeners seeking to maximize kitchen scraps or extend their harvest. This inquiry focuses on taking a dormant bulb, or the root-bearing basal plate, and reintroducing it to soil to encourage new growth. The process utilizes the plant’s natural tendency to sprout from its base. Understanding the onion’s life cycle is the first step in determining the outcome, as the initial bulb is a storage organ whose regrowth serves a purpose distinct from growing a new, single, large bulb.
The Immediate Answer: Why Plant a Whole Onion
Planting a whole, mature onion bulb will not typically yield a single, large new bulb ready for long-term storage. The primary outcome is the production of green, hollow leaves, commonly known as scallions or green onions. These greens sprout rapidly from the bulb’s center and provide a continuous, fresh harvest. The mature bulb often splits into a cluster of smaller, less uniform bulbs, rather than forming one large replacement bulb.
Another possible result of replanting a mature bulb is encouraging the plant to complete its biennial life cycle by bolting, which means sending up a flower stalk and producing seeds. Onions are biennials, growing the bulb in the first year and flowering in the second. Replanting a mature bulb often tricks it into entering this second, reproductive stage. If the goal is to collect seeds, allowing the plant to bolt is the desired outcome. If the goal is a new bulb harvest, bolting is undesirable because it redirects the plant’s energy away from bulb enlargement.
Step-by-Step Planting Guide
To successfully replant a mature onion, select a healthy bulb that shows signs of sprouting or has a clean, intact basal plate (the flat end where the roots emerge). The planting location should receive full sunlight for at least six to eight hours daily, as onions require high light to thrive. The soil must be loose, well-draining, and amended with aged compost or organic matter to provide necessary nutrients and prevent waterlogging.
When planting, ensure the basal plate is pointed down and the top third of the bulb is left exposed above the soil line. If planting only the root end, bury it just deep enough so the cut edge is covered and the new green shoot is fully exposed. Spacing can be tight, approximately three to four inches apart, especially if the intention is only to harvest the greens rather than encourage large bulb development. After planting, provide a thorough initial watering to settle the soil around the developing roots.
Harvesting and Maintenance
Newly planted onions require consistent moisture, especially during the initial phase of green growth, but the soil should never become soggy. Overwatering or poor drainage can quickly cause the original bulb to rot. Applying a balanced, all-purpose fertilizer every two to three weeks during the leafy growth stage supports the continuous production of greens.
For a continuous harvest of greens, use scissors or kitchen shears to cut the hollow leaves approximately one inch above the bulb. This allows the plant to regenerate new leaves for multiple harvests without pulling up the entire bulb. To collect seeds, cease cutting the greens and allow the plant to naturally send up a stalk and flower. The plant is ready to harvest when the flower head dries out and the seeds can be easily collected.
Alternatives to Bulb Planting
For gardeners whose primary aim is to produce a crop of large, uniform, storage-ready onion bulbs, planting a mature bulb is not the most efficient method. The standard alternatives involve starting with either onion sets or seeds.
Onion Sets
Onion sets are small, immature bulbs grown during the previous season. They offer a convenient shortcut for gardeners and mature quickly, making them a favored option for fast results. However, sets are often limited in variety and are more prone to bolting than seeds.
Seeds
Starting with seeds provides the greatest variety of cultivars and is the most cost-effective option for large-scale planting. Onions grown from seed tend to develop deeper root systems and produce larger, stronger bulbs less prone to bolting. Seeds require a much longer growing season, often necessitating an indoor start ten to twelve weeks before the last expected frost, making them more time-intensive than sets.