Cutting the tops off onions is a precise step in harvesting that directly influences the bulb’s ability to store long-term. The timing is crucial for a successful cure, the drying process that prepares the onion for months of dormancy. Cutting too early invites rot, while waiting too long can lead to the onion starting to regrow. The goal is to signal the plant to seal the neck and concentrate its sugars, maximizing both flavor and shelf life.
Reading the Plant’s Signals for Maturity
The onion plant provides distinct visual cues that indicate the bulb has finished its growth cycle and is ready for harvest. The primary signal is the natural “lodging” or falling over of the green tops. This occurs because the neck, the transition zone between the leaves and the bulb, softens and weakens.
This softening signals that the plant has ceased transferring energy from the foliage to the bulb. The leaves will begin to turn yellow or brown as the plant starts to dry down. Growers should wait until at least half to two-thirds of the tops have fallen over naturally before harvesting.
Harvesting before this natural signal means the bulb has not fully matured, resulting in a thin outer skin that offers poor protection and reduced storage life. Conversely, leaving the bulbs in the ground for too long after the tops have fully dried can be risky. They may start to develop secondary roots or become susceptible to rot if the soil is wet. The neck should feel completely dry and papery before the final trimming for storage.
The Proper Technique for Topping and Curing
Topping, the physical act of cutting the tops, must follow the initial harvesting and curing period. Once pulled from the ground, onions must be cured in a warm, dry, and well-ventilated location for several weeks. This drying process allows the neck to seal naturally, making the outer layers papery and dry.
Using clean, sharp shears or scissors, the dry foliage is trimmed down to a specific length. It is important to leave a stub of at least one to two inches of the neck attached to the bulb. This short, dry stub acts as a natural plug, sealing the vulnerable neck area.
This seal prevents moisture from entering the bulb, which promotes rot, and blocks potential entry points for pathogens and fungal spores. Cutting too close to the bulb before or after curing compromises this protective barrier. Successful long-term storage depends on the integrity of this neck seal created by the topping process.
When Cutting Tops Prematurely Is Acceptable
While cutting the tops of mature bulbs prematurely is detrimental to long-term storage, specific contexts allow for this practice. Immature onions, often harvested for fresh use, can have their green tops cut without concern for curing. This includes small bulbs like spring onions or scallions, which are intended for immediate culinary use.
Gardeners may also choose to harvest larger bulbs early for fresh eating, before the tops have naturally fallen over. These bulbs will not form the protective, papery skin needed for storage, and must be used within a few weeks. Similarly, the green foliage may be trimmed from young plants during the growing season to encourage stronger root development, with the trimmings used like chives.
Any onion topped while the foliage is still green and succulent will not successfully cure. These onions have a “thick neck” that remains moist and open, making them highly susceptible to rot. Therefore, any onion harvested and topped before natural maturity must be consumed quickly, as it lacks the necessary biological defenses for extended dormancy.