The onion foliage is the plant’s primary engine, using photosynthesis to convert light energy into the sugars necessary for bulb growth. The size of the mature bulb is directly correlated with the amount of healthy green foliage present during the growing season. Understanding the precise timing of cutting the tops, whether for maintenance or final harvest, dictates both the final yield and the bulb’s long-term storage potential.
Identifying the Right Time to Harvest
The readiness of an onion bulb for harvest is signaled by a distinct physiological change known as “lodging” or “neck softening.” This occurs when the flow of nutrients from the leaves to the bulb ceases. The neck, where the green leaves emerge from the bulb, weakens, causing the entire top to flop over onto the ground.
The general rule is to wait until 50% to two-thirds of the plants in the patch have naturally lodged and begun to yellow. During this final stage, the plant actively pulls stored sugars and nutrients from the dying leaves back into the bulb. This transfer is crucial for flavor development and long-term dormancy.
Cutting the tops prematurely prevents this final energy transfer, resulting in a smaller bulb with a thick, green neck. This thick, uncured neck will not dry properly during curing, providing an entry point for moisture and pathogens. This severely reduces the onion’s ability to store for more than a few weeks.
Trimming Foliage to Encourage Bulb Development
Mid-season trimming of onion foliage is a practice some gardeners employ, though it carries specific risks and is distinct from the final harvest cut. This maintenance is primarily done to redirect the plant’s energy from excessive leaf production toward bulb swelling. Trimming may also be considered if the leaves become excessively tall or dense, creating poor air circulation that could harbor disease.
If trimming is chosen, it must be done sparingly and only early in the season, before the bulb begins its rapid enlargement phase. Removing too much green tissue, the photosynthetic surface area, directly reduces the total energy the plant can produce. This limits the potential size of the mature bulb, as larger foliage generally correlates with a larger final bulb.
Any mid-season cuts should be executed with caution, ensuring that no more than the top third of the foliage is removed. This practice is often used to manage overly vigorous growth in young plants or to prevent tall leaves from snapping. It is not a substitute for the natural lodging signal, which remains the sole indicator that the bulb’s growth cycle is complete.
Cutting Tops for Curing and Storage
The final cut of the onion tops occurs only after the bulbs have been harvested and fully cured. Curing is a necessary drying process that seals the outer skin and dries down the neck of the bulb. This step transforms the succulent neck tissue into a dry, papery seal that locks out pathogens and moisture, preparing the onion for long-term storage.
The tops can be trimmed after the bulbs have cured for several weeks, once the outer skins are papery and the necks are completely dry to the touch. Using a clean, sharp shear, the foliage should be cut off, leaving a stem of approximately one to two inches attached to the top of the bulb. This stub of dried neck material ensures the protective seal remains intact.
Leaving this short stem prevents an open wound at the bulb’s apex, which would invite rot-causing organisms. If the tops are removed too close to the bulb before the neck is fully dried, the raw, moist tissue is exposed, creating a high-risk scenario for bulb decay. For those who choose to braid their onions, the dried tops are left long and woven together, serving the same protective function.