A green stem emerging from the top of a stored onion signals the beginning of sprouting, a natural biological process. This growth is the onion bulb responding to environmental cues, attempting to transition back into a growing plant. The green shoot, essentially a new leaf, indicates the onion has broken dormancy and is using its stored energy. Understanding the cause and prevention of sprouting helps maintain quality and extend the shelf life.
The Biological Reason for Sprouting
The stored onion bulb is botanically a biennial plant structure, completing its life cycle over two years. The bulb is a storage organ that allows the plant to survive unfavorable conditions, like winter. Once harvested, the bulb enters a period of dormancy, maintained by internal plant hormones like abscisic acid (ABA).
This dormancy is temporary and breaks when conditions become favorable for growth. Environmental triggers that signal sprouting are elevated temperatures and high humidity or moisture. Temperatures that are too warm, such as those in a kitchen pantry, or temperature fluctuations can trick the onion into believing spring has arrived.
The green stem is the shoot tip elongating into a new leaf, which can develop into a flower stalk if planted. As the sprout grows, the onion mobilizes stored carbohydrates like fructans from the fleshy layers to fuel cell division. This process depletes the bulb’s stored energy to support the new growth.
Proper Storage to Halt Green Growth
Preventing green stems requires maintaining storage conditions that preserve the onion’s dormant state. The environment should be cool, dry, and dark. The recommended temperature range for long-term storage is between 32 and 40°F (0 and 4°C), which effectively inhibits sprouting.
If refrigeration is unavailable, a cool room, basement, or garage can work, ideally held between 45 and 55°F (7 and 13°C). Storing whole bulbs in the refrigerator is discouraged because high humidity encourages rooting and sprouting, and the cold can cause the texture to soften. Low humidity (65% to 70%) is necessary to prevent moisture accumulation that promotes growth and decay.
Proper air circulation helps maintain consistent temperature and prevents moisture buildup on the bulb surface. Onions should never be stored in plastic bags, which trap moisture and heat, accelerating sprouting. Using mesh bags, baskets, or ventilated crates allows the necessary airflow. Keeping the bulbs dark is important because light encourages photosynthesis in the new growth, resulting in the green color.
Are Sprouted Onions Safe to Eat?
The presence of a green stem does not mean the onion is unsafe to consume. Unlike potatoes, which produce the toxic compound solanine when they sprout, all parts of a sprouted onion are safe to eat. There are no harmful toxins associated with onion sprouting, so discarding the entire bulb is unnecessary.
However, sprouting alters the quality of the bulb. As the onion uses stored sugars to fuel the green shoot, the bulb loses moisture and firmness, becoming softer and slightly dehydrated. The reallocation of sugar reserves often results in a less sweet and more pungent or slightly bitter flavor.
To use a sprouted onion, simply cut out the green core or sprout to reduce the stronger flavor. The green shoots are edible and can be chopped and used like chives or scallions, adding a mild onion flavor to dishes. It is best to use sprouted onions quickly in cooked dishes with other strong flavors, where texture and pungency changes are less noticeable.