Onion plants, whether purchased as small bulbs (sets) or as bare-root transplants, are highly perishable living materials requiring immediate attention upon arrival. While they can be temporarily stored, the primary goal should always be to get them into the ground quickly. Every day spent in storage reduces the plant’s vigor and increases the risk of bolting or rot, ultimately impacting the final harvest. Understanding this brief planting window is crucial for a successful onion crop.
The Critical Storage Timeframe
The maximum safe storage time varies significantly depending on the form purchased. Bare-root transplants, which are actively growing seedlings shipped without soil, are the most vulnerable. They should be planted within a week of receiving them. Although these dormant seedlings can survive up to three weeks on stored energy, planting within three to eight days minimizes transplant shock and encourages root establishment.
Onion sets, which are small, immature bulbs, are far more resilient. They can generally be stored for several weeks, or up to a few months, if kept under ideal, cool conditions. Prolonged storage increases the chance of premature sprouting or bolting, which diverts energy away from forming a large edible bulb. Sets should be planted as soon as the soil is workable in early spring to prevent this issue.
Optimal Storage Conditions
The best temporary storage environment must be cool, dry, and well-ventilated to prevent the growth of molds and bacteria. Temperatures between 35°F and 50°F are ideal for slowing the plant’s metabolism without causing freeze damage.
Bare-root transplants must be removed immediately from their shipping box and any rubber bands to allow air circulation around the roots and tops. Spread the transplants in a single layer in a shallow cardboard box or tray, away from direct sunlight, to prevent moisture buildup. Onion sets should be stored in breathable containers, such as mesh bags or old nylon stockings. Never store sets or transplants in sealed plastic bags, as trapped humidity creates an anaerobic environment that quickly leads to rot.
Signs of Deterioration and What to Avoid
Monitoring stored plants for signs of decline is necessary to mitigate loss. Visual cues include excessive yellowing or wilting of the green tops, which indicates the plant is rapidly using its stored energy.
More concerning signs are soft, mushy, or slimy bulbs, which suggest bacterial soft rot. This decay often starts at the neck and can be identified by a pungent, sulfurous odor. Black mold, indicated by sooty-black spore masses, thrives in high humidity and temperatures over 85°F.
The most common storage mistake is leaving bare-root transplants bundled or in their original sealed packaging. This mistake suffocates the plants and concentrates moisture, accelerating fungal and bacterial growth. Any plant displaying these symptoms should be removed immediately to prevent the spread of pathogens to healthy stock.
Preparing Stored Plants for the Garden
Before planting, bare-root transplants stored for more than a few days will benefit from a brief rehydration soak. Placing the roots in a shallow container of water for a few hours helps the plant absorb moisture and recover from dry shipping conditions.
The roots are often long and tangled from shipping and should be trimmed to about a half-inch to one inch in length. Trimming the tops by cutting off about one-third of the green growth encourages the plant to focus energy on new root development rather than supporting excessive foliage. This preparation reduces transplant stress, resulting in a stronger seedling less prone to wind damage. Once prepared, handle the plants gently and place them into the garden soil promptly.