Onion sets are small, immature bulbs grown from seed the previous year, dried for storage, and then replanted. They offer gardeners a quick, reliable way to grow mature onions, typically being less prone to disease and bolting than starting from seed. Proper storage is necessary to keep these small bulbs dormant, preventing premature sprouting, drying out, or rot before the optimal planting window. Successful storage maintains a balance of cool, dry conditions that halts biological activity without causing desiccation. This process ensures the sets remain firm and viable until the soil is ready for spring planting.
Essential Preparation Steps
Upon acquiring onion sets, immediate preparation is required for successful storage. First, conduct a thorough inspection to eliminate any compromised bulbs that could jeopardize the entire batch. Discard any sets that appear soft, mushy, or show signs of mold, rot, or physical damage immediately, as a single spoiled set can quickly spread pathogens.
Sets must be completely dry before long-term storage, a process called curing. If the sets are damp, spread them out in a single layer in a warm, well-ventilated area for several days. This drying process toughens the outer skin and seals the neck of the bulb, acting as a protective barrier against moisture loss and fungal infections during storage.
Maintaining Viability: Environment and Method
The environment where onion sets are kept must be cool, dry, and dark to suppress the natural urge to sprout or initiate root growth. The ideal temperature range for dormancy is just above freezing, generally between 35 and 40 degrees Fahrenheit (2 to 4 degrees Celsius). Temperatures above this range encourage premature sprouting, while temperatures below freezing can damage the internal structure of the bulb.
Maintaining low humidity (60 to 70 percent) is equally important to prevent mold and rot. The storage method must prioritize air circulation to achieve this low-humidity environment. Sets should be stored in breathable containers such as:
Mesh bags
Old nylon stockings
Shallow crates with perforated sides
Avoid sealed plastic bags or airtight bins entirely, as these trap moisture and create conditions favorable for decay. Storing them in a dark location, such as a garage or unheated basement, also helps suppress light-triggered sprouting.
Duration of Storage
The successful storage duration for onion sets is typically governed by the local planting window, usually lasting from a few weeks up to a few months. Most gardeners aim to keep sets dormant until the soil becomes workable in early spring. Varieties with better storage properties can last longer, but sets generally maintain good viability for approximately one to three months under ideal conditions.
Sets should be planted as soon as the weather allows to ensure a successful harvest. Attempting to store them for extremely long periods, such as until the following year, is generally unsuccessful, as the bulbs will eventually attempt to grow or dry out. Extended storage also increases the likelihood that resulting plants will “bolt,” sending up a flower stalk that diverts energy away from bulb formation.
Recognizing and Addressing Spoilage
Regular monitoring is necessary to catch and correct changes that indicate failing storage. A soft, mushy texture, especially near the root or neck, is a clear sign of internal rot caused by excess moisture, requiring immediate removal of the affected set. Signs of mold, dark spots, or a foul odor signal that the environment is too humid or ventilation is insufficient.
If sets begin to shrivel and become desiccated, the storage environment is too dry. Conversely, the appearance of green shoots indicates that the temperature is too warm, signaling the start of the growing season. Corrective action involves immediately discarding all damaged bulbs and moving the remaining sets to a cooler location or increasing air circulation to reduce humidity.