The onion (Allium cepa) is a staple ingredient cultivated and selectively bred by humans for at least 7,000 years. Ancient Egyptians viewed its concentric rings as a symbol of eternal life. The plant’s practical durability and long storage life ensured its wide distribution, making it the second most-produced vegetable globally, following only the tomato.
The Onion’s Botanical Identity
The edible portion of the onion plant is a bulb, a specialized type of modified underground stem. This bulb functions primarily as a food storage organ, allowing the plant to survive dormant periods and harsh conditions. The familiar fleshy rings that make up the bulb are actually the bases of the plant’s leaves, which are compressed and thickened to store nutrients.
The true stem of the onion is the basal plate, a small, flattened disc at the bottom of the bulb. Fibrous roots emerge downward from this plate, anchoring the plant and absorbing water and nutrients. The papery outer layers, sometimes called the tunic, are modified leaves that dry out to protect the inner storage layers from pests and moisture loss. Although botanically a biennial, the onion is generally harvested as an annual in its first growing season.
Methods of Starting Onion Growth
There are three primary materials used to begin growing onions, each offering a different balance of convenience and variety. Seeds are the most economical option and provide access to the largest selection of unique varieties. They require a long growing period, often 90 to 150 days from sowing to harvest. Gardeners in northern climates often start seeds indoors 10 to 12 weeks before the last frost to ensure the seedlings are large enough for a substantial bulb harvest.
Sets are small, dormant onion bulbs grown from seed in the previous year and are the easiest and quickest method for home gardeners. These miniature bulbs are planted directly into the soil in early spring. Sets are usually limited to just a few common varieties, and they have a higher tendency to “bolt,” or prematurely send up a flower stalk, which halts bulb enlargement.
Transplants are small, actively growing seedlings that are typically shipped in bundles. They represent a middle ground between seeds and sets. Transplants are planted directly into the garden as soon as the soil is workable in the spring, offering a good selection of varieties and a lower risk of bolting compared to sets.
Ideal Soil and Environmental Needs
Successful bulb formation depends on the correct environmental cues and a specific soil structure. Onions require full sun exposure, meaning at least six hours of direct sunlight daily, as light is the primary trigger for bulb development. The soil must be loose, well-drained, and rich in organic matter, ideally a sandy-loam or silt-loam, to prevent waterlogging which can lead to bulb rot.
The distinction between long-day and short-day varieties is an important environmental requirement for proper growth. Onion varieties are classified based on the number of daylight hours needed to initiate the switch from growing leaves to forming a bulb. Short-day onions require 10 to 12 hours of daylight and are best suited for southern latitudes, while long-day varieties need 14 to 16 hours of daylight and are grown in northern regions.
Consistent moisture is necessary because onions have shallow root systems and compete poorly with weeds. The plants require about one inch of water per week, especially during the vegetative stage before the bulb begins to swell. Once the tops begin to yellow and fall over, which signals the completion of the bulbing process, watering should be stopped to allow the bulbs to cure and develop their protective outer skin.