Onion sets offer a distinct advantage over planting from seed because they bypass the lengthy germination period and mature much faster. Utilizing a raised bed provides an ideal environment for these plants, offering superior control over soil conditions. The elevated structure naturally ensures excellent drainage, which is required for healthy bulb development, and allows the soil to warm up earlier in the spring. This combination creates an optimized system for a reliable onion harvest.
Preparing the Raised Bed and Onion Sets
Successful bulb production begins with selecting the appropriate planting time and location. Onions require a minimum of six to eight hours of direct sunlight each day, so the raised bed should be situated in a full-sun area to promote optimal growth and bulb size. The timing for planting onion sets is typically in early spring, as soon as the soil becomes workable, though some regions also support a fall planting for an earlier spring crop. Spring planting must occur early enough to allow the foliage to develop before the long daylight hours trigger bulbing.
The soil medium in the raised bed is of concern because onions are shallow-rooted and dislike competition. A depth of at least 15 inches is recommended to allow for root development and proper drainage. The ideal growing medium is loose, well-draining, and rich in organic matter, such as a mix of compost, well-aged manure, and a sandy loam or silt-loam soil. This composition ensures the bulbs can easily expand without resistance.
Onions thrive in a slightly acidic to neutral pH range, ideally between 6.0 and 6.8, which should be confirmed through a soil test before planting. Before planting, the onion sets should be firm to the touch. Any sets that are soft, moldy, or excessively large (over one inch in diameter) should be discarded or planted separately, as they are more likely to bolt.
Step-by-Step Planting Process
The mechanics of planting onion sets involve precise attention to depth and spacing to ensure healthy bulb formation. After preparing the soil, each set must be placed in the ground with the root plate facing down and the pointed tip facing up. Planting depth should be shallow, just enough to cover the bulb’s base, typically about one to two inches deep. It is important to leave the neck of the bulb, where the leaves emerge, exposed above the soil line to prevent rot and allow the bulb to expand freely.
The spacing between individual onion sets directly impacts the final bulb size. For mature, full-sized bulbs, space the sets approximately four to six inches apart. If the sets are planted closer, such as two to three inches apart, the resulting bulbs will be smaller; this tighter spacing is sometimes used for a continuous harvest of green onions or smaller bulbs. Once the sets are placed, gently firm the soil around each one to establish good contact between the roots and the medium. A thorough initial watering will help settle the soil further and remove any air pockets that could impede root growth.
Essential Care for Growing Onions
After planting, consistent moisture and nutrient management are necessary to encourage large bulb growth. Onions have a shallow root system and are not efficient at drawing up water, requiring about one inch of water per week through rain or irrigation. The raised bed soil should be kept consistently moist but never waterlogged; excessive moisture can lead to root diseases. Using a drip system or soaker hose is often effective in a raised bed, delivering water directly to the root zone without wetting the foliage, which helps prevent fungal issues.
Weeding must be a priority because onions cannot compete well for moisture and nutrients. The shallow roots are easily damaged, so hand-weeding is preferable to deep cultivation near the plants. Fertilization should be managed carefully, beginning with a balanced, phosphorus-heavy fertilizer at planting to promote strong root development, such as a 10-20-10 product. Once the foliage is actively growing, apply a nitrogen-rich fertilizer every few weeks until bulbing begins. Nitrogen applications must cease once the bulbs start to swell, as continued nitrogen encourages foliage growth at the expense of bulb sizing and reduces storage potential.
Recognizing When to Harvest
Maturation is signaled by the plant’s foliage, providing visual cues for when the bulbs are ready. The most reliable sign is when the tops begin to yellow and naturally flop over at the neck. This indicates the plant has stopped growth and is transferring stored carbohydrates from the leaves down into the bulb. Wait until approximately two-thirds of the tops have fallen over before beginning the harvest.
Once the tops have fallen, the neck of the onion will begin to soften and dry. The bulbs should then be carefully lifted from the soil using a garden fork to avoid bruising. After removal, the onions must be cured for storage by allowing them to dry in a warm, dry, and well-ventilated area for about one to two weeks. Curing is complete when the necks are completely dry and the outer skins are papery, which seals the bulb and extends its storage life.