How to Grow Onions

Selecting the right variety for your growing region is important to the success of growing onions. There are short-day, intermediate-day and long-day varieties. Choose the one best suited for your area. For more information on growing zones and the best fertilizer for onions, check out our blog How to Select the Best Onions and Leeks.

Soil Preparation and Organic Fertilizer

Onions prefer loose, well-drained soils that are high in fertility, slightly acidic (pH between 6.2-6.8), adequately irrigated, and in full sun. The looser the composition of your soil, the larger your onion bulbs will grow. Prepare your bed by turning under animal manure or compost. Make sure that it is fully broken down before planting. Compost composed of cedar or redwood is not an acceptable substitute for high quality compost.

Onions are heavy feeders, so make sure to provide plant food with plenty of nitrogen, phosphorus and ammonium sulfate. A good rule of thumb is to add one cup of equal parts blood meal and bone meal granule fertilizer every 10 feet of row. Be sure to use a quality organic fertilizer to improve soil ph for your onion crop.

Planting & Growing Onion Transplants

The potential for fungal diseases like downy mildew and pink root can be greatly reduced by avoiding beds where onions, garlic and other alliums have been grown within the last two years. This time period is a basic rule of thumb but, in general, "the longer the better."

As gophers are a major pest in onion beds, use gopher traps, wire barriers or wire baskets prior to planting.

Onion transplants can be grown in the fall or spring (planting time depends on your growing region). Onion transplants are often wilted when they arrive, but like other members of the hardy lily family, they will survive for about 2-3 weeks after being pulled from the soil. If you cannot plant them immediately upon receipt, either refrigerate them after soaking the roots in water, or mound soil around the roots and keep them moist until planted.

Before planting, trim the tops to approximately 3” and roots to ¼”. The roots will begin to grow rapidly once planted. Plant onion transplants 1-2” deep and 4-6” apart. Plant close as 3” apart if smaller onions are desired. Rows should be 18-24” apart or 12” apart if planting for commercial production.

If planted on raised beds which are approximately 20” wide, transplants should be planted in double-rows 2-4” from each edge. “Scatter planting” among vegetables in inter-planted gardens is sometimes utilized to ward off a variety of pests, but onions must not be forced to face heavy competition from surrounding neighbors.

Apply a layer of mulch such as straw, to help maintain moisture and protect the plants during the winter. Onions are hardy to 20°F, but in cold climate regions, protect your plants with a thicker layer of mulch (at least 2" deep).

Onion Sets

Onions are easy to grow from sets. Keep in mind though, onion sets perform the best in long day growing regions. Plant 1” deep and 1-3” apart. Harvest young plants for use as scallions, thinning to 3-4” spacing. Onions should be mulched and supplied with ample phosphorus while growing.

Mulch deeply (up to 8”) in cold winter areas but only lightly in milder climates. Mulching will suppress weeds, maintain soil moisture and protect bulbs from “heaving” (working their way out of the soil) during extreme temperature cycles.

Weed suppression is critical for onions; you can grow weeds or onions, but not both. Regular irrigation is necessary anytime rainfall is not sufficient to provide the 1” of water per week required to keep bulbs from splitting in hot dry soil or tasting bitter at harvest. Water up until the time you harvest!

Beds kept weed free and properly irrigated will require little additional care.

Harvesting & Storing

Onions are mature and ready to harvest when their tops have yellowed and begin to fall over. Finish bending the tops horizontal to the ground by hand or with a rake for those that have not completely fallen over. This bending will stop the sap from diverting energy into the leaves while the bulb matures.

Harvest bulbs after the tops have turned brown. Place the tops of one row over the bulbs of another to keep them from becoming sunburned. When the outer skins have dried (curing should be between 10-14 days), complete harvesting by clipping the roots, wiping off any remaining soil, and cutting the tops back to 1” above the bulb.

Onions keep best when kept separated; individual foil wrapped specimens can last up to a year under refrigeration. Pungent onion varieties, which have low water content, will keep longer than sweeter, moister types. Hanging an onion in a mesh bag, in a cool, dry, well-ventilated location and tied off to separate onions from each other, is the recommended method of preserving onion bulbs for maximum shelf life.

Plant an onion this fall or spring for enjoyment of a fresh, home-grown onion next summer!