How to Plant Onions: A Growing Guide

How to Grow and Care for Onions

The fundamental considerations and subtle nuances of growing and caring for onions are beyond the scope of this introduction to plant care. Please see our Resource Center where we offer additional instructional videos and articles, as well as our bookstore.

Soil Preparation to Grow Onions

Onions prefer loose, well-drained soils that are high in fertility, slightly acidic, adequately irrigated, and exposed to full sunlight. The looser the composition of your soil, the larger your onion bulbs will grow. Prepare your bed by turning under animal manure or plant residue-based compost. Please make sure that this material is fully broken down before planting.

Compost composed of cedar or redwood is not an acceptable substitute for high-quality compost.

The potential for fungus 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. The longer the break, the better. As gophers are a major pest in onion beds, use gopher traps, wire barriers, or wire baskets prior to and during planting.

Growing Onions from Seed

Onions are easy to grow from seed and can be started about 10–12 weeks before you are ready to transplant them outside. If you live in very cold winter regions, start seeds indoors in February or plant directly in the garden in warm winter regions. Use a good-quality seed starting mix and sow about 1/2” deep. Onion seeds can take up to a week to germinate and are slow growing. Keep the soil moist and, once seeds have germinated, supply them with a grow light or place in a sunny window.

When seedlings are about 2” tall, feed them with a very dilute solution of fish and kelp fertilizer (follow label directions). Once you are ready to transplant them out in the garden (daytime temps at least 50°F), trim the tops to about 4” and plant 2” deep, 4–6” apart. The soil should be loose and have a good supply of phosphorus. Keep the bed well weeded, as onions do not like competition from weeds or other plants.

Planting Onion Transplants

Onion transplants can be grown up from seed or purchased as plants—typically available in fall and spring. Fall is the optimum time to plant onions in mild climates. Onion transplants are often wilted when they arrive. 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 the onions immediately, remove the transplants from the plastic bag and spread them out in a cool, dry location until ready for the garden. The sooner you plant, the better!

Before planting, trim onion transplant tops to approximately 3” and roots to 1/4”. Roots will begin to grow rapidly once planted. Plant onion transplants 1–2 inches deep (to the top of the white section) and 4–6 inches apart. Plant as close as 3” apart if smaller onions are desired.

Rows should be 18”–24” apart, or 12” apart if planting for commercial production. If planting on raised beds approximately 20” wide, plant in double rows 4” from each edge.

“Scatter planting” among vegetables in interplanted gardens is sometimes used to ward off pests. Onions must not face heavy competition from surrounding plants.

Planting Onion Sets

Onions are easy to grow from sets. 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 suppresses weeds, maintains soil moisture, and protects onion bulbs from “heaving” (working 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 does not provide the 1” of water per week required to keep bulbs from splitting or tasting bitter at harvest. Water consistently until harvest!

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

Harvesting & Storage

Onions are ready to harvest when their tops have yellowed and begun to fall over. Finish bending the tops horizontal to the ground by hand or with a rake. This stops 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 burning in full sun. When the outer skins have dried, complete harvesting by clipping the roots. Wipe off soil and cut tops back to 1” above the bulb.

Onions keep best when stored cool and dry. Pungent onion varieties, which have low water content, keep longer than sweeter, moister types. Hang onions in a mesh bag in a ventilated area. Wrap individual bulbs in foil for refrigeration storage up to a year.

Enjoy onions on your table and planting onion bulbs in your garden. We have many onion sets for sale and onion seeds for sale.

Check out our Garlic Resource Center for more information about garlic and the varieties we offer. Or use our Garlic Selector Tool for a customized garlic list matched to your climate, gardening style, and flavor preferences.

Looking for more onion-specific guidance? Don’t miss our article on Tips for Growing Large, Healthy Onions, where we cover soil prep, watering, spacing, and fertilization for bigger harvests.