How To: Grow Onions
You really need to "know your onions," as the saying goes, to avoid planting the wrong ones for your zone at the wrong time of year.
When you’re learning how to grow onions, keep in mind that they can be started from seed, purchased seedlings, or sets (bulblets). While sets progress fastest, they unfortunately also bolt most easily and generally make smaller onions than seedlings do.
Gardeners in zones up to and including USDA Zone 6 should grow long-day types, which require 14 to 16 hours of light per day, and plant them early enough in spring so they mature during summer. Southern gardeners should opt for short-day onions, which need only 10 to 12 hours of daylight, planting them in autumn or winter so they mature before summer.
BEFORE YOU BEGIN
If you prefer to start your own seedlings, sow them indoors about 10 weeks before your last spring frost date, hardening them off before setting them out six weeks later. In parts of the South where frost is rare, you can sow onion seeds directly into the garden in mid-autumn.
STEP 1: Site selection considerations.
Onions grown for their bulbs require a location that receives more than six hours of sunlight per day and has fertile, fast-draining soil. That loam should have a pH between 6 and 6.8. Green or bunching onions, raised for their stems rather than their bulbs, can be planted in partial shade, if necessary, where they receive at least three to six hours of sun per day. If your ground contains a lot of clay and tends to be soggy in spring, grow your onions in a fast-draining raised garden bed to avoid the problems caused by working heavy soil when it is wet.
STEP 2: Soil preparation.
Sometimes you have no choice but to grow onions in clay soil without a raised bed or in soil too sandy to retain moisture well. If that’s the case, spread 3 inches of compost over the ground in autumn (for long-day onions) or in spring (for short-day onions), and till it into the top 6 inches of soil. Compost helps make heavy soils lighter and light soils more water retentive. Alternatively, sow a fall cover crop, such as winter rye, mowing it off and tilling it under in spring at least two to three weeks before you plant your onions.
STEP 3: Planting.
Another important consideration when learning how to grow onions is when to plant them outside. Plant onion sets or transplant seedlings a month before your last spring frost. Simply push sets into the loose soil with your fingers, 4 inches apart, making sure that the root ends are down and the pointy tips lie just beneath the surface of the soil. Those destined to be harvested as green onions can be planted 2 inches apart.
For seedlings, trim the tops back to 3 or 4 inches before using a trowel to set each little plant about 1 inch deep in the ground.
STEP 4: Cultivation and fertilizing.
Fertilize the soil with ½ cup of bonemeal or superphosphate per 10 feet of row, placing it 2 to 3 inches beneath the sets or seedlings when you plant them. About three weeks after planting and once a month thereafter, water the onions with fish emulsion, mixing 2 tablespoons into 1 gallon of water. When the plants begin to make bulbs, you should stop fertilizing.
If you mulch your onions, keep the mulch off the tops of the bulbs; they mature best when exposed to the sun. A better choice would be to simply weed the onions by hand.
STEP 5: Watering.
Make sure the onion patch receives at least 1 inch of water per week, either via rainfall or irrigation with a soaker hose laid down beside the rows of seedlings or sets. Because onions have shallow roots, if the soil feels dry down to 1 inch beneath the surface, it is time to water.
STEP 6: Harvesting.
A couple of weeks before harvest time, when the onions’ bulbs have fully developed, bend their tops over—being careful not to break them—to encourage those bulbs to “ripen.” Once their greenery has browned and shriveled, the onions are ready for harvest.
Pull them up and place them in a dry, airy location out of direct sun for 7 to 10 days. If you prefer, you can leave them lying in the garden for a few days before you gather them up, positioning them so that the foliage of one covers the bulb of the next to protect the bulbs from sunburn.
STEP 7: Storing.
Brush any dirt off your onions and pluck off their dried tops and roots before storing them in mesh bags. They keep best in a dry location where the temperature remains cool but above freezing, preferably at about 45 degrees Fahrenheit, such as in a root cellar or an unheated garage. Onions should not be refrigerated.
When selecting commercial seedlings or sets, opt for seedlings whose green tops are pencil-sized in width or sets no more than ¾ inch in diameter. Smaller seedlings may freeze too easily, while larger sets will bolt too easily.
Note that onions prefer cool weather while growing their leaves and warmer weather when they begin to “bulb up,” but they don’t like temperatures above 85 degrees. Onions grow slowly, usually taking 90 to 110 days to mature. If you follow the above instructions, your onions’ bulbs should begin to develop around the summer solstice (June 20, 21, or 22) in the North, or by March or April in the South.