Most people are familiar with onions (allium cepa), the edible bulb in the allium family. Their strong flavor makes them a favorite of chefs in various cuisines. Growing them in your home garden takes some patience, but you'll be rewarded with onions that taste and smell better than the ones you buy at the grocery store.
How Onions Grow
Early in the season, onions send up tubular, hollow leaves, before beginning to form bulbs. Most onions are biennial, so you will seldom see an onion flower. Multiplier onions or Egyptian walking onions are a perennial variety that sends up a flower stalk with a clump of tiny bulbs or bulbils on top. When the top becomes too heavy to stand, it falls over. The bulbils take root, forming new plants and giving the perennial onion its description of walking.
Onions are actually biennials, but they never make it to their second year unless you plan to save seed. They are grown as annuals.
Both the size and shape of onions vary by type and growing season. Bulbing onions are greatly affected by day length, as described below. The bulb size is related to the size and number of the leaves. Each leaf translates to a ring of onion. Larger leaves make larger rings.
So choosing the right type of onion for your day length will give your onion tops time to form before the onion bulb begins developing: more leaves, more bulb.
Onions need a spot in full sun to develop properly.
The time required for the bulbs to mature depends on the variety and whether they were started from seed. You can harvest onions at any stage. The plants you thin from a row can be used as green onions.
However, onion bulbs are ready when about half the tops have fallen over and the bulbs’ skins have a papery feel. Bulbs allowed to remain in the ground until 50 percent or more of the green tops have fallen over will store longer.
Once you see half the tops are down, very gently coax the remaining leaves down, without breaking them off the bulb. Then allow the bulbs to sit in the ground and cure for a couple of days before you lift them. You’ll have better luck digging the onion bulbs, rather than pulling. You don’t have to go deep, just enough to loosen the remaining roots.
Shake and brush off any loose soil and let the bulbs finish curing in a warm, dry place with good air circulation. Leave the leaves on. You can use fresh onions at any time now.
For storing onions, wait until the outside onion skins dry and the neck —where the leaves meet the bulbs— starts to shrivel. Then you can store them in a cool, dry location, like your basement. Onions keep longer in cool temperatures (under 40 degrees) but should not be allowed to freeze.
Store onions in mesh type bags or by braiding the tops together and hanging. Just make sure they are not piled on top of each other and not getting any air.
If you are buying transplants or sets, you will find types suitable to your area in any good nursery, although they will sometimes only be labeled as yellow or red.
Onions are categorized according to day length: how much daylight there is when onions stop forming tops or new green leaves and start making bulbs.
Short-day onions begin forming bulbs when the day length is 10 to 12 hours. These varieties tend to do better in the southern and western U.S:
- Granex: Large, globe-shaped, sweet onions sometimes called the Vidalia onion. Early producer.
- Texas Grano 1015Y: An improved strain of 'Yellow Grano' with treat disease resistance and good storing ability.
- Cipollini: Flat, doughnut-shaped onions. Store up to 5 months.
Long-day onions begin forming bulbs when the day length is 14 to 16 hours. They tend to do better in northern states.
- Italian Red Torpedo: Heirloom onion. Reddish-purple with an elongated in shape. It can also be grown as a short day onion.
- Redwing: Red color holds up well. Nice solid bulb. Late season onion, so best from transplants.
- White or Yellow Sweet Spanish and other Spanish types: Long-season varieties. Includes ‘Walla Walla’ and the earlier Olympic (F1).
Choose a site with at least six hours of direct sunlight.
Onions can be started three ways: by sets (tiny bulbs), transplants and direct seeding. Though planting onion sets are the most popular way to grow them, you'll have better results transplanting seedlings you start indoors ahead of time.
Starting Onions from Sets
Onion sets should be about the size of a marble. Larger sets don’t always adjust well and could bolt or split. For similar reasons, don’t buy sets that have already sprouted. And as with all bulbs, onion sets should be firm and healthy-looking.
Sets can be planted early in the season, before the last frost, but after the soil has dried and warmed a bit. Plant onion sets pointed end up and cover with about 2 inches of soil. Depending on the mature size of your variety of onion, space about 3 to 4 inches apart.
Transplants generally result in larger onions than sets. You can buy transplants or start your own indoors from seed. Start onion seed about 12 weeks before your transplant date. You can plant thickly and thin at transplant time. Keep the soil moist. As the tops grow, keep them trimmed to about 4 inches.
Transplants or onion seedlings will need to be hardened off before planting outdoors. Wait until all danger of frost has passed before planting.
Don’t bury transplants too deeply. Plant them right on the surface of the soil, spaced about 4 inches apart. Keep onions well watered throughout the season. The bulbs need regular water to swell in size. Transplant grown onions are the type you see pushing up out of the ground.
Direct seeding onions require a longer season, so warmer climates will have better luck. To direct seed, follow the directions above for seeding transplants.
Pests and Problems
- Rot: In damp soils, you may encounter neck or stem rot or bulb rot. Avoid with good soil drainage and air circulation.
- Splitting: Bulbs will split or double if the soil is allowed to remain dry while the bulbs are forming.
- Thrips: Small yellowish-brown flying insects that feed on leaves and can cause twisting and curling. Repeated attacks cause the plant to stop growing, so bulbs don’t mature. Plant resistant varieties and don’t plant near grain crops. Neem and insecticidal soaps provide temporary control.
- Onion root maggots: Eggs are laid near the base of onion plants. The larvae hatch and burrow into the stems, feeding on the plants below the soil and eventually killing the plants. Rotate plants yearly to avoid infestation. Covering new seedlings will prevent eggs from being laid. Diatomaceous earth is also effective.