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Growing Onions

Growing Onions

Aisla Craig Exhibition Okra

Aisla Craig Exhibition Okra item #02721


There are both bulbing and bunching types of onions.

Bunching onions are hardy onions that are harvested as green onions or scallions. They are sometimes called scallions or spring onions. There are two different species of onions harvested as bunching onions, Allium fistulosum and Allium cepa. There are also hybrid varieties between the two types. Allium fistulosum varieties have a more pungent flavor and do not form bulbs, while cepa and hybrid types are milder and may form small bulbs.

Bulbing onions (Allium cepa var. cepa) begin producing bulbs in response to the length of the day, so it is important to choose varieties with the proper day length response for your area to ensure good yields. Planting varieties with the wrong day length will result in greatly reduced yields and overly small bulbs. Long day types begin bulbing when days are 14 to 16 hours long, and they are the best choice for northern areas (between 37 and 47 degrees latitude). Short day types produce bulbs when days are 10 to 12 hours long. They are the best choice for southern regions, where they are planted in winter or very early spring. Intermediate day varieties are the most adaptable types. They begin bulbing when days are 12 to 14 hours long and will generally produce acceptable sized bulbs in all areas except the far southern US. They are the best types for growers in areas between 32 and 42 degrees latitude.

How to Plant

Bunching onions can be direct sows outdoors after the last spring frost. Plant seeds 1/4 to 1/2 inch deep and about an inch apart. As the plants grow, thin to 8 to 12 inches between plants. Plants that are removed during thinning can be used as green onions. For an extended harvest, sow every 2 to 3 weeks through the summer. Most varieties of bunching onions are hardy enough to overwinter their first season, forming a clump in their second year.

Bulbing onions can be grown from seeds, sets, or plants. All can be planted as soon as the soil can be worked, which is usually 4 to 6 weeks before the last frost in spring. Onion seed can also be started indoors 6 to 8 weeks before outdoor planting time. Space sets and plants about 3 to 4 inches apart in rows, with 18 to 24 inches between rows. If your goal is the largest sized onions, use 6 inch spacing in the rows.


Vigorous growth will produce the largest sized bulbs. Onions are shallow rooted and do not compete well with weeds. Cultivate carefully and shallowly to avoid damaging the onion roots when weeding. Applying an inch or two of organic mulch, like chopped leaves or straw, can help maintain soil moisture and reduce weed problems. Onions benefit from more regular, light fertilizing about every 2 weeks. Harvest onions after the leaves lodge (fall over) and begin to die back. Dig the onions and allow them to cure for 10 to 14 days at 70 to 80 degrees. Store at 50 to 60 degrees in a dark, well ventilated area.

Fertilizer Recommendations

Onions benefit from regular fertilization. Side dressing or broadcasting with a granular fertilizer like Jung’s Jump Start Onion Fertilizer 10-20-10 at planting helps get plants off to a strong start. A balanced liquid fertilizer like AlgoPlus All Purpose 6-6-6 is also effective. When growth begins, fertilize every 2 weeks until mid-summer.

Common Problems

Onions are generally free from pests and disease problems. Onion maggot is sometimes a pest. This small fly lays eggs at the base of plants and maggots tunnel into the plant tissue. Onion maggots can be excluded from plants by using floating row covers or fleece tunnels over plants. This will prevent onion maggot flies from laying eggs on the plants. Neem oil can also be used. Thrips are small insects that sometimes feed on onion leaves, causing yellow to silver streaking of the leaves. Thrips can be controlled with insecticidal soap or neem oil.

Alternative Products

Shallots are a bulbing vegetable related to onions, but with a more elongated shape and milder flavor. Leeks are an onion relative that looks like a giant green onion. They are prized for their mild onion flavor.

Complimentary Products

Other vegetable root crops are potatoes and carrots.

Product Recommendations

Floating row covers can be used to protect onions from insect pests like onion maggot.

Onion Facts

Onions have been eaten by humans for at least 7,000 years. The largest onion according to the Guinness Book of World Records was grown by Tony Glover in the UK in 2014. It weighed 18 pounds 11.84 ounces!

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