A quick guide to onions
- You can grow onions from seed, transplants or small bulbs called "sets."
- Plant onions early in spring.
- Soil for onions should be well-drained and high in organic matter.
- Onions are shallow-rooted and require constant moisture for proper growth.
- Harvest onions when about half the tops are falling over and dry.
- Curing is essential if you want to store onions. You must dry them in a warm, dry, well-ventilated area.
Long-day and short-day onions
Onions (Allium cepa) have relatives that include garlic, chives, leeks and shallots.
Storage onions grown in Minnesota generally are long-day types that require 14 or more hours of daylight to form bulbs. All onions require full sun for the best growth. Overcast skies and cool temperatures during the growing season will delay bulb formation.
Sweet or mild onions are short-day onions. Although you can grow them in Minnesota, they will generally develop small bulbs. Bunching onions, including scallions and Egyptian walking onions, have green stalks.
Soil testing and fertilizer
- Have your soil tested.
- Onions grow best in well-drained soil with a pH between 6.0 and 7.0 and high organic matter.
- Apply phosphorus (P) and potassium (K) according to soil test recommendations. Many Minnesota soils have enough phosphorus.
- Unless your soil test report specifically recommends additional phosphorus, use a low- or no-phosphorus fertilizer.
- You can increase your soil’s organic matter content by adding well-rotted manure or compost in spring or fall. Do not use fresh manure as it may contain harmful bacteria and may increase weed problems.
- Onions require a good supply of available nitrogen, but too much nitrogen can result in late maturity, large necks that are difficult to cure, soft bulbs, green flesh and poor storage quality.
- Side-dress with fertilizer after root systems are well-developed.
- Do this once or twice during the growing season, with urea (45-0-0) at a rate of 0.25 to 0.5 pounds per 25 feet of row.
- Spread the fertilizer alongside the row of onions, about six inches away from the plants, and scratch it into the soil.
- Do not use “Weed and Feed” type fertilizers on vegetables. They contain weed killers that will kill vegetable plants.
Planting
Direct seed as soon as the soil is workable in the spring.
- Sow seeds in a 2-inch wide band
- 1/4 to 1/2 inch deep
- in rows 12 to 18 inches apart.
After seedlings emerge, thin to 3 to 4 inches apart.
Transplanting
Some seed companies sell onion transplants. They can tolerate light frosts, and you can plant them when temperatures reach 50° F.
You can also raise your own transplants by starting seed indoors 10 to 12 weeks before planting outside.
Start seeds indoors in February
- Sow seeds 3/4 of an inch deep.
- Keep evenly moist.
- Loosen plants when ready to transplant.
Transplant in May
- Trim roots to 1/2 inch.
- Trim tops to 4 inches long.
- Plant 2 inches deep, 3 to 4 inches apart, in rows 12 to 16 inches apart.
You can also plant onions from sets, which are small bulbs grown the previous year. Most of the sets available in Minnesota are of the short-day type. If using sets, plant them as soon as the soil is workable in the spring.
How to keep your onion plants healthy and productive
- Onions are shallow-rooted and require constant moisture for good growth.
- If the planting does not receive one inch of rain each week, soak the soil thoroughly at least once a week.
- An inch of water will wet a sandy soil to a depth of 10 inches, a heavy clay soil to 6 inches.
- If your soil is sandy, it is important to water more often than once a week.
- Use a trowel to see how far down the soil is wet. If it is only an inch or two, keep the water running.
- Stop watering when bulbs have reached full size and the tops begin to fall.
- Using a hoe or hand tool, make a shallow cut to kill weeds just below the soil level before they become a problem.
- Do not hoe or chop too deeply. Injured onion bulbs are more prone to disease.
- Mulch with herbicide-free grass clippings, weed-free straw, or other organic material to a depth of 3 to 4 inches to help prevent weed growth and reduce the need for frequent cultivation.
- Harvest onions when about half the tops are falling over and dry. Undercut and lift bulbs with a spading fork.
- You can leave onions on the ground for several days if the weather will be dry and warm, or bring them indoors to cure.
- Curing is essential if you plan to store your onions.
- Keep the onions in a warm (75°F - 90°F), well-ventilated area for two to four weeks, until outer bulb scales are dry and the neck is tight. Poor curing will result in decay during storage.
- When properly dry, you can braid the onions or cut the tops off.
- Store onions in a cool, dry area. Do not let them freeze. They will start to sprout if kept above 40° F.
Managing pests and diseases
Many things can affect onion bulbs and leaves. Changes in physical appearance and plant health can be caused by the environment, plant diseases, insects and wildlife. In order to address what you’re seeing, it is important to make a correct diagnosis.
You can find additional help identifying common pest problems by sending a sample to the UMN Plant Disease Diagnostic Clinic. You can also Ask a Master Gardener and share pictures to get input.
- Onion maggot bores into plant stems, causing the plants to turn yellow and wilt.
Many of the same cultural practices help prevent a wide variety of onion diseases.
- Several kinds of rot can infect onions, including Fusarium basal rot, Botrytis neck rot and bacterial soft rot.
- Resistant varieties are available for Fusarium basal rot.
- Harvest onions on dry, cool days to limit diseases post-harvest.
Reviewed in 2024