How to Grow Onions

Start with day length, not the seed rack

An onion grows leaves until the days reach a certain length, then switches to building a bulb. Every leaf it has made by then becomes one ring of that bulb, which is why Texas A&M and Nebraska Extension both stress leaf growth before bulbing. Iowa State puts it simply: the bigger the top when bulbing begins, the bigger the bulb. If you plant a variety whose trigger comes too early for your latitude, it bulbs while the plant is small. If the trigger never arrives, the plant just keeps making leaves.

Varieties fall into three groups. The exact cutoffs vary a little by source:

• Short-day onions start bulbing at about 10 to 12 hours of daylight (Iowa State; Texas A&M says 11 to 12, Tennessee 10 to 13).
• Intermediate-day onions respond to roughly 12 to 14 hours (Iowa State; Texas A&M says 12 to 13).
• Long-day onions need 14 hours or more. Texas A&M puts it at 14 to 16, while Maryland Extension and Virginia Tech give 15 to 16.

Summer days get longer the farther north you are, so latitude decides which group will work for you. University of Tennessee Extension says short-day onions are typically grown south of 35° N. A University of Maryland Extension article puts the line at about 36° N, with long-day types doing best north of it. Roughly, that band crosses Tennessee, central North Carolina, northern Arkansas, central Oklahoma and northern Arizona.

The North (the upper Midwest, the Northeast, the northern Plains and the Northwest): grow long-day onions. Iowa State calls them the best choice for Iowa and other upper Midwest states and notes that short-day varieties planted there give small bulbs, because there are few leaves when bulbing starts. Tennessee Extension notes that long-day onions are typically the harder storage types.

The transition zone (the Mid-Atlantic, Virginia, Kentucky, Tennessee): intermediate types are the safest bet. Tennessee Extension recommends intermediate-day onions for dry bulbs in Tennessee and suggests Candy, Super Star, Zoey and the long/intermediate Cabernet. For Maryland, the Maryland Grows article names Sierra Blanca, Superstar and Candy as the most reliable choices. Maryland Extension adds that long-day types can also do well there. Virginia Tech notes that this middle band is not a major onion-producing area and that yields there run lower than in the North or the Deep South.

The Deep South and Gulf Coast: grow short-day onions, planted in fall. Clemson says short-day varieties suit all of South Carolina, intermediate types are worth trying in the Upstate, and long-day onions cannot be grown well anywhere in the South. Clemson’s list for South Carolina gardens is Candy, Granex (white), Stockton Sweet Red, Texas Early Grano and Yellow Granex. UF/IFAS recommends Granex for Florida. It is the same cultivar sold as Vidalia when grown in that region of Georgia.

Texas spans all three bands. Texas A&M lists short-day choices such as TX 1015Y, Granex, Texas Early White and Red Burgandy, intermediate types such as Caballero, Cimarron and Sierra Blanca, and long-day types such as Armada, Sterling and Tango. Pick the group that matches your part of the state.

The Intermountain West: Utah State recommends Utah Sweet Spanish, Candy, Walla Walla, Cabernet (red) and Sweet Sandwich for Utah. It says they keep about 2 to 4 months.

This choice also decides how long the onions will keep. Tennessee Extension points out that short-day and intermediate onions are milder and sweeter but do not store as well, and Clemson says the same of short-day types. If you want onions in the pantry until spring, northern long-day storage types are the ones to grow.

Sets, transplants or seed

Sets are small dormant bulbs grown the previous summer. Utah State calls them the easiest to handle. The catch is that an onion is a biennial, and a set has already lived through one short season. Nebraska Extension notes that plants from sets often send up flower stalks in midsummer, and once they do the bulb stops growing. Buy small sets. Vermont and Montana Extension suggest sets under 3/4 inch, Utah State says sets over 5/8 inch bolt more, and Clemson’s limit is 1 inch. Vermont’s garden-planting guide suggests sorting the bag. Plant sets smaller than a dime for storage bulbs, and plant the larger ones close together for green onions. It also notes that torpedo-shaped sets tend to make round onions and round sets tend to make flat ones.

Transplants are bundles of young seedlings sold in late winter and spring. Maryland Extension says they give the best results for most Maryland gardeners. Nebraska Extension prefers them over sets for storage onions because seedlings do not bloom in their first year. Clemson’s ideal transplant is about 6 inches tall and half as thick as a pencil. Clemson also advises keeping the bundles dry: don’t soak the roots or heel them into soil. Store them somewhere cool and dry, and they will hold for up to three weeks.

Seed costs the least but takes the longest (Texas A&M). In the North it is usually started indoors. Maryland Extension and Utah State allow about 8 weeks before setting out, the Maryland Grows video 8 to 12, and Nebraska 10 to 12. Nebraska suggests 12 to 14 hours of supplemental light a day with the lights a few inches above the seedlings. Maryland says it is fine to snip the tops if they get too long. Direct sowing works too: sow 1/4 to 1/2 inch deep as soon as the soil can be worked, sow thickly, and eat the thinnings. In South Carolina, Clemson recommends fall direct seeding only in the Coastal and Central regions, not the Piedmont.

When to plant

In cold-winter areas, onions are among the first crops of spring. Transplants and sets go out a few weeks before the last frost. Sources differ slightly here: Vermont and Montana Extension say 2 to 4 weeks ahead, while Vermont’s planting guide and the Rhode Island calendar put sets in 4 to 6 weeks ahead. Use the dates for your zone above. Nebraska’s rule of thumb is to plant once the soil is dry enough to work and daytime highs reach about 50°F. Transplants handle light frost.

Timing matters in both directions. Plant too late and the long days arrive before the plants have much leaf, so the bulbs come out small. Utah State warns that late-planted onions may never bulb at all. Plant too early and a long cold spell can make the plants bolt (send up a flower stalk). In warm-winter regions the season is turned around, with planting in fall and harvest in late spring. When to plant onions covers timing region by region.

Soil, spacing and depth

Onions want full sun and loose, well-drained soil with plenty of organic matter. Clemson notes that clay soil tends to give hotter-tasting onions, while sandy soil needs more fertilizer and water. Recommended pH ranges overlap: Clemson and Virginia Tech say 6.2 to 6.5, Vermont 6.2 to 6.8, and Tennessee 6.0 to 7.0. Where native soil is heavy, Nebraska suggests raised beds 4 to 6 inches high.

Spacing sets the size of the bulb. Maryland Extension’s guideline is 2 inches apart for green onions, 4 inches for medium bulbs and 8 inches for large ones. Most sources settle on 3 to 4 inches in rows 12 to 18 inches apart. Wide rows also work, with plants about 4 inches apart each way. Utah State warns that crowded plantings make small bulbs unless you add extra water and fertilizer. Thin seeded rows before the plants start competing or bulbing.

Plant shallow. Sets go about 1 inch deep with the point up (Maryland, Vermont). Texas A&M plants sets and transplants 3/4 inch deep and says never more than 1 inch. Clemson sets transplants slightly deeper, 1 to 1-1/2 inches. A bulb planted too deep has to push against the soil, so stay on the shallow side.

Water: the main factor in bulb size

Onion roots are shallow, so the plants dry out quickly. Maryland Extension says bulb size is directly proportional to the water the crop gets over the season. Utah State says drought cuts yield, shrinks bulbs and changes flavor. Utah State says to water deeply enough to wet the top 12 inches of soil, and Montana State suggests 1 to 2 inches of water a week. Texas A&M finds that once a week is usually enough in a Texas spring, with more during dry, windy spells. Drip line run down the rows makes this easy, and it helps most in the arid West.

Moisture matters most once the bulbs begin to swell. Dry soil at that stage causes split and double bulbs (Virginia Tech, Clemson). At the end of the season the rule reverses. Utah State says to stop watering when the tops start to fall over, and Clemson stops a week before harvest. Wet soil at maturity delays curing and leads to rot in storage.

Feeding: early and often, then stop

Onions are heavy feeders. Without a soil test, Texas A&M mixes 2 to 3 pounds of 10-10-10 per 100 square feet into the top 3 to 4 inches of soil. When the plants have 5 to 6 leaves, it adds about 1/2 cup per 10 feet of row, scattered between rows and watered in. Utah State side-dresses with 1/2 pound of 21-0-0 per 100 square feet in mid-May and again in late June. Maryland and Virginia Tech time a side-dressing for 1 to 2 weeks after the bulbs begin to enlarge. In Florida’s sandy soils, UF/IFAS feeds with nitrogen monthly.

Stopping on time matters just as much. Utah State does not fertilize after mid-July, because late nitrogen pushes new leaves and hurts storage. Clemson adds that late nitrogen delays maturity and causes thick necks and doubled bulbs. In fall-planted Southern gardens, heavy nitrogen in fall makes plants more prone to winter kill.

Weeds, and why you should never hill onions

Onions have shallow roots and compete poorly with weeds. Utah State says the first two months matter most, when the onions are growing slowly. Pull weeds by hand or hoe very shallowly. Texas A&M suggests getting them while they are 3 to 4 inches tall. Clemson says to stop cultivating once onion roots reach into the space between rows, because cutting them stresses the plants and lets disease in. A mulch of straw, shredded leaves, grass clippings or compost holds moisture and smothers weeds.

Do not pull soil up around the bulbs. Maryland and Clemson both say hilling encourages stem rot. Bulbs naturally rise partly out of the ground as they swell. Clemson goes further: a month before harvest it gradually pulls soil away so that about a third of each bulb is exposed, which helps the necks dry down.

Common problems and fixes

• Seed stalks (bolting). The plant got a long cold spell after it was already growing well, and larger plants are more likely to bolt (Clemson). A Maryland Grows article adds that soil above 75°F can also trigger flowering before bulbing begins. Clemson says the result is small bulbs with thick necks that are hard to cure and generally unusable. Next time, use small sets, plant transplants about pencil-thin, plant at the right time for your area, and go easy on fall fertilizer.
• Small or no bulbs. Check, in order: the wrong day-length type, late planting, dry soil and crowding.
• Split or double bulbs. Usually caused by dry soil during bulbing or heavy late nitrogen.
• Thrips. These tiny insects hide where the leaves join the bulb. Feeding leaves silvery streaks, and badly hit leaves twist and die. Utah State recommends a hard spray of water, mulch and compost, and protecting the natural enemies that feed on thrips. Tennessee reserves insecticides for heavy infestations.
• Onion maggot. White larvae tunnel into roots and bulbs. Clemson notes the flies are drawn to decaying organic matter, so keep beds clean and don’t overdo fresh organic matter. Rotate beds and remove infested plants.
• Pink root and basal rot. These soil fungi turn roots pink or rot the base of the plant. Use long rotations, avoid overwatering, and solarize the bed (Utah State). Clemson’s rule is to grow onions in the same spot no more than once every four years.
• Neck rot in storage. Avoid late watering, handle bulbs gently and cure them thoroughly.

Regional notes

Cold North (zones 3-5). The season is short, so use transplants or indoor-started seed of long-day varieties and get them in as soon as the soil can be worked. Every early leaf counts. Utah State describes a second option for the Intermountain West: special overwintering varieties sown in mid-to-late August and mulched for winter, which mature the following June or July.

Mid-Atlantic and Upper South. Grow intermediate types in spring. Tennessee Extension says fall-transplanted onions are possible but have not been tested enough there to recommend.

Humid South and Gulf Coast. Grow short-day onions from fall to late spring in raised rows for drainage. Clemson recommends rows because they also make it easier to pull soil away from the bulbs at harvest. Expect sweet onions that don’t keep as long as northern storage types.

Arid West and low desert. Drip irrigation and mulch make the difference. In the Phoenix area, the University of Arizona calendar plants bulb onions in fall and winter. Seed takes about 7 to 8 months there, and sets 4 to 5.

Harvest and storage

Bulb onions are ready about 100 to 120 days from seed (Utah State). Maryland gives 85 to 120 days and UF/IFAS 100 to 130. Harvest once most of the tops have flopped over and the necks are softening. Lift the bulbs, cure them in a shady, airy spot until the necks are dry and the skins papery, then trim the tops and store them cool and dry. When to harvest onions covers the signs, curing and storage temperatures in detail.