Red onions (Allium cepa) are biennial bulbs that most gardeners grow as annuals, prized for their deep purple-red skin, mild bite, and long storage life. They can be started from tiny bulbs called onion sets, from nursery transplants, or from seed sown indoors eight to ten weeks before the last average frost date, and they belong to the same allium family as garlic, leeks, and shallots. Harvest timing decides how long that work pays off: knowing when to harvest onions, how to cure them, and how to store them separates a bin of crisp keepers from a bag of soft bulbs by midwinter.
Planting Windows: Day Length, Variety, and Frost Dates
Onions are photoperiodic: the bulb only starts to swell once day length crosses a threshold built into the variety. Plant the wrong day-length class for your latitude and you get tall tops with no bulb, or bulbs that size up early and stay small. The three classes are short-day, intermediate-day, and long-day, and the dividing line in North America sits near the 37th parallel, roughly the latitude of northern California, Tennessee, and Virginia.
The table below summarizes the three classes, the day length each one needs to bulb, and the storage life you can expect.
| Day-length class | Day length needed to bulb | Best growing regions | Typical storage life |
|---|---|---|---|
| Short-day | 10 to 12 hours | Southern states, zones 8 to 13 | 1 to 3 months |
| Intermediate-day | 12 to 14 hours | Middle latitudes, zones 6 and 7 | 3 to 5 months |
| Long-day | 14 to 16 hours | Northern states and Canada, zones 5 and colder | 6 to 12 months |
Choose the class by latitude, then set the planting date by frost. Sets and transplants go into the ground as soon as the soil can be worked, usually two to four weeks before the last average frost date. Seed follows a different schedule: start it indoors eight to ten weeks before that date so seedlings reach transplant size. In mild-winter regions, plant sets six to eight weeks before the first hard freeze so roots establish before dormancy. The full picture of when to plant onions by day length, variety type, and timing matters more than any calendar date, because a two-week swing in planting can shift the harvest by a month.
Matching the variety to your latitude
Read the catalog description instead of trusting the name. A sweet onion bred for Texas is a short-day type, while the same label sold in Michigan usually describes a long-day hybrid. Red onions are bred in all three classes, so confirm before ordering. Gardeners on a zone boundary should pick the longer-day class and plant early, because extra daylight triggers bulbing sooner.
Using frost dates as your anchor
Your state extension service publishes the last average frost date for your county; that date anchors both indoor sowing and outdoor planting. For direct-seeded rows in mild climates, wait until the soil reaches about 50°F (10°C); cold, wet seed beds rot more seed than they germinate.
Checking soil temperature before planting
A soil thermometer removes the guesswork. Onion seed germinates fastest near 68°F (20°C). If the bed is still cold, hold off a week rather than planting into mud.
Planting Methods: Sets, Transplants, and Seed
Three starting options exist, each trading off speed, cost, and bulb size. Sets are small dormant bulbs that sprout within days, the fastest and most beginner-friendly route. Transplants come from a nursery in bundles and give the most uniform crop. Seed costs the least and offers the widest variety selection, but adds eight to ten weeks of indoor growing before plants reach the garden.
- Sets: ready in about 90 to 110 days, lowest cost for a small patch, but more prone to bolting if stored warm before planting.
- Transplants: most uniform size, easiest to space, ready in 70 to 90 days from planting out.
- Seed: widest variety choice, lowest cost per plant, needs 8 to 10 weeks of indoor start plus 100 to 120 days in the ground.
Whatever the starting material, plant shallow. Sets go about 1 inch deep with the tip just below the surface; transplants sit slightly deeper at 1.5 inches. Space rows 12 to 18 inches apart and bulbs 4 to 6 inches apart, because crowding is the most common cause of small bulbs. The same bed can double as a scallion patch: pull young plants early, before the bulb forms. Scallions and green onions are simply onions harvested early, and the method for growing green onions is nearly identical with tighter spacing.
Spacing and depth rules
Follow the 4-6-12 rule: 4 to 6 inches between bulbs and 12 to 18 inches between rows. Onion roots are shallow and compete poorly, so tight rows mean smaller bulbs and more weeding. In a grid layout, give each bulb a full 6-inch square. Sets planted tip-up sprout fastest; an upside-down set still grows, just days later.
Planting in blocks vs. rows
Block planting, where bulbs are staggered like a checkerboard, fits the same number of plants into roughly two-thirds of the row length and shades the soil earlier, which suppresses weeds. Rows make weeding and harvesting easier. Choose blocks for small patches and rows for runs longer than 20 feet.
Growth Timelines and Seasonal Care
Onions are slow starters and fast finishers. A set reaches harvest in about 90 to 110 days, a transplant in 70 to 90 days, and a seed-started plant in 120 to 150 days counting the indoor start. The seed, set, and transplant timelines shift with variety and weather, but the pattern stays consistent: leaves grow through early spring, the bulb swells as days lengthen, and the tops flop over when the bulb is mature.
A week-by-week care calendar
- Weeks 1 to 4: keep soil evenly moist, watering deeply once or twice a week if rain falls short of 1 inch.
- Weeks 5 to 8: side-dress with nitrogen while leaves are building.
- Weeks 9 to 12: stop nitrogen once bulbs swell and stay on top of weeds.
- Weeks 13 and beyond: watch for tops that flop, the first sign the crop is ready.
Watering rules
Red onions grow best with about an inch of rainfall per week. When rain does not deliver, water deeply once or twice a week instead of sprinkling daily, because shallow watering keeps roots at the surface. Stop watering about two weeks before harvest; dry soil firms the skin and speeds curing.
Feeding and weed control
Onions feed lightly at first and heavily in the six weeks before bulbing. Apply a balanced fertilizer at planting, then side-dress with nitrogen once plants have four to six leaves. Weeds are the bigger problem: onions cannot outcompete them, and a weedy row can cut yield by half. Hand-pull or hoe shallowly, because the roots sit close to the surface.
Pest and disease troubleshooting
The usual suspects are onion thrips, onion maggots, and fungal diseases such as downy mildew and purple blotch. Thrips are tiny, pale insects that streak leaves with silver in hot, dry weather; onion maggots attack the base of the plant in cool, wet springs. Rotate onions on a three-year cycle, keep the bed free of allium debris, and pull any plant showing twisted, yellowing growth. A well-spaced, healthy planting resists most of these problems better than any spray.
Raised Beds, Containers, and Soil Preparation
Red onions want full sun, six to eight hours of direct light each day, and moist, well-drained soil with a neutral pH between 6.0 and 7.0. Heavy clay holds water and rots bulbs; sandy soil drains so fast that plants stress between waterings. Raised beds solve both problems by lifting the root zone above the grade. When you build permanent bed walls, the choice between red bricks and solid concrete blocks changes how the bed behaves for decades: brick warms quickly in spring and holds heat into the evening, while concrete blocks cost less and stack faster.
Building a raised bed for onions
Keep the bed narrow enough to reach the middle from either side, about 3 to 4 feet wide, filled with garden soil amended with compost. Eight to twelve inches of depth is plenty because onion roots stay shallow; morning sun dries the leaves early and lowers disease pressure.
Growing in containers
Containers work for a small harvest. Choose a pot at least 8 inches deep with drainage holes and give each bulb a 4-inch square of soil, so a 12-inch pot holds about six bulbs. Use potting mix rather than garden soil, which compacts, and water more often because pots dry out faster.
Expanding the Onion Patch: Irrigation, Fencing, and Hiring Help
Once the patch grows past a dozen rows, hand watering stops being practical. A drip line on a timer delivers the inch-per-week target without wetting the leaves, which also cuts fungal pressure. Fencing becomes part of the plan too: rabbits and groundhogs can strip a patch overnight, and a 3-foot welded-wire fence buried 6 inches stops most of them. For bigger jobs such as trenching a water line or erecting a deer fence, many homeowners hire a contractor. Before signing, learn the signs that a contractor is overcharging you, from inflated markups to vague change orders.
Drip irrigation basics
Lay drip tape along each row before or right after planting and run it 20 to 30 minutes per session, two or three times a week. A battery timer at the spigot automates the system for under $50.
Fencing and critter control
Welded-wire fencing 3 feet tall keeps rabbits out; sinking the bottom edge a few inches stops diggers. Row cover over newly planted sets blocks onion maggots, which lay eggs at the base of young plants.
Spacing, Layout, and Measuring Accuracy
Layout errors do not show up until harvest, when the row runs out of space or the bed comes up a plant short. A tape measure is the cheapest insurance: mark the row ends, pull a string line, and space bulbs against a stick cut to your target spacing. The odd symbols on the blade have real purposes, and learning what the marks on measuring tapes mean keeps rows straight.
Marking straight rows
Drive a stake at each end of the row and pull a mason line tight between them. Plant along the line, then move it for the next row. For blocks, transfer the spacing with a marked stick.
Using a measuring stick for repeat spacing
Cut a scrap board to your exact spacing, such as 6 inches, and use it as a walking gauge: lay it against the last planted bulb, drop the next at the far end, and repeat. Faster than measuring each bulb, and just as accurate.