How to Grow Onions from Sets: A Complete Guide

Choosing the right onion sets or seed

Most home gardeners start onions from sets — small, partly-grown bulbs about 1/2 to 3/4 in (1–2 cm) across that have already survived the trickiest early stage of growth. The alternative is starting from seed, which gives you a much wider choice of varieties and costs less per plant, but means sowing indoors 8–10 weeks before your last frost and hardening off transplants. A third option, potted or bare-root transplants, is the fastest of all but usually the most expensive per plant.

For most kitchen gardens, sets are the easiest starting point: cheap, forgiving, and already far enough along that failure rates are low. Reliable, widely available set varieties for northern gardens include the yellow Stuttgarter, a strong keeper with a slightly flattened shape; Copra, bred specifically for long storage; and Red Wing, a red variety that also stores unusually well for its color. Walla Walla Sweet is a popular sweet onion, but it's a poor keeper and best eaten within a couple of months of harvest.

Yellow, red or white — what's the real difference?

Yellow onions are the most widely grown, with the strongest flavor and the best storage life of the three. Red onions are milder and sweet enough to eat raw, but they don't keep as long and are best used within a few months. White onions have the mildest flavor of all and are popular in dishes where you don't want visible onion color, but they're also the most disease-prone and the shortest-lived in storage.

Daylength matters more than most gardeners realize

Onions form bulbs in response to daylength, not just warmth. Short-day varieties, bred for regions closer to the equator, almost never bulb properly once you're north of about 35°N latitude — which covers essentially all of USDA zones 3–6. Gardeners in these zones should stick to long-day or intermediate-day varieties, which are what most northern seed catalogs and garden centers stock by default. If you're ordering seed or sets from a supplier based further south, it's worth double-checking the daylength type before you buy.

Sets, seed or transplants — which suits you?

When should you plant onion sets?

Soil temperature is what matters, not the calendar. Onion sets want soil that has reached at least 50°F (10°C); planting into cold, wet ground raises the risk of rot, and an early cold shock also makes bolting far more likely later in the season. It's better to wait a few extra days than to gamble on soil that's still cold.

As a rule of thumb, plant sets 2–4 weeks before your average last frost date, as soon as the ground can be worked. In practice that's often March in milder regions and April to early May across most of USDA zones 3–6. If the soil still feels cold when you push a finger a few inches in, give it another week.

Pre-sprouting sets for a head start

You can gain two to three weeks by pre-sprouting sets indoors. Spread them in a shallow tray of lightly moist potting mix and keep them somewhere bright and cool, around 46–54°F (8–12°C), for two to three weeks before planting out. This lets roots and the first green shoots get moving without exposing the plant to frost. Handle pre-sprouted sets gently — the new roots are fragile and snap easily.

Good to know. Pre-sprouted sets usually give an earlier, more even harvest, but they're more sensitive to cold right after transplanting because they're already actively growing. Keep row cover on hand if frost is forecast in the first couple of weeks.

Soil, site and bed preparation

Onions want full sun and loose, well-drained soil with a pH of 6.0–6.8. Heavy clay holds too much moisture around the bulb and raises the risk of rot, though it can be improved by working in sand and well-rotted compost before planting. Dig or till the bed deeply and evenly — stones and clumps force young bulbs to grow lopsided.

Work in about an inch (2.5 cm) of well-rotted compost across the bed before planting, ideally the previous autumn. Avoid fresh manure right before planting: too much available nitrogen produces lush, leafy growth but soft, poor-keeping bulbs, and raises the risk of fungal disease.

Raised beds help on heavy soil

If you're gardening on clay, or anywhere water tends to pool, build a raised bed 4–6 in (10–15 cm) high. Onions tolerate dry spells far better than standing water, and a raised bed also warms up faster in spring, which can buy you several extra days at planting time.

How to plant onion sets step by step

1. Prepare the bed. Dig or till the soil loose and level, removing stones and weed roots. Rake the surface smooth.
2. Mark out rows. Space rows 10–12 in (25–30 cm) apart for most home gardens. Tighter spacing, down to about 8 in (20 cm), works if you're diligent about watering.
3. Push the sets in, pointed end up. Space bulbs 4–6 in (10–15 cm) apart, pressing each one into loose soil so the tip sits level with the surface or pokes slightly above it.
4. Cover lightly if needed. Add no more than an inch (2.5 cm) of soil on top if a set has sunk in too far. Planting too deep often produces misshapen or split bulbs.
5. Water in thoroughly. A good soak right after planting settles the soil around the roots and gets root growth started.
6. Protect against a late cold snap. If frost is forecast in the next couple of weeks, drape row cover loosely over the bed and remove it during the day when conditions allow, so plants still harden off naturally.
7. Watch for birds. Crows and other curious birds sometimes pull up freshly planted sets before the roots have anchored. A row cover or a few strings over stakes usually solves it.

Watering and feeding

Watering: consistency beats quantity

Onions have a shallow, relatively weak root system that stays mostly in the top 6–8 in (15–20 cm) of soil, which makes them sensitive to both drought and waterlogging. Aim for the equivalent of about 1 in (2.5 cm) of water a week during active growth, more during extended heat or drought. If the soil feels dry a few inches down, it's time to water.

Stop watering entirely once the tops begin to yellow and flop over near the end of the season. Continued watering at that point only delays maturity and prevents the bulbs from curing properly before storage.

Feeding in two phases

For the first six to eight weeks, while the plant is building leaves, onions benefit from a nitrogen-rich feed to drive strong top growth — and stronger tops ultimately build a bigger bulb. Apply a balanced or nitrogen-leaning fertilizer every two to three weeks according to the label.

Once the bulb starts visibly swelling, usually by midsummer, stop nitrogen feeding completely. Continued nitrogen late in the season produces soft bulbs with poor storage life and a higher risk of rot. If the bed was well amended with compost before planting, most home gardeners don't need to feed at all after this point. For more on how nitrogen, phosphorus and potassium each affect bulb crops, see the guide to understanding NPK fertilizer.

Weeding and season-long care

Onions compete poorly with weeds because their narrow leaves cast very little shade on the soil below. Weeds left unchecked can cut your harvest in half without it being obvious until you dig the bulbs up. Hand-weed or hoe at least once a week for the first six weeks, and go carefully near the plants — onion roots are shallow and easily damaged by an aggressive hoe.

Mulch reduces the workload

A thin layer of straw or grass clippings between rows suppresses weeds and evens out soil moisture, but keep it a couple of inches back from the bulbs themselves — trapped moisture against the bulb increases the risk of rot.

Should you bend the tops over during the season?

No. Unlike some other alliums, onion tops should be left standing until they fall over naturally at harvest time. Bending or breaking the leaves early removes energy the plant would otherwise put into the bulb, and the wound gives fungal disease an easy way in.

Why is my onion bolting instead of forming a bulb?

Bolting means the plant sends up a flower stalk instead of putting its energy into the bulb. A bolted onion stops sizing up and stores far worse than a normal one. Two triggers explain most cases.

An early cold shock

Exposing a young onion plant to a stretch of near-freezing temperatures makes it behave as though winter is approaching, and it responds by trying to flower and set seed before it dies — a survival response that backfires in a vegetable garden. Protect newly planted sets with row cover if frost is expected in the first two to three weeks after planting.

Drought and heat stress later on

A period of severe drought triggers the same flowering reflex, essentially out of self-preservation. Even, consistent watering through the season, especially during heat waves, noticeably lowers the risk.

If you spot a flower stalk forming, snap it off right away. That won't fully solve the problem, but it stops the bulb from splitting and means you can harvest and eat the onion straight away rather than trying to store it, since a bolted onion won't keep well regardless.

Common onion pests and diseases

Most onion crops sail through the season with few problems, but here are the ones that show up most often, along with symptoms and fixes.

Onion maggots

Symptoms: plants that suddenly wilt and yellow, often in early summer. Digging up the bulb reveals small white larvae feeding at the base. Cause: the larvae of the onion fly, drawn to the scent of onions, especially freshly planted or damaged ones. Fix: cover the bed with row cover right after planting and keep it on for the first several weeks, rotate planting sites each year, and remove affected plants entirely rather than composting them.

Downy mildew

Symptoms: grayish-purple, fuzzy patches on the leaves in damp weather, often followed by leaves collapsing and yellowing. Cause: the fungus thrives in high humidity and poor air movement between plants. Fix: keep recommended spacing so air can circulate, water in the morning and directly at the soil, and remove affected leaves as soon as you spot them.

Onion rust

Symptoms: small orange-yellow pustules or streaks on the leaves. Cause: a fungal disease that rarely kills the plant outright but can weaken it and reduce bulb size in heavy infestations. Fix: remove the worst-affected leaves, avoid overfeeding with nitrogen, and make sure plants aren't crowded.

White rot

Symptoms: white, fluffy fungal growth at the base of the bulb, sudden wilting of the tops, and the whole bulb rotting. Cause: a soil-borne fungus that is unusually persistent — spores can survive in soil for over a decade. Fix: there's no cure. Remove and discard affected plants in household trash, never in the compost, and avoid growing any onion, garlic or leek in that spot for many years.

Common mistake. Composting onions suspected of having white rot. Home composting doesn't get hot enough or last long enough to kill the spores, which then spread to wherever that compost ends up. Bag and trash affected plants instead.

Companion planting and crop rotation

Onions and carrots are a classic pairing. Onion scent is said to disrupt carrot rust fly's ability to find its host, while carrot scent works similarly against the onion fly. The effect isn't absolute, but many gardeners see fewer pest problems when the two crops are interplanted rather than grown in separate blocks. Full details on growing the companion crop are in the guide to growing carrots, and the broader logic behind pairing crops is covered in the guide to companion planting.

Peas and beans, on the other hand, are poor neighbors for onions. Alliums release root compounds that can suppress legume growth, and the effect runs both ways. Keep a little distance between them in a shared raised bed, or give them separate beds entirely.

Rotate the spot every year

Never grow onions, garlic, leeks or shallots in the same spot two years running. Alliums share the same pests and diseases — particularly onion maggot and white rot — both of which can persist in soil between seasons. A rotation of at least three to four years between allium crops on the same bed noticeably cuts down on problems; the reasoning behind rotating any vegetable family is explained in the guide to crop rotation. If you're also growing garlic, many of the same rules apply; the guide to growing garlic covers how it fits into the same rotation.

When is an onion ready to harvest?

Onions are ready when roughly half to two-thirds of the tops have yellowed and flopped over on their own — not because you bent or trampled them, which only damages the plant early and can hurt storage life. This typically happens 12–16 weeks after planting, though the exact timing depends on variety and how warm the summer has been.

Stop watering as soon as you see the first leaves start to yellow. Wait about another week before harvesting, giving the bulb time to finish sizing up and begin hardening its outer skin, which matters a great deal for how well it later stores.

Lifting bulbs without damaging them

Use a garden fork to lift onions from underneath rather than pulling on the tops, which tear away before the roots let go. In hard or clay soil, loosen around each bulb before lifting. Bruised or nicked onions won't heal and rot quickly — sort these out and use them first, since they won't store.

How long do onions keep in storage?

Properly cured and stored, yellow onions typically keep four to eight months, red onions two to four months, and white onions the shortest of all, often just four to eight weeks. The difference comes down to how thick and tight the outer skin is — the thicker the skin, the longer the storage life.

1. Field-dry if the weather allows. If the forecast is dry and warm, leave bulbs lying on top of the soil for a day or two to begin drying before moving them.
2. Move to a sheltered spot to finish curing. Lay bulbs in a single layer on a screen, in shallow crates, or hang them in bunches somewhere warm, dry and well ventilated, out of direct rain — a garage, porch or shed works well.
3. Cure for two to three weeks. The neck should become completely thin and papery, and the outer skins should rustle when dry. Never rush this by putting onions in direct sun, which scorches the skin.
4. Trim roots and tops. Cut roots close to the base and trim tops to 1–2 in (2.5–5 cm), or braid the necks if you want to hang bulbs in bunches.
5. Sort out damaged bulbs. Onions with soft spots, cuts or thick necks won't keep long — use these within a few weeks instead of storing them.
6. Store cool, dark and dry. A temperature of 32–50°F (0–10°C) with good airflow is ideal. Mesh bags, braided bunches or crates with airflow work far better than plastic bags, which trap moisture and cause rot.

Avoid storing onions next to potatoes. Potatoes release moisture that makes onions sprout early, and gases from the onions can in turn speed up sprouting in the potatoes.

Planting calendar by USDA zone

The dates below are rough guidelines based on typical average last frost dates for each zone. A cold or warm spring can shift these by a week or two in either direction — always check that the soil itself has warmed before planting.

Gardeners in zone 3 and the coldest parts of zone 4 should prioritize early-maturing, long-day varieties and consider pre-sprouting sets indoors to guarantee bulbs finish sizing up before autumn frost ends the season.

Common mistakes

• Planting too deep. The bulb has to fight its way up through too much soil and often ends up misshapen or split. The tip should sit level with the soil surface, not several inches under.
• Letting weeds get ahead of you. Onions' shallow roots compete poorly. Weeds can cut your harvest in half without it being obvious until you dig the bulbs.
• Feeding nitrogen after the bulb starts swelling. Produces soft bulbs that rot quickly in storage instead of curing properly.
• Bending the tops over early. Many gardeners think this speeds up ripening, but it only damages the plant and can hurt storage life. Let the tops fall over on their own.
• Planting too early into cold, wet soil. Raises the risk of rot and makes bolting far more likely later on.
• Skipping the cure before storage. An onion put away with a damp neck often molds within weeks. The neck needs to be fully dry and papery first.
• Growing onions in the same spot every year. Quickly builds up onion maggot and white rot in the soil. Rotate on at least a three-to-four-year cycle.
• Storing in plastic bags. Trapped moisture causes rot within a short time. Use mesh bags or ventilated crates instead.

Frequently asked questions

Can you grow onions from a grocery-store onion?
Yes, if it's already sprouting you can plant a grocery-store onion, but results are usually worse than with proper sets. Store onions aren't bred for planting, may carry disease, and are more likely to bolt or grow unevenly. It works in a pinch, but real sets give far more reliable results.

How many onions do you get from one set?
Normally just one large bulb per set, unlike seed potatoes, which produce several tubers. Some varieties or growing conditions occasionally produce a split bulb with two fused onions, but that's the exception rather than the rule.

Can you grow onions in a container or raised bed?
Yes, onions do well in both as long as the container is at least 8–10 in (20–25 cm) deep and the soil stays evenly moist. Containers dry out faster than garden soil, so expect to water more often, especially in hot weather.

Why are my onions so small?
The most common causes are planting too close together, insufficient sun, weed competition, or uneven watering during summer. Bulb size tracks leaf growth closely, so anything that limits the leaves early in the season limits the bulb later.

Can you eat onions before they're fully mature?
Yes, young onions with green tops make excellent scallions at any point in the season. They just won't store the way a fully mature, cured bulb with a dry neck will — eat them within a few days instead.

Do onions need to be watered every day?
No. Onions want steady moisture, not daily watering, under normal conditions. Check the soil a few inches down; if it feels dry, water. During extended heat or drought, daily watering may be needed, especially in containers.

What should you do if an onion has already bolted?
Snap off the flower stalk as soon as you notice it — that keeps the bulb from splitting. The onion is still edible, but it won't store well, so harvest and use it soon rather than trying to keep it long-term.

Is there any difference between growing sets and growing from a mature bulb?
Sets are specifically grown and selected as planting stock — uniform in size, reliably viable, and tested against premature bolting. A mature bulb from the grocery store hasn't been through any of that selection, which is exactly why results from it are so much less predictable.

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