How to Plant Onion Bulbs (Sets) — Depth, Spacing & Timing

Onion sets are the fastest, most reliable way to grow full-size bulbs at home. Each set is a small dormant onion grown from seed the previous season — all the hard early work is already done. Plant them correctly in early spring and you’ll be lifting heavy, papery-skinned storage onions in less than four months.

The most common mistakes happen at planting: wrong depth, wrong end up, and wrong variety for the latitude. This guide fixes all three and walks through every other step in the process.

Quick answer

Plant onion sets with the pointed end facing up, 1 inch (2.5 cm) deep, 4 to 6 inches (10–15 cm) apart, in rows 12 to 18 inches (30–45 cm) apart. Plant in early spring, 4 to 6 weeks before your last frost. Choose long-day varieties if you live north of 36°N, short-day varieties south of that line. Harvest at 90 to 120 days, when the tops yellow and fall over on their own.

Sets vs seeds vs transplants — which should you choose?

There are three ways to start onions. Sets win for most home growers.

Method	Pros	Cons
Sets	Fastest to plant, cold-tolerant, widely available, forgiving	Slightly more prone to bolting; fewer variety choices
Seeds	Cheapest; widest variety range	Slow (10–15 weeks indoors before transplant date); requires grow lights
Transplants	Almost as fast as sets; good variety range	More expensive; must be purchased at the right time

Sets are small, pre-grown bulbs sold in garden centres from late winter. They go from planting to harvest in 90–120 days, they tolerate light frost, and they need no indoor seed-starting. If you want reliable, big storage onions with minimal effort, sets are the right choice.

Long-day vs short-day — the single most important variety decision

Onions are photoperiod-sensitive: they stop producing leaves and start forming a bulb only once day length hits a specific threshold. Plant the wrong type for your location and you’ll either get a plant that stays leafy all season without bulbing, or one that bulbs too early before the leaves are large enough to feed a big bulb.

Type	Bulbs when days reach	Best latitudes	Example varieties
Long-day	14–16 hours	North of ~36°N (northern US, Canada, UK, northern Europe)	Walla Walla, Yellow Sweet Spanish, Copra
Short-day	10–12 hours	South of ~36°N (southern US, Mediterranean, southern hemisphere equivalent)	Vidalia, Texas Early Grano, Red Creole
Day-neutral	12–14 hours	Any latitude; good for intermediate zones	Cabernet, Highlander

Rule of thumb: If you live in the northern United States, Canada, the UK, or northern Europe, buy long-day sets. If you are in the American South (Texas, Georgia, Florida, California’s central valley), buy short-day sets. Day-neutral varieties are a safe choice if you are uncertain or between latitudes.

When to plant by zone

Onions are one of the first crops you can put in the ground each spring. They tolerate light frost (down to about 28°F / -2°C) and actually need cool early-spring weather to build foliage before long days trigger bulbing.

USDA Zone	Approximate planting window
Zones 3–4	Late March – late April
Zones 5–6	Mid-February – late March
Zones 7–8	Late January – late February
Zones 9–10	October – January (fall planting for short-day types)

Target 4 to 6 weeks before your average last frost date. The soil should be workable — not frozen solid — but it does not need to be warm. Onion sets can go in when soil temperature is as low as 50°F (10°C).

Soil preparation

Good soil is the biggest yield driver. Onion sets can’t push big bulbs through compacted, waterlogged, or nutrient-poor ground.

What to aim for:

• Loose and well-drained. Loosen the top 6 inches (15 cm) with a fork or tiller. Onion roots are shallow; they don’t need deep tillage, but they do need loose texture.
• Fertile. Work in a 2-inch (5 cm) layer of compost or well-rotted manure before planting. Onions are moderate feeders and compost gives them a slow, steady nutrient base.
• pH 6.0–7.0. Outside this range, nutrients lock up. If you’re unsure, a basic soil test (available at most extension offices or garden centres) tells you within 48 hours.
• Not recently allium-planted. Avoid beds where onions, garlic, leeks, or shallots grew in the past three years — shared diseases like white rot and onion smut persist in the soil.

If your native soil is heavy clay, plant in a raised bed filled with a 50/50 mix of topsoil and compost. The drainage difference is significant, and raised beds warm up faster in spring.

Step-by-step: depth and spacing

1. Choose healthy sets

Pick firm, dry sets with no mould, no soft spots, and no cracks in the outer skin. Sets about the size of a marble (roughly 0.75 inch / 2 cm in diameter) are ideal. Very large sets (over 1 inch / 2.5 cm) are more prone to bolting — use them for green onion harvests rather than full bulbs.

2. Dig your row or holes

Use a trowel or garden dibber to make a shallow trench or individual holes exactly 1 inch (2.5 cm) deep. Measure once, plant consistently. Shallower than 1 inch and late frost heaves the sets out; deeper than 2 inches and emergence slows noticeably.

3. Orient the set — pointed end up

This is the step most beginners get wrong. The pointed tip is the growing point — that’s where the green shoot will emerge. The flat, slightly concave end is the root plate. Drop every set in with the point facing straight up. Upside-down sets do eventually right themselves, but they waste 2 to 3 weeks doing it and the bulb comes out small.

4. Space correctly

Place sets 4 to 6 inches (10–15 cm) apart within the row, with rows 12 to 18 inches (30–45 cm) apart.

• 4-inch spacing works if you plan to pull every other plant as a green onion once the tops are 8–10 inches tall, leaving the remainder 8 inches apart for full bulb development.
• 6-inch spacing is better if you want every plant to develop into a full storage onion from the start.

Row spacing of 12 to 18 inches gives you room to weed, fertilize, and avoid trampling the shallow bulbs as they size up.

5. Backfill and firm gently

Cover the sets with soil and press down just enough to make firm contact — not packed tight. Overly compacted soil restricts bulb expansion. The tip of the set should be just at or 1 inch below the surface.

6. Water in and mulch

Water gently after planting to settle the soil. Once shoots are 2–3 inches tall, apply a 1-inch (2.5 cm) layer of straw or shredded leaves between the rows. Mulch conserves moisture, moderates soil temperature, and cuts down significantly on weeding. Keep it away from the necks of the emerging shoots.

Watering

Onions have shallow roots and are moderately drought-sensitive during bulb development, but they hate waterlogged soil.

• After planting through early growth: water whenever the top inch of soil feels dry. In cool spring weather that’s typically every 5 to 7 days.
• During bulb sizing (roughly 60 days to harvest): consistent moisture is most critical here. Irregular watering at this stage causes bulbs to crack or form rings inside. Aim for 1 inch of water per week from rain or irrigation.
• Last 2 weeks before harvest: stop watering. Dry conditions harden the outer skins and dramatically improve storage life. Onions harvested from wet soil rot quickly in storage.

Fertilizing schedule

Onions are moderate nitrogen feeders — they need enough nitrogen to build big leafy tops (since bigger tops mean bigger bulbs) but too much nitrogen late in the season pushes leaf growth at the expense of bulb sizing.

Timing	What to apply
At planting	Work in a balanced granular fertilizer (e.g. 10-10-10) or nitrogen-rich organic feed at the manufacturer’s rate
When tops are 6 inches tall	Side-dress with a nitrogen-rich fertilizer (blood meal, ammonium sulfate, or organic balanced feed)
6 weeks before expected harvest	Switch to a low-nitrogen, higher-phosphorus/potassium feed to encourage bulb hardening
When tops begin to yellow	Stop all feeding

Avoid high-nitrogen fertilizers in the final 4 to 6 weeks — they keep the tops green long after the bulb should be hardening, which leads to thick necks that don’t store well.

Harvest cues

Onions tell you when they’re ready. Watch for these signals:

• Tops yellow and flop. The green leaves gradually yellow from the bottom up, then the neck collapses and the top falls over. This is the primary cue.
• Half to two-thirds of tops have fallen. At this point, stop watering and wait a few more days. You can gently bend any stubborn upright tops to speed the process.
• Outer skin is papery and dry. Lift one bulb and check the outer layers — they should be dry and papery, not green or soft.
• Timeline. Most sets produce harvest-ready bulbs 90 to 120 days after planting. Long-day varieties typically run longer than short-day types.

To harvest, use a garden fork to gently lift bulbs from below rather than pulling by the tops — the neck is fragile at harvest and can tear, opening a wound for rot. Lay the bulbs in a single layer in a warm, dry, airy spot out of direct sun (a covered porch, a garage, or a shed) for 2 to 3 weeks to cure. Once the necks are completely dry and the outer skins are fully papery, trim the roots and tops, and store in a mesh bag or open crate in a cool, dry place. Well-cured long-day onions store for 4 to 6 months; short-day types are best used within 1 to 2 months.

Common mistakes to avoid

• Planting the wrong variety for your latitude. A long-day variety planted in Georgia will never form a decent bulb. Match the type to your location before you buy.
• Planting too deep. Sets pushed 3 or 4 inches down struggle to emerge and the bulbs develop irregularly. Stay at 1 inch.
• Planting flat-end up. Takes seconds to fix and makes a meaningful difference in bulb shape and size.
• Watering at harvest time. Wet bulbs going into storage is the leading cause of onion rot. Stop watering at least two weeks before you plan to lift.
• Skipping the cure. Uncured onions rot within weeks. Two to three weeks of warm, dry air transforms a fresh onion into a months-long pantry staple.
• Feeding nitrogen late. Late nitrogen pushes thick necks and soggy necks don’t store. Switch to a low-nitrogen feed 6 weeks out.

Related reading

• How to plant onions that have sprouted — if a store-bought onion has already sprouted in your kitchen, this guide shows you how to replant it for green onion tops within a month.
• How to plant garlic cloves — garlic’s timing, depth, and spacing rules are similar; plant them in the same bed in alternating rows to maximise space.
• How to plant sprouted garlic — put a sprouted garlic bulb in the ground instead of the compost bin and harvest a fresh head in late summer.
• Track watering intervals and set a harvest reminder with the free Tazart plant care app — it adjusts watering schedules for your local weather so you never miss a critical dry-down window before harvest.

A note on conditions

Every garden performs differently. Soil type, drainage, your local frost dates, the specific variety you chose, and the weather during your growing season all shift how onion sets develop and how long they take to mature. Use the depths, spacings, and timelines above as a well-tested starting point, then adjust based on what your bed actually produces in the first season — that’s how every gardener dials in their onion patch.