To plant onions in a garden, prep loose, fertile soil, set sets or transplants shallowly, space 3–4 inches apart, and keep water and weeds in check.
What You Need Before You Start
Onions like full sun, loose soil, and steady moisture. A bed that drains after rain beats a soggy low spot. Aim for a neutral to slightly acidic pH. Mix in finished compost and a balanced starter fertilizer based on a soil test. If you garden in a cooler zone, check timing with the USDA Plant Hardiness Zone Map so your crop has enough growing days.
Pick A Planting Method
You can start from seed, buy pencil-thick transplants, or push small dormant bulbs called sets. Seeds give the widest choice and storage champs, but they take the longest. Sets are quick and easy, handy for first-timers. Transplants split the difference and help you chase a named variety.
| Method | Pros | When To Use |
|---|---|---|
| Seeds | Cheapest, widest variety choice, great keepers | Start indoors 8–12 weeks before outdoor date; sow shallow in bands, then thin |
| Transplants | Fast start, named cultivars | Set out when days reach about 10°C–15°C; tolerate light frost |
| Sets | Easiest for beginners, quick bulbs | Plant early spring as soon as soil can be worked |
For direct sowing, place seed 1/4–1/2 inch deep in rows 12–18 inches apart; thin to 3–4 inches once seedlings stand strong. For sets or transplants, sit them 3–4 inches apart and keep the growing point just above the soil line.
Planting Onions Outdoors: Step-By-Step
1) Time It Right
Plant as soon as the ground is workable in spring in cool and temperate areas. In mild winters, fall planting is common for short-day types. Seeds go in first; sets and transplants follow when the bed can be raked fine.
2) Prep The Bed
Rake to a crumbly tilth. Raised rows or mounded beds help drainage and warm sooner. Work a complete fertilizer into the top few inches per label rates. Many extension guides suggest balanced blends like 10-10-10 before planting and a light side-dress of nitrogen later in the season.
3) Set Depth And Spacing
Keep depth shallow. Bury only the roots and the lower inch on transplants. With sets, cover so the tip is just at or slightly below surface. Space plants 3–4 inches within the row and keep rows 12–18 inches apart for airflow and easy hoeing.
4) Water Smart
Onions have small root systems, so even moisture matters. Target about an inch per week from rain and irrigation. Drip lines or a slow sprinkler pass keep leaves dry and disease pressure lower. More water is needed during bulbing, then ease off as tops bend.
5) Feed For Steady Growth
Bulbs size in direct proportion to healthy leaves. Start with a balanced pre-plant feed, then side-dress with nitrogen mid-season. Many state sheets suggest one or two light feedings, then stop late in the season to avoid soft bulbs in storage.
Daylength And Variety Choice
Bulb cues are tied to daylength. Short-day types start bulbing with about 11–12 hours of light, intermediate around 12–14, and long-day around 14 or more. Grow the group matched to your latitude; planting a short-day type far north makes bulbs start too soon and stay small. A long-day type far south may never bulb well.
Quick Picks By Region
South and frost-light areas use short-day or day-neutral lines for late fall or late winter planting. Middle bands use intermediate types for spring planting. Northern gardens lean on long-day types set out in spring. Seed catalogs list the group on each variety page.
Soil, Beds, And Spacing That Work
Loose, rich soil grows wider bulbs. Break clods, remove rocks, and rake flat. Keep rows 12–18 inches apart. For wider beds, run two to four rows per bed, 10–12 inches between rows, and 3–4 inches between plants. Pull soil away late in the season so bulb shoulders can dry.
Fertilizer Targets
A common plan is 2.5–3 lb of 10-10-10 per 100 sq ft worked into the top layer before planting, with a light side-dress about a month later if growth stalls, especially on sandy plots. Follow local soil test advice first.
Mulch And Weed Control
Weeds steal light fast. Lay a thin mulch of clean straw or shredded leaves once the soil warms. Keep mulch pulled back from young stems to prevent rot. Hoe lightly; deep cultivation can nick shallow roots. Label rows so spacing checks stay easy and stays tidy.
Broad Timing Guide For Different Starts
Seeds: start indoors 8–12 weeks before your outdoor date, then transplant sturdy, trimmed seedlings once nights settle above freezing. Sets and nursery starts go outside when soil can be worked and daytime highs hold near 10–15°C. Many guides note that these plants shrug off light frosts.
| Stage | Water Target | Feeding Note |
|---|---|---|
| Seedling/Establishment | Even moisture; never soggy | Starter fertilizer in top few inches at bed prep |
| Leaf Build (Pre-Bulb) | About 1 inch per week | Light nitrogen side-dress; avoid heavy doses |
| Bulbing/Curing Prep | Ease off as tops bend | Stop nitrogen to improve storage quality |
Common Mistakes To Dodge
Planting The Wrong Daylength Type
Mismatch the group and you get marble-size bulbs or plants that never bulb. Check product pages or packets for short-, intermediate-, or long-day labeling.
Setting Starts Too Deep
Deep planting slows growth and can rot the base. Keep the growing point near the surface and firm soil gently around the roots. Multiple extensions echo this shallow rule.
Letting Weeds Run
Shallow roots leave onions poor at competing. A weekly pass with a stirrup hoe and a thin mulch keeps rows clean and bulbs sizing up.
Late Nitrogen
Feeding late gives lush leaves and soft bulbs that store poorly. Time side-dressings to the middle of the season and stop several weeks before harvest.
Water, Pests, And Problems
Smart Watering
Consistent moisture keeps bulbs even and reduces split skins. A simple rain gauge in the bed helps you decide when to irrigate.
Thrips And Disease
Tiny thrips rasp leaves and slow growth. A strong water spray on leaf bases knocks numbers back. Good spacing and dry leaves after irrigation help with leaf spots. Rotate with non-alliums to keep soil issues down.
Bolting
Cold snaps and drought swings trigger flower stalks, which reduce bulb quality. Plant at the right time, keep moisture steady, and pick varieties suited to your latitude to lower the risk.
Harvest, Cure, And Store
When half the tops flop and the necks soften, bulbs are near ready. Loosen soil with a fork and lift carefully. Cure in a warm, airy spot out of direct rain until necks dry and skins rattle, then trim roots and leaves. Store in mesh bags in a cool, dry, dark place with airflow. Stop watering once tops fall so the outer skins toughen.
Raised Beds, Containers, And Small Spaces
Shallow roots make onions friendly to boxes and troughs at least 8–10 inches deep. Use a loose mix, keep spacing, and feed lightly as you would in ground beds. Group plants by size so smaller ones don’t get shaded by big leaves.
Simple Calendar By Climate
Cold winters (long-day): start seed late winter; plant out early spring; harvest mid to late summer. Middle zones (intermediate): start late winter; set out in spring; harvest summer. Mild winters (short-day): set out late fall or late winter; harvest late spring. Pair dates with your zone map.
Quick Reference: Depth, Spacing, Rows
Seeds: 1/4–1/2 inch deep; rows 12–18 inches; thin to 3–4 inches. Sets: tip near surface; 3–4 inches apart. Transplants: bury roots and the lower inch; 3–4 inches apart; keep rows open for airflow. These ranges show up again and again in extension sheets.
Trusted How-To For Later
Bookmark a step-by-step page from the University of Minnesota’s vegetable team for seed depth, row spacing, thinning, and frost tolerance. It pairs well with your zone map when planning your start date. Growing onions.
Troubleshooting In Season
Symptom Guide
Leaves pale? Feed a light nitrogen dose and water in. Bulbs splitting? Hold moisture steady and skip late feed. Scorched tips? Add mulch and irrigate earlier. Thick necks? Stop nitrogen sooner. Soft bulbs later often trace to rough harvest or weak curing.
When Plants Bolt
A seed stalk can pop up after a cold snap on young plants or after stress. Once it appears, bulbs won’t keep long. Pull and eat those first and let the rest finish. Stagger planting dates and use sets or transplants sized like a pencil to lower risk.
Better Curing And Storage
Dry air and movement matter. Cure on mesh racks in shade for two to three weeks, turning bulbs so skins dry evenly. When necks are tight and papery, clip leaves to an inch and trim roots. Store cool, dry, and check often.
Plan Your Next Crop
Rotate beds yearly with non-alliums to keep pests and soil diseases in check. Follow onions with leafy greens or legumes next season. Keep notes on spacing, dates, and which varieties sized up best at your latitude. Simple records make the next round smoother and help you time seed starting, transplanting, and harvest with more precision.