How To Plant Onions In Your Garden? | Tried-And-True Tips

Onions reward a little planning. Pick the right variety for your latitude, prep a loose bed, and use tight spacing with regular water. This guide walks you through timing, soil prep, spacing, feeding, watering, and the simple cues for harvest and curing. You’ll also find quick tables so you can act fast in the yard without guesswork.

Planting Onions At Home: Step-By-Step

Onions can start from three entry points: seed, sets (small dormant bulbs), or nursery transplants. Seeds offer the widest variety choice. Sets bring speed and convenience. Transplants split the difference. Whichever path you choose, give onions full sun, good drainage, and a weed-free row.

Choose The Right Day-Length Group

Bulb formation is triggered by day length. Short-day types bulb with about 11–12 hours of daylight. Day-neutral or intermediate types respond near 12–14 hours. Long-day types need roughly 14–16 hours. Matching this to your latitude keeps plants growing leaves first, then bulbing at the right time. That timing is what makes bulbs size up well. Authoritative growing pages explain these thresholds and give regional tips on which group suits where, such as the University of Maryland overview of day-length needs and the RHS guide to sowing and spacing.

Onion Planting At-A-Glance

Method	When To Start	Spacing & Depth
Seeds (indoors or direct)	Indoors 8–12 weeks before last frost; direct in early spring once soil can be worked	Sow 0.6–1.3 cm deep; thin to 7–10 cm in row; rows 20–30 cm apart
Sets (small bulbs)	As soon as soil is workable in spring; fall in mild-winter regions	Plant 2–5 cm deep; 5–8 cm apart; rows 30–40 cm apart
Transplants	After hardening off, a few weeks before local frost-free date	Set roots and lower white portion below soil; 7–10 cm apart; rows 30–40 cm apart

Why it matters: Uniform spacing drives uniform bulbs. Shallow depth prevents buried necks that store poorly. These measures mirror spacing ranges used by university kitchen-garden guides, including seed depth and row spacing for seeds from RHS, and set/transplant spacing common to land-grant extension pages.

Prep A Loose, Fertile Bed

Onions like a crumbly, well-drained loam with a pH near 6.0–6.8. Work in finished compost and rake a level surface. Raised rows shed spring moisture and warm a touch faster. Avoid fresh manure. Keep rocks and clods out of the top 5 cm so that swelling bulbs aren’t misshapen. Extension guides note the pH range and drainage preference for dependable bulbing, matching the needs listed by Ohio State’s kitchen-garden factsheet.

Sow, Set, Or Transplant

From Seed

Sow indoors in open flats or cell trays, 2–3 seeds per cell. Keep evenly moist and bright. Trim tops to about 10 cm to prevent tangling and to promote sturdy stems. Harden off before planting outside. For direct sowing, lay shallow drills and cover with a fine layer of soil; thin in stages to your final in-row spacing.

From Sets

Sort sets by size. Use marble-sized pieces for bulbing; tiny ones can become spring onions. Pointed end up. Push gently into the bed so the neck sits just below the surface. Firm the soil so sets stay put in spring winds.

From Transplants

Clip tops to 10 cm if long and wispy. Plant so the roots and lower white portion are covered. Press soil around each to prevent air pockets. Water in well, then mulch lightly once the bed settles.

Dial In The Spacing You Want

For full bulbs, keep 7–10 cm between plants. For bunching onions, sow thicker and harvest every second plant for kitchen use, leaving the rest to size up. Row spacing near 30 cm keeps cultivation simple and airflow steady. These measurements fall within the ranges listed by Minnesota and UK kitchen-garden references and keep rows easy to weed.

Pro Tips That Make Bulbs Bigger

Onions build one ring per true leaf. More healthy leaves before bulbing equals fatter rings later. The simple way to stack leaves is to remove stress early: steady nutrients, steady water, and zero competition.

Fertilize On A Schedule

Mix a balanced fertilizer into the bed before planting. Then spoon-feed nitrogen during the leaf-building stage. A light side-dress every 2–3 weeks through early summer keeps growth even. Stop feeding when necks thicken and bulbs start to swell. University guides often suggest spring side-dressing to keep foliage pushing; that timing matches the way onions actually bulk up.

Water For Even Growth

Shallow roots mean onions prefer frequent, moderate watering rather than deep, rare soakings. Keep moisture consistent from emergence through bulbing. Uneven water leads to small bulbs and split skins. Oregon State’s production notes point out that light, regular irrigation early keeps nutrients available near the root zone where onion roots are active.

Mulch, But Keep Necks Clear

After seedlings stand upright, add a thin mulch to hold moisture and block weeds. Straw, chopped leaves, or a fine compost layer all work. Keep mulch away from the neck to prevent rot. As bulbs push up, let the top of each bulb breathe and dry.

Keep Weeds Out From Day One

Weeds steal light and nitrogen during the precious leaf-building window. Hand-weed weekly while plants are small. A narrow hoe between rows pays off. Sharp tools and a slow pass after every watering keep the bed clean with very little effort.

Choose Varieties With Your Goal In Mind

Day-length match comes first. After that, pick for flavor, storage, or color. Thick-necked sweet types taste great fresh but store for a shorter window. Firmer storage types dry down hard and keep for months in a cool, airy spot. Regional lists from extension services and the RHS help you sort options without guesswork.

Timing Your Planting By Climate

Cool springs let onions grow leaves before long days arrive. In cold-winter zones, sow indoors late winter and transplant a few weeks before the last frost. In mild-winter regions, sets or transplants can go in during fall so plants root, pause in midwinter, then finish in spring.

Seed Starting Calendar

Count back 8–12 weeks from your last frost date for indoor sowing. Harden off and set out about 2–4 weeks before that frost-free date if the bed drains well. Minnesota’s extension notes this common timing for reliable transplants, and the approach lines up with standard home-garden practice.

Fall Planting Where Winters Are Mild

Short-day and some day-neutral types suit fall beds in the South and coastal zones. Plant sets or transplants in late fall so roots establish before true cold. Growth resumes as days lengthen, then bulbs fill fast in spring.

Diagnose Common Problems Fast

Onions are straightforward once the basics are set. A few patterns still show up in many beds. Use these quick fixes.

Bolting (Flowering Too Soon)

Cold snaps after planting can trigger a flower stalk, especially with sets. Choose fresh, small sets and keep planting soil on the warmer side in spring. If a stalk forms, pinch it off and plan to eat those bulbs first.

Small Bulbs

Most small bulbs trace back to a day-length mismatch, crowding, or stress during early growth. Revisit spacing, feed lightly but steadily, and confirm that your selection fits your latitude.

Soft Necks Or Rot

Water that lingers at the neck invites trouble. Ease off irrigation late in the season, stop fertilizing as bulbs swell, and keep mulch clear of the neck. Harvest on a dry stretch so tops cure cleanly.

Harvest, Cure, And Store For Flavor That Lasts

When 50–80% of tops flop and skins turn papery, it’s time. Lift bulbs gently with a fork, keeping skins intact. Lay them in a single layer in shade with a breeze for a week or two, then trim and move to longer-term storage.

How To Read The Plant’s Signals

Tops fold over at the neck when bulbing is complete. Color deepens and outer skins dry. A test lift should show firm bulbs with tight wrappers. Let the bed stand an extra few days if skins still feel tender.

Simple Curing Setup

Spread bulbs on mesh racks, greenhouse benches, or cardboard over a porch floor. Shade beats direct sun. Flip once during the first week for even drying. After 7–14 days, trim roots and tops to 2–3 cm, or braid soft-neck types while tops still bend.

Storage That Actually Works

Keep cured bulbs in a cool, dry, airy space. Mesh bags, crates, or shallow boxes are perfect. Sweet types keep for a shorter spell. Firm storage cultivars last longest. Check monthly and cook any bulbs that soften.

Harvest And Curing Timeline

Sign You See	What You Do	Notes
Half the tops have fallen	Stop feeding and reduce water	Slows neck growth so skins tighten
Most tops down; skins papery	Lift bulbs and lay in shade	Handle gently to keep wrappers intact
Necks dry and tight after 7–14 days	Trim or braid; move to storage	Cool, dry airflow extends shelf life

Reference-Back Details For Precision Growers

Spacing guides for home beds commonly land at 7–10 cm in row for bulbing and 20–30 cm between rows, with sowing depths near 0.6–1.3 cm for seed. UK and US kitchen-garden pages align on those numbers. For quick lookups on depth and thinning ranges, the RHS onions page lists seed depth and thinning stages, while Minnesota’s home-garden page shows row and plant spacing for sets and transplants along with transplant trimming tips; see University of Minnesota’s onion guide.

Troubleshooting Cheat Sheet

Feeding And Watering

• Side-dress lightly every 2–3 weeks during leaf build, then stop once bulbs swell.
• Keep soil evenly moist from sprout to bulb set. Short, frequent watering early; deeper drinks later as roots spread.
• Mulch thinly to steady moisture and block weeds. Pull mulch away from necks late in the season.

Weeds, Pests, And Damage

• Weed weekly during the first month. A clean bed early pays dividends.
• Thrips scrape leaves in hot, dry spells. A firm spray of water knocks them back. Keep air moving and avoid lush, late nitrogen.
• Prevent maggot trouble with crop rotation and clean harvests. Remove cull bulbs and plant residue after curing.

Why Day-Length Matching Matters

Plant a group suited to your latitude so plants make leaves during short days, then bulb when the clock says go. Short-day types shine in fall beds across mild-winter zones. Day-neutral types bridge a wide band of regions. Long-day types thrive in the North with long summer days. That simple match is the best single step you can take for plump bulbs.

Quick Planning Template For Your Bed

1. Pick a day-length group that fits your latitude.
2. Choose a start method (seed for choice, sets for speed, transplants for a head start).
3. Prep the bed with compost and a light pre-plant feed; rake level.
4. Plant with purpose: 7–10 cm in row, 30 cm between rows; shallow depth for seeds and necks just under the surface for sets.
5. Feed and water on rhythm through early summer, easing off as bulbs swell.
6. Harvest on cues: flopped tops and papery skins, then cure in shade.

Sources used: This guide draws on peer-reviewed kitchen-garden pages and national horticulture references for spacing, timing, and day-length groups, including the University of Minnesota onion guide and the Royal Horticultural Society’s onion page, which align on sowing depth, thinning, and row spacing for home beds.