Plant onion sets or transplants in sunny, well-drained beds, spaced 2–4 inches apart and shallow, then feed, weed, water, harvest, and cure.
What You Need Before Planting
Onions thrive with full sun, loose soil, and modest, steady moisture. Pick a spot that drains well and sees at least six hours of direct light. Roots are shallow, so ground that crusts or stays soggy slows growth and invites rot. A raised row or bed gives you a head start in spring and sheds extra water after storms.
Plan for the form you’ll plant. Seeds are fine for long seasons or indoor starts. Sets (small dry bulbs) and young transplants jump ahead in cool spring weather. Choose a size goal too. Tight spacing yields more small bulbs and spring onions. Wider gaps grow fewer but larger bulbs. Both approaches work; match spacing to your kitchen plans.
Choose The Right Onion Type
Bulbing is triggered by day length. Pick varieties that fit your latitude so bulbing starts at the right time. Short-day strains suit lower latitudes. Intermediate types suit middle latitudes. Long-day types suit higher latitudes. If day length doesn’t match your location, plants either stay leafy or bulb too soon, which caps size.
Daylength Matchup For Home Gardeners
| Onion Type | Best Latitudes | Typical Planting Window |
|---|---|---|
| Short-Day | ~25°–33° N/S | Late fall–winter in mild zones; late winter–early spring in cooler areas |
| Intermediate (Day-Neutral) | ~32°–42° N/S | Early–mid spring once soil is workable |
| Long-Day | ~40°–55° N/S | Early spring as soon as ground can be worked |
Variety tags often say “short,” “intermediate,” or “long.” That label matters more than any catchy name. If you’re near the border between zones, trial two types across a bed. You’ll hedge against weather swings and see which bulbs size up best in your microclimate.
Planting Garden Onion Sets: Timing & Prep
Get sets or transplants in the ground as soon as the soil can be worked in spring. Earlier planting builds leaves before days lengthen, which pays off later because each healthy leaf translates to a layer on the bulb. Cool nights are fine; hard freezes are not. Aim for loose, crumbly soil that stays just moist after watering.
Sort sets by size. Smaller sets lean toward bulbs. Large sets tend to bolt. Keep only firm, dry, papery bulbs. With transplants, trim ragged tips and long roots before planting, then tuck them in so the white shank is buried and green leaves sit above the surface.
Soil Prep And Bed Setup
Onions feed lightly but steadily. Work in finished compost to improve texture and drainage. A soil test guides any extra nutrients. If you add fertilizer at planting, mix it in and keep salts away from roots. Flat beds work, yet a low ridge sheds spring rains better and warms sooner.
Weeds steal light and nutrients fast because onion roots stay near the surface. Shape the bed so you can reach the center without stepping in. A narrow hoe or hand fork helps you slice weeds just under the crust. A thin mulch of straw or shredded leaves blocks light for weed seeds and keeps soil even between waterings.
Step-By-Step Planting
Depth And Spacing
Sets: press in just deep enough to cover the tip with about ¾–1 inch of soil. Transplants: set the white base just under the surface with leaves upright. Space plants 2–4 inches apart for standard bulbs, tighten to 2 inches for lots of small bulbs, or loosen to 6–8 inches for jumbo keepers. Keep rows 12–16 inches apart if you use rows, or plant in blocks at even spacing.
Orientation And Firming
Pointy end up for sets. Firm the soil so each plant stands straight and the neck stays above any mulch. Water gently to settle air pockets, then top off any shallow spots that expose the set. Good contact between roots and soil beats heavy tamping.
Watering, Feeding, And Weeding
Steady moisture builds leaves. Think frequent, light drinks on sandy ground and deeper, less frequent watering on loam. The goal is even growth without stress swings. A light side-dress of nitrogen after plants root in and again at early bulbing keeps leaves vigorous. Stop nitrogen once bulbs start to swell so necks dry down cleanly at the end.
Keep beds tidy. Hand-pull small weeds after rain while the ground is soft. A stirrup hoe glides along the surface and spares shallow roots. Avoid burying the neck with mulch at midseason; that trapped moisture can invite disease and thick necks that cure poorly.
Daylength, Latitude, And Variety Picks
Bulb initiation depends on daylength thresholds. Short-day types trigger near the 10-hour mark, intermediate near 12–13 hours, and long-day near 14 hours. Matching those triggers to your latitude times bulbing for late spring into early summer. If bulbs start too soon, size stalls. If bulbing starts late, plants may never finish before heat, disease, or fall rains arrive. Learn more about daylength classes from the Delaware Cooperative Extension explainer.
Common Problems And Simple Fixes
Bolting (Premature Flowering)
Cold snaps can push plants to send up seed stalks. Large sets bolt more than small ones. Plant early, choose smaller sets, and avoid deep burial. If a stalk appears, clip it low. That bulb won’t store long, so use it first.
Thrips
Thrips feed in leaf folds and scar tissue, which slows growth and shrinks bulbs. A strong rinse aimed down the neck helps dislodge early outbreaks. Keep beds evenly watered and avoid heavy nitrogen that spurs tender growth. For ID and thresholds, see the UC IPM thrips guide. Sticky, dusty plants attract pests, so rinse foliage during dry spells and keep weeds down around bed edges.
Neck Rot And Soft Bulbs
Thick necks hold moisture and cure slowly. Stop feeding after bulbing begins, keep water steady but not heavy late in the season, and space for air flow. Pull only when tops start to fall and skins feel papery. Rough handling bruises scales and opens the door to storage losses.
Harvest, Cure, And Store
When half to two-thirds of tops flop and bulbs have colored skins, lift with a fork and ease them out. Leave the tops attached during drying. Lay onions in a single layer on mesh or slats in a shaded, breezy spot. Warm, dry air cures necks and tightens skins. Once necks are crisp and outer layers rustle, clip tops to about an inch. Then shift bulbs to a cool, dry, airy place. A simple shelf or crate stack works. Detailed curing steps are outlined by Iowa State’s guidance on harvesting and storing onions.
Sweet types store poorly, so eat those first. Firm yellow keepers last longest. Check stored crates now and then and cull any soft bulbs so rot doesn’t spread.
Fertilizer And Water Plan By Growth Stage
| Stage | What To Do | Why It Matters |
|---|---|---|
| Establishment (Weeks 1–3) | Keep soil evenly moist; light starter feed near but not on stems | Fast root set and early leaf growth |
| Leaf Build (Pre-bulbing) | 1 inch of water per week; light nitrogen side-dress; steady weed control | Each healthy leaf adds a ring to the bulb |
| Bulbing And Finish | Hold moisture steady; stop nitrogen; ease off water near harvest | Clean necks and firm skins for better curing |
Soil tests guide exact rates. If you see pale, slow leaves early, a light side-dress helps. If you see lush, floppy growth late, skip nitrogen so necks dry down on time. Excess salts burn roots and stall plants, so keep any granular feed off the stems and water it in gently.
Bed Layouts That Work
Rows suit larger plots and tools. Blocks suit small spaces and reduce path area. A two-line row on a low ridge packs plants densely but still lets air move. Keep drip lines along the row to water the root zone without soaking foliage. In blocks, weave a soaker hose back and forth across the bed for even coverage.
Match spacing to your harvest plan. If you want both spring onions and bulbs, plant tight and thin by pulling every other plant in late spring. Those thinnings are tender and sweet, and the remaining plants gain space without replanting.
Container And Small-Space Tips
Use a deep window box or a low tote with holes drilled in the base. Fill with a high-quality potting mix, not ground soil, so drainage stays consistent. Plant in a tight grid if you want bunching onions or small bulbs. For larger bulbs, give each plant more room. Keep containers on pot feet so the base drains after rain.
Containers dry fast, so check moisture often. A light mulch of shredded leaves slows evaporation. Feed lightly every few weeks during leaf build, then stop once bulbs swell. Rotate the box every week so all sides catch the sun.
Simple Weekly Rhythm
Early Season
Weed after each rain. Top off mulch where you see bare soil. Check that necks stay above the mulch line. Replant any set that frost heaves up. A quick rinse knocks dust and tiny pests out of the leaf folds.
Midseason
Watch for thrips by spreading leaves near the neck and looking for tiny tan insects and silvery streaks. If numbers climb, rinse plants and clear weeds along the edges to remove shelter. Keep water steady and skip heavy nitrogen at this stage.
Late Season
Back off water when tops start to bend. Lift bulbs on a dry day. Cure under cover with good air flow. Clip, sort, and store. Label crates by variety so you use short keepers first.
Troubleshooting Chart
What You See And What To Do
- Tops feel sticky, leaves look silvered: likely thrips. Rinse plants, clear weeds, and check again in a week.
- Lots of seed stalks: large sets or cold snaps. Use those bulbs first; next season choose smaller sets.
- Necks stay thick at harvest: late nitrogen or crowding. Give plants space and stop feeding once bulbs swell.
- Soft bulbs in storage: rough handling or poor curing. Dry longer before clipping and store cooler with airflow.
- Bulbs small even with time: daylength mismatch or shade. Pick a variety for your latitude and move to full sun.
How This Guide Was Built
The steps here align with recommendations from land-grant extensions that test spacing, depth, curing, and pest care in real gardens. Daylength classes, staged feeding, shallow planting for sets, and careful curing are shared across those resources. The linked pages above show deeper charts, photos, and regional tips if you want a second opinion before planting day.