Root & Relay

Growing guide

Onion Growing Guide

Allium cepa

Build a reliable onion crop around the correct day-length type, loose well-drained soil, careful planting depth, steady moisture, early nitrogen, weed control, and a dry curing period.

Day-length type determines bulb formation: long-day, intermediate-day, and short-day onions suit different latitudes and planting windows.

For planting dates, weather, and guidance tailored to your garden, ask Karpos.

Ask Karpos
Light
6-8 hr direct sun
Temperature
Match day-length type to latitude and season
EC context
Use soil testing before nutrient or pH corrections
Local planning
Use Karpos for location-specific timing and conditions.

Inherent properties

What to expect

These crop traits help set realistic expectations before you choose a bed, pot, grow bag, or container plan.

Growth habit
Bulb onions form shallow roots and expand a storage bulb at the base of the leaves. They compete poorly with weeds and need consistent moisture.
bulbTrigger
Bulbing begins when the cultivar receives its required day length, so variety choice must match latitude and season.
storageQuality
Bulbs store best after a complete dry cure, when skins are papery and necks are shriveled.

Planting and setup

Start with the right setup

Choose the site, soil, timing, and growing approach before the season begins.

Match the day-length type

Long-day onions need about 15 to 16 or more hours of daylight, short-day onions need about 11 to 12 hours, and intermediate-day onions need about 12 to 14 hours. Select the type that matches latitude and season.

Prepare loose soil

Use loose sandy loam or loam around pH 6.0 to 6.8. Mix in aged compost or well-rotted manure, and avoid fresh manure that can introduce weeds and harmful microbes.

Choose sets or transplants

Sets are small immature bulbs that are easy to plant. Transplants give more cultivar control and are useful when a specific day-length type is needed. Plant as soon as the soil is workable for the local season.

Set the planting depth

Space plants 4 to 6 inches apart in rows 12 inches apart. Bury sets with the point barely showing or up to about 1 inch deep, and set transplants about 1 inch deep.

Build early leaf growth

Start with balanced fertility, then side-dress young plants with nitrogen every 2 to 3 weeks while leaves are building. More healthy leaves can support a larger bulb.

Protect shallow roots

Hand-weed carefully and use clean straw or grass-clipping mulch to reduce competition and conserve moisture without burying the bulb neck.

Start here

Quick-start conditions

Use these as practical starting points, then adjust for cultivar, container, weather, and measured plant response.

Light
Use full sun with at least 6 to 8 hours of direct light for strong leaf growth and bulb development.
Temperature
Choose long-day, intermediate-day, or short-day onions to match local daylight and planting timing.
Soil or media
Use loose sandy loam or loam around pH 6.0 to 6.8, amended with finished compost or well-rotted manure rather than fresh manure.
Water
Provide about 1 inch of water per week and keep the shallow root zone from drying and cracking, while avoiding yellowing from overwatering.
Spacing
Plant sets or transplants 4 to 6 inches apart in rows 12 inches apart, with sets barely covered and transplants about 1 inch deep.
Harvest
Stop watering and feeding as tops begin lodging, lift bulbs on a dry day, and cure them in a warm, dry, well-ventilated area for 2 to 3 weeks.

Seasonal care

Keep the crop on track

Timing shifts with cultivar, climate, and crop stage, so use these as seasonal checkpoints.

Planting and establishment

Set the correct day-length type into workable soil, keep the shallow bed evenly moist, and remove weeds before they compete with the young roots.

Leaf building

Maintain steady moisture and side-dress with nitrogen every 2 to 3 weeks while the plant is producing green leaves and before bulb formation begins.

Bulb formation

As daylight reaches the cultivar trigger, watch the base for swelling. Keep soil moisture even, but avoid late heavy feeding that can delay storage maturity.

Lodging and dry-down

When about half to two-thirds of the tops naturally fall over, stop watering and fertilizing. Lift bulbs on a dry day after the necks begin to weaken.

Curing and storage

Spread bulbs in a warm, dry, sheltered, well-ventilated place for 2 to 3 weeks. Trim only after skins are papery and necks are fully shriveled, then store in breathable mesh.

Why it matters

What Root & Relay Watches

  • day length and cultivar fitA mismatched day-length type may grow leaves without forming a useful bulb, or begin bulbing before enough leaf mass develops.
  • soil texture and planting depthLoose soil supports bulb expansion, while deep planting or heavy clay can restrict the bulb and encourage rot.
  • moisture and shallow rootsOnions need consistent moisture near the surface but do not tolerate saturated soil. Dry cracking and yellowing leaves both signal a problem.
  • leaf growth and late nitrogenHealthy early leaves support bulb size, but excessive or late nitrogen can delay maturity and reduce storage quality.
  • lodging and cure drynessNatural top fall signals maturity. Bulbs need a full dry cure before trimming or storing to prevent neck rot.

Functional utility & care

Simple Routine

  • Record day-length type, cultivar, set or transplant source, planting date, spacing, and the local daylight or seasonal window.
  • Check the top inch of soil and water at the base often enough to keep the shallow root zone evenly moist without waterlogging it.
  • Hand-weed carefully, maintain light mulch, and inspect leaves and necks for stress, pests, disease, or soft tissue.
  • Feed only while the plant is building leaves, then stop fertilizer and irrigation as tops lodge and bulbs mature.
  • Cure lifted bulbs until skins are papery and necks are dry, then trim and store them in a breathable container.

Decision support

When Karpos Can Help

  • Which onion day-length type fits this latitude and season?
  • Should I use sets or transplants for this onion variety?
  • Are these yellow leaves caused by overwatering or normal maturity?
  • When should I stop nitrogen and irrigation before harvest?
  • Are these onion bulbs fully cured and ready for storage?

Research notes

Evidence and sources

These are source-attributed starting points for soil, raised-bed, and home-garden decisions. Hydroponic nutrient-solution references are intentionally excluded from this soil guide. This page is not a substitute for a local soil, water, or plant-health diagnosis.

Evidence: university Extension and horticulture references reviewed; day-length type, planting method, and storage timing vary by cultivar and latitude. Last reviewed: 2026-09-19