How to Grow Onions: The Complete Beginner's Guide

Onions are one of the most rewarding vegetables a home grower can choose. They are relatively undemanding, store exceptionally well, and the homegrown flavor far surpasses anything from a supermarket. This guide covers every stage of the process, from choosing the right type for your location through to storing your harvest for months of use.

Understanding onion types: day length is everything

Onion bulb formation is triggered by the number of hours of daylight, and varieties have been bred to respond to specific day-length ranges. Plant the wrong type for your latitude and you will end up with poor bulbs — or no bulbs at all.

Long-day onions

Long-day varieties require 14–16 hours of daylight to trigger bulbing — summer conditions in northern latitudes, roughly above the 40th parallel. Planted in spring, they form bulbs as summer days lengthen. Long-day onions tend to have stronger, more pungent flavors and store exceptionally well.

Short-day onions

Short-day varieties initiate bulbing when days reach 10–12 hours, which occurs in late winter and spring in southern latitudes. Typically planted in autumn and harvested in spring, they are often sweeter and milder, but do not store as long.

Day-neutral onions

Day-neutral varieties bulb when days reach 12–14 hours, making them adaptable across a wide range of latitudes. An excellent choice for middle latitudes or for anyone unsure which type suits their location.

Three ways to start onions

From seed

Starting from seed offers the widest variety selection and lowest cost per plant, but requires the most lead time. Sow seeds indoors 10–12 weeks before your outdoor planting date in a light, sterile seed-starting mix. Seedlings are ready to transplant when they are pencil-thick and about 15–20 cm tall.

From sets

Onion sets are small dormant bulbs grown the previous season and dried for replanting — the most popular method for home gardeners. Easy to handle, quick to plant, and reliably productive without any indoor seed-starting. The main limitation is more restricted variety selection compared to seeds.

From transplants

Bare-root transplants offer a middle ground: better variety selection than sets, less lead time than starting from seed. Handle them carefully — the roots are delicate and must not dry out before planting.

Soil preparation and planting

Onions thrive in loose, fertile, well-drained soil — heavy clay soils encourage rot and misshapen bulbs. Work the bed to a depth of 20–25 cm, removing stones and large clods, and incorporate generous amounts of well-rotted compost. Onions need a sunny site with at least six hours of direct sun daily.

Plant sets or transplants so the base sits about 2–3 cm below the soil surface. Space storage onions 10 cm apart in rows 25–30 cm apart; for green onions, 2–3 cm spacing is fine. Water in well after planting.

Growing onions in containers

Choose a container at least 20 cm deep with good drainage holes and fill with quality potting mix combined with compost. Plant sets or transplants 5–10 cm apart. Water regularly and feed every two to three weeks with a balanced liquid fertiliser. Container-grown onions may produce slightly smaller bulbs, but the flavour will be equally good.

Onion care through the season

Watering: onions have shallow root systems and need consistent moisture, especially during bulb formation. Water deeply once a week, providing about 2–3 cm. Inconsistent watering causes bulbs to split or become misshapen. Reduce watering as harvest approaches.

Feeding: if you prepared the soil with compost, additional feeding may not be necessary. For poorer soils, side-dress with compost when plants are about 15 cm tall. Avoid high-nitrogen fertilisers once bulbs start forming.

Weeding: onions compete poorly with weeds. Keep the bed weed-free, especially when plants are small. Cultivate shallowly to avoid damaging shallow roots. Mulching with straw or grass clippings helps suppress weeds and retain moisture.

Bolting: if flower stalks appear, cut them off at the base immediately — the onion can still be used but should be eaten soon, not stored. Bolting is more common with large sets or plants stressed by temperature fluctuations.

Common pests and diseases

Onion maggots — small white larvae feeding on roots and bulbs — are most common in cool, wet springs. Prevent them with crop rotation and floating row covers.

Thrips cause silvery streaks and distorted growth. Control with insecticidal soap or a strong jet of water. Keep the area weed-free.

Downy mildew appears as pale patches on leaves in humid conditions. Space plants for good air circulation and avoid overhead watering.

White rot causes leaves to yellow and wilt with white mold at the bulb base. There is no cure — remove and destroy affected plants and avoid growing onions in that spot for at least eight years.

Neck rot appears in storage as soft, sunken areas around the neck. Prevent it by curing onions thoroughly and storing in dry conditions with good airflow.

When and how to harvest

Green onions (scallions) are harvested before bulbs form, typically about 8–10 weeks after planting sets.

For storage onions, wait until about half to three-quarters of the tops have fallen over naturally, then stop watering. After a week or two, when the tops have dried and turned brown, lift the onions with a garden fork — do not pull by the tops, as the necks may break.

Curing onions

After harvest, leave onions in the garden for a day or two if the weather is dry, then move to a well-ventilated shaded location — a shed or covered porch works well. Spread in a single layer or hang in bunches. Good air circulation around each bulb is critical. Curing takes two to four weeks — it is complete when the necks are tight and dry, the outer skins are papery, and the roots are dry and wiry. During curing, remove any onions with thick necks; they will not store well.

Storing onions

Ideal storage conditions: cool (0–4°C), low to moderate humidity, good ventilation, and darkness. Mesh bags, baskets, or nylon stocking chains all allow good airflow. Never store onions in plastic bags or with potatoes — potatoes release moisture and gases that cause onions to sprout and rot. Storage life: pungent storage onions 6–8 months; sweet onions 1–3 months; red onions 3–4 months. Check stored onions regularly and remove any showing signs of sprouting or rot.

Common problems and solutions

Small bulbs: overcrowding, insufficient water, or wrong day-length variety. Space properly, water consistently, choose varieties suited to your latitude.

No bulbs formed: planted the wrong day-length variety. Switch to day-neutral varieties appropriate for your region.

Splitting bulbs: inconsistent watering or excess nitrogen. Water evenly and avoid high-nitrogen fertilisers after bulb formation begins.

Thick necks: planted too deep or given too much nitrogen. Plant at the correct depth and reduce nitrogen once bulbs start forming.

Regional growing notes

Cold climates (zones 3–4): long-day varieties, start seeds indoors early or use sets, plant as soon as soil can be worked in spring, harvest before heavy frosts.

Temperate climates (zones 5–6): long-day or day-neutral varieties. Plant sets or transplants in early spring.

Warm climates (zones 7–8): short-day or day-neutral varieties. In zones 7 and warmer, plant in autumn for spring harvest.

Hot climates (zones 9–10): short-day varieties only, planted in autumn for winter and spring harvest.

Growing onions is deeply satisfying. From small sets or tiny seeds, you can produce dozens of bulbs that last through winter. The variety of flavors, colors, and sizes available to home growers far exceeds anything from a store.