At a glance
Onions are one of the most practical crops a UK kitchen garden can produce. A well-grown bed of onions planted in March and harvested in July can supply a household with cooking onions for 6-9 months – displacing a year’s worth of supermarket purchases from a single growing season. They are not the most exciting crop to grow but they are among the most useful, and the satisfaction of lifting a full row of large, dry, storable onions in mid-summer is genuinely rewarding.
Growing onions successfully in the UK is largely a matter of timing and soil preparation. Get those two things right and onions are an undemanding crop that mostly looks after itself through the growing season.
Best onion varieties for UK gardens
| Variety | Type | Skin colour | Storage | Verdict |
|---|---|---|---|---|
| Stuttgarter Giant | Sets | Brown | Excellent – up to 9 months | Most reliable UK set variety |
| Sturon | Sets | Brown | Excellent | Best all-round flavour |
| Red Baron | Sets | Red | Good – 4-6 months | Best red variety |
| Ailsa Craig | Seed | Brown | Good | Classic show onion, large |
| Centurion F1 | Sets | Brown | Very good | Good bolt resistance |
Grow sets rather than seed for your first onion crop. Onion seed requires sowing in January or February under cover, pricking out, potting on and careful hardening off before planting – a 5-6 month growing project from seed to harvest. Onion sets – small immature onion bulbs – are pushed directly into the ground in March and produce a full-sized onion by July. For most UK gardeners sets are the practical, reliable choice that delivers excellent results with minimal effort.
Sets vs seed
| Method | Start date | Difficulty | Cost | Verdict |
|---|---|---|---|---|
| Sets (small bulbs) | March outdoors | Very easy | ~£3-5 per 250g | Best for most UK gardeners |
| Seed indoors | January under cover | Moderate | ~£2-3 per packet | More variety choice, lower cost per plant |
| Seed direct outdoors | March-April | Easy but slow | ~£2-3 per packet | Unreliable in UK – not recommended |
Planting
Plant onion sets outdoors from mid-March to mid-April once the soil is workable. Onions need an open, sunny position in well-drained fertile soil. They perform poorly in waterlogged or freshly manured ground – the latter encourages leafy growth at the expense of bulb development and increases disease risk.
- 1Prepare a firm, fine seedbedRake the bed to a fine tilth and firm it gently – onions prefer firm soil rather than the loose, rich conditions preferred by many other vegetables. Remove any stones or large clods.
- 2Push sets in with the pointed end upwardPlant sets so the tip is just visible at the soil surface – approximately 1cm below the surface. Planting too deep increases rot risk. Space sets 10cm apart in rows 25-30cm apart.
- 3Check and re-firm after frost or bird activityFrost can lift newly planted sets and birds find them irresistible to pull out. Check every few days after planting and push any displaced sets back into position. A covering of fine mesh for the first 2-3 weeks prevents bird problems entirely.
Never grow onions in the same ground two years running. White rot – a devastating soil-borne fungal disease – builds up rapidly in soil where onions and their relatives are grown repeatedly. There is no cure once established. A strict 3-4 year rotation moving onions, shallots, leeks and garlic around the garden each year is the only effective prevention. White rot can persist in soil for 20+ years once established.
As an Amazon Associate I earn from qualifying purchases. Prices correct at time of publishing.
Care through the season
- Keep weed-free throughout – onions are poor competitors against weeds and their yields drop significantly in a weedy bed. Hand weed carefully to avoid disturbing the shallow root system – a hoe used carefully between rows is effective from late April onwards.
- Water sparingly – onions need less water than most vegetable crops. Water in very dry spells during bulb development (June-July) but avoid overwatering which encourages leafy growth and reduces storage life. Never water in the final 4-6 weeks before harvest.
- No feeding needed in prepared soil – onions grown in soil that has had compost or manure incorporated before planting need no additional feeding. Feeding with high-nitrogen fertiliser produces large soft bulbs that store poorly.
- Remove any bolting plants – if an onion sends up a flower stem it has bolted and will not develop a proper bulb. Pull and use immediately – they will not improve and taking them out prevents cross-contamination with neighbours.
Harvesting and curing
Onions are ready to harvest when the foliage naturally topples over and begins to turn yellow – typically July to August. Do not bend the tops over manually as was traditionally advised; wait for it to happen naturally as this indicates the bulb has stopped growing and begun to ripen.
Lift onions on a dry day using a fork, being careful not to spear the bulbs. Lay them on the surface of the bed or a rack in full sun to dry for 1-2 weeks in good weather. In wet weather bring them under cover – a greenhouse, cold frame or garage with good airflow is ideal. The outer skin should become papery and dry and the neck should feel completely dry before storing.
Storage
Properly cured onions store for 6-9 months in the right conditions. The ideal storage environment is cool (5-10°C), dry and with good airflow – a garage, shed or cool spare room is ideal. Avoid storing in the kitchen where warmth and humidity dramatically reduce storage life.
Traditional onion ropes or strings look attractive and provide good airflow around each bulb. Alternatively, single layers in wooden trays or net bags work well. Check stored onions monthly and remove any showing signs of softening or mould immediately – one rotting onion accelerates deterioration in its neighbours. For more on productive vegetable growing read our guides on how to grow garlic UK and how to grow leeks UK.
As an Amazon Associate I earn from qualifying purchases. Prices correct at time of publishing.
Share on socials:
