For most UK gardens, onion sets planted in spring are best planted from March to April, once the soil is workable and no longer waterlogged. If you are growing onions from seed, sow indoors from January to March or direct outdoors from March to April.
Suitable overwintering onion sets can also be planted from September to November, but they are much less forgiving of wet, poorly drained ground.
The calendar is only a guide. In practice, soil condition matters more than the exact date — if the ground is cold, sticky or holding water, wait until it dries out before planting.
Best Time to Plant Onions in the UK
For most UK gardens, the main onion planting windows are:
- onion sets planted in spring: March to April
- Onion seed indoors: January to March
- Direct-sown onion seed: March to April
- Autumn onion sets: September to November, using suitable overwintering varieties
For spring sets, April is often more reliable than early March, particularly on heavy or wet ground. Planting a little later into workable soil is usually better than putting sets into cold, saturated ground.
Onion Planting Times at a Glance
| Method | When to plant or sow | Best for | Key point |
|---|---|---|---|
| onion sets planted in spring | March–April | Easiest route | Wait until soil is workable |
| Onion seed indoors | January–March | More variety choice | Give seedlings plenty of light |
| Direct-sown seed | March–April | Simple seed-grown crop | Sow once soil is workable |
| Autumn onion sets | September–November | Earlier harvest | Best on free-draining ground |
Planning your onion bed?
Once you’ve chosen your planting window, mark out your onion bed or rows in the free BYF Allotment Planner. It helps you see how much space the crop will take and plan what will grow alongside it through the season.
When to Plant Onion Sets in the UK
For most UK growers, March to April is the main window for planting onion sets planted in spring.
March can work well in mild areas or on light, free-draining soil, but there is no advantage to planting into cold, wet ground simply because the calendar says spring has started. On heavier soils, waiting until April often gives the sets a cleaner start.
May is still usable if spring has been particularly wet or cold, although later-planted onions may produce slightly smaller bulbs.
Before planting, check that the soil:
- Crumbles rather than smears or sticks
- Is not holding standing water
- Has started to warm after winter
Once the sets are planted, the next job is keeping them growing steadily.
When to Sow Onion Seeds in the UK
Onions grown from seed need a longer season than sets.
For most UK growers:
- Sow indoors: January to March
- Direct sow outdoors: March to April
- Plant indoor-raised seedlings out: March to May, once hardened off and conditions are suitable
Starting indoors gives the plants more time to develop before summer and provides a much wider choice of varieties. Early seedlings do need good light, though — sowing very early in warm, dim conditions can produce weak, stretched plants.
Direct sowing is simpler, but wait until the soil is workable and no longer waterlogged.
Seeds vs Sets: Which Should You Choose?
| Onion seeds | Onion sets |
|---|---|
| Wider choice of varieties | Easiest way to start |
| Lower risk of bolting | Faster and more forgiving |
| Good option for storage onions | Less preparation needed |
| Need a longer growing season | Best choice for many beginners |
When to Transplant Onion Seedlings Outdoors
Plant indoor-raised onions outside from March to May, depending on weather and soil conditions.
Harden them off first, then plant once the ground is workable and not waterlogged. There is little benefit in rushing seedlings into cold, saturated soil.
Planting Onions in Autumn
Autumn planting can give you an earlier crop the following year, but it is less forgiving than spring planting.
Plant suitable overwintering onion sets from September to November. The aim is to let them establish roots before winter rather than produce lots of top growth.
Autumn onions work best in:
- Free-draining soil
- Raised beds or lighter ground
- Sheltered positions
- Areas that do not stay waterlogged through winter
On heavy or persistently wet soil, spring planting is usually the safer option.
Autumn vs Spring Onion Planting
| Autumn planting | Spring planting |
|---|---|
| Earlier harvest | More reliable overall |
| Greater risk on wet ground | Lower risk of winter rot |
| Can bolt after winter stress | More consistent bulbs |
| Needs good drainage | Works in a wider range of gardens |
For most growers, spring planting remains the safer default.
When Can You Plant Onions Each Month? UK Guide
Onions can be started at several points in the year, depending on whether you are growing from seed, spring sets or overwintering sets.
Use the calendar as a guide rather than a fixed rule — weather and soil conditions can shift the practical planting window by a week or two.
Month-by-Month Onion Planting Guide
January
Sow onion seed indoors if you want a long growing season. Too early for outdoor sets.
February
Continue sowing seed indoors. Outdoor planting will usually still need to wait.
March
Start planting spring sets where the soil is workable. Onion seed can also be direct sown outdoors.
April
The most reliable month for planting onion sets planted in spring in much of the UK. Direct sowing can continue.
May
Late spring sets can still be planted, particularly after a cold or wet spring. Bulbs may finish slightly smaller.
June–July
Too late for a normal maincrop of bulb onions. Spring onions are a better option for quick crops.
August
Generally too late to start full-sized bulb onions. You can still sow spring onions.
September
Start planting suitable overwintering onion sets where drainage is good.
October
A good month for overwintering sets in suitable soil.
November
Late window for hardy overwintering sets where the ground remains workable and free-draining.
December
Generally leave bulb onion planting until spring.
How Soil and Weather Change the Planting Date
The dates above are useful starting points, but soil condition matters more than being a week or two early.
- Wet ground: Wait if the soil is saturated or sticking heavily to your boots. Onion sets left sitting in cold, wet soil are more likely to stall or rot.
- Heavy clay: These soils often take longer to dry and warm in spring, so planting in April can be more reliable than rushing into March.
- Light or free-draining soil: These soils warm and drain faster, so planting may be possible a little earlier.
- Cold snaps or prolonged rain: Delaying planting for a week or two is usually better than putting onions into poor conditions.
A simple rule is:
If the soil is crumbly, drains properly and is easy to work, plant. If it is cold, sticky or holding water, wait.
Frequently Asked Questions (UK)
Yes, particularly on light or free-draining soil. On cold, heavy or waterlogged ground, waiting until April can give spring sets a better start.
For many gardens, yes. By April the soil is usually warmer and easier to work, making it a reliable month for onion sets planted in spring.
Yes. Spring sets planted in May can still crop, although they may produce somewhat smaller bulbs than onions planted earlier.
For a normal crop of full-sized bulb onions, generally yes. Spring onions are a better choice for sowing at this time.
Yes. September is the start of the normal window for suitable overwintering varieties. Good drainage is particularly important.
Yes, suitable overwintering varieties can still be planted in November if the soil is workable and free-draining. Avoid planting into cold, saturated ground.
Established onions are fairly hardy. The bigger problems are prolonged winter wet, repeated freeze-thaw conditions and stress that can contribute to bolting later.
Use them as a guide. The packet cannot account for your soil, local weather or whether the ground is actually ready to plant.
Once your onions are planted, see How to Grow Onions in the UK for watering, feeding, bulb development and common problems. Later in the season, use When to Harvest Onions to judge when the bulbs are ready to lift.
If you want to complete the cycle, see How to Collect Onion Seeds for saving seed for future crops.
