Gaps in the cob
That is poor pollination. Plant in a block, not a row, and do not strip off the tassels. Baby corn is the one type you grow in rows, because you do not want the kernels to swell.
UK crop guide
Sweetcorn
Sweetcorn needs a warm summer, but modern varieties do crop in Britain if they get a head start and a sunny, sheltered site. The RHS expects cobs to ripen from August, often two per plant, and about six to nine cobs from a square metre.
Seed is best sown indoors. Outdoor sowing is only really worth trying in the south, in warm light soil, because a late start may not leave enough season to ripen the cobs.
- Sow indoors in April and May. Mid to late spring, 2.5cm deep, somewhere at 18 to 21°C.
- Plant or sow outside in May and June. Plant out in late May or early June after frost. Outdoor seed only in late spring or early summer, and mainly in the south.
- Harvest in August and September. Cobs ripen from August. Test a kernel before you pick the whole crop.
Sow indoors
From mid to late spring, sow one seed 2.5cm deep in a deep pot or module of peat-free compost. They germinate well at 18 to 21°C, on a warm windowsill or in a heated propagator, usually within two weeks. Sow an early, a mid-season and a late variety together if you want a longer harvest, or sow one early variety in two or three batches.
Keep them in bright light. They are ready to move once they are at least 8cm tall.
Sow or plant outdoors
Only try this in a southern garden with warm, light soil, and only from late spring or early summer when the soil is above 10°C. Warm the bed with a cloche or fleece first. Sow two or three seeds 2.5cm deep at each station, 34 to 45cm apart in a grid, and thin to one. Mice, slugs and a cool summer are the reasons this fails. Baby corn is the exception to the grid: sow it 20cm apart in rows, because those cobs are eaten before the kernels swell and should not be pollinated.
Plant out
Plant in late May or early June, after the last frost, once plants are 8 to 15cm tall and have been hardened off. Sweetcorn is wind-pollinated. A single long row pollinates badly and you get cobs with gaps. Plant a block, with plants 34 to 45cm apart each way, so every plant has neighbours.
Do not mix a supersweet variety with an older type nearby. Cross-pollination spoils the extra sweetness. In a cold garden, choose an early variety. Water in dry spells, especially as the flowers open and the cobs swell. Tall plants can rock in the wind. A cane, or a string around the outside of the block, plus a bit of soil mounded at the base, keeps them upright.
Spacing and depth
- In the block
- 34 to 45cm each way
- Depth
- 2.5cm
- Baby corn
- 20cm apart, in rows rather than a block
Harvest
Cobs start to ripen in late summer. When the silks turn brown, peel a little husk and press a kernel with a fingernail. Watery juice means wait. Creamy juice means pick. Thick, doughy juice means the cob is past its best.
Twist the cob off the stem and cook it soon. The sugar starts turning to starch as soon as it is picked, which is the reason a home-grown cob tastes different from a shop one. For popcorn, leave the cobs to dry on the plant, then finish drying them indoors until the kernels are hard.
Common problems
Something ate the cobs
Birds, mice, squirrels, rats and badgers all take sweetcorn. Sow indoors so mice do not get the seed, and net the plants or bag the cobs once they form.
A cool summer
Cobs that never ripen were usually sown late, planted in shade, or grown from a late variety in a short season. An early variety, an indoor sowing and a sunny block are the three things that fix that next year.
What you'll need
Kit for this crop, based on the method above.
- Peat-free multi-purpose compost
For pots, modules and containers once seedlings need more body.
- Module trays or small pots
For crops started under cover. Deep pots suit peas, beans and sweetcorn.
- Horticultural fleece
For early sowings and for covering tender plants if a late frost is forecast.
- Cloches
Useful for warming a row before you sow, and for protecting early carrots and salads.
- Netting
Birds pull up new garlic and onion sets, and they take strawberries and sweetcorn.
- Canes or hazel poles
Runner beans need supports about 2.5m tall. Cordons of tomatoes need a cane or a string.
- Garden compost or well-rotted manure
Dug or mulched in before planting. Do not use fresh manure.
Common questions
Can I grow sweetcorn in a single row?
You can, but the cobs will be patchy. Pollen has to blow from the tassels onto the silks. A block of at least a few plants each way is what the RHS recommends. Baby corn is the type that is sown in rows.
Is sweetcorn worth growing in the north?
It is more reliable in a long, warm summer, and outdoors it is a gamble in a cold district. Choose an early variety, sow indoors, and use the frost page so you are not planting out in May if your area is an early June garden. A greenhouse or polytunnel removes most of the risk.
On the calendar
Related crops
Sources
- RHS: How to grow sweetcorn. Checked 11 October 2026.
- RHS: Direct sowing tender veg outdoors. Checked 11 October 2026.