Sweetcorn is one of those crops that does brilliantly when you get the timing right and goes badly wrong when you do not. It is not a difficult plant to grow, but it does need warmth, and the UK gives you a fairly narrow window to work with. Sow too early and the seeds rot. Plant out too soon and the seedlings stall. Wait too long and the cobs do not have time to ripen before autumn arrives.
The good news is that the right timing is genuinely simple once you know it. Most UK gardeners can grow good sweetcorn with a bit of planning and a sheltered spot.
This guide covers exactly when to plant sweetcorn in the UK, when to sow indoors, when to plant outside, and when to harvest. For the bigger picture across the whole growing year, When to Plant Vegetables in the UK covers timing for every common UK crop. And for everything else worth getting in the ground this month, What to Plant in May in the UK has the full list.
When to Sow Sweetcorn Seeds Indoors in the UK
Most UK gardeners start sweetcorn indoors. It is much more reliable than direct sowing because you can control the temperature properly, which is exactly what sweetcorn seeds need to germinate.
The best time to sow sweetcorn seeds indoors is from mid-April to early May. This gives you about four to six weeks of indoor growing before the seedlings are ready to plant out, which is exactly when outdoor conditions are warm enough.
Sweetcorn seeds need a soil temperature of at least 16°C to germinate properly. On a windowsill or in a propagator this is easy to achieve. In cold compost outdoors in April it almost never happens, which is why direct sowing fails so often early in the season.
Sow individual seeds in deep pots or modules about 2 to 3cm deep. Sweetcorn does not like having its roots disturbed so giving each seed its own pot from the start saves you trouble later. Keep them somewhere warm and bright until they are ready to harden off.
For more on why soil temperature matters so much in spring, When Is Soil Warm Enough to Plant Vegetables is worth a read.
When to Plant Sweetcorn Outside in the UK
Sweetcorn goes outside from late May to early June in most parts of the UK. The soil needs to be warm and overnight temperatures should be reliably above 10°C. In southern England this can be from late May. In the Midlands and northern England it is usually early June. In Scotland and exposed upland areas, mid-June is more sensible.
Before they go out for good, sweetcorn plants need hardening off like any tender crop. A week or two of gradual outdoor exposure during the day makes a big difference to how quickly they settle in once planted out. How to Harden Off Plants in the UK covers the full process if you have not done it before.
One thing that is genuinely unique about sweetcorn is how it needs to be planted. Sweetcorn is wind-pollinated, which means the pollen needs to drift from one plant to another to produce filled cobs. Planting in a single long row almost guarantees a poor crop because the wind blows pollen away rather than between plants. Always plant sweetcorn in a block or grid pattern with at least 3 rows side by side. Even a small block of 9 plants in a 3 by 3 grid works much better than a row of 12.
Space plants about 45cm apart in each direction. Water them in well and give them a sheltered, sunny spot.
If you are not sure when frost is finally past in your area, UK Last Frost Dates by Postcode is worth checking.
Can You Grow Sweetcorn in Pots in the UK?
Yes, but with a couple of important rules. Sweetcorn can grow well in pots if the pots are big enough and you grow several together rather than just one or two.
The pollination problem above applies in pots too. A single sweetcorn plant in a pot on its own will not pollinate properly and the cobs will be poor. If you want to grow sweetcorn in containers, get at least 4 to 6 large pots and group them close together so the wind can move pollen between plants.
The pots themselves need to be at least 30cm wide and the same deep, ideally bigger. Sweetcorn has surprisingly hungry roots and small pots simply do not give them enough room. Use a good quality multipurpose compost and water consistently. Can You Grow Vegetables in Pots in the UK? covers what works well in containers.
How Long Does Sweetcorn Take to Grow?
From sowing to harvest, sweetcorn takes around 3 to 4 months in the UK, depending on the variety and the weather. Seeds sown indoors in mid-April will usually be ready to harvest from late August through September.
The plant itself grows surprisingly fast once it gets going. By July it is often head height. By August the cobs are forming and you start watching for the right moment to pick them.
When to Harvest Sweetcorn in the UK
Harvest time is usually late August to early October depending on where you are and when you planted. The trick is knowing when each cob is properly ready, because once it is ripe it does not stay at peak quality for long.
There are three classic signs that sweetcorn is ready to harvest:
The tassels at the top of each cob turn brown and dry. The cob feels full and firm when you squeeze it. And if you peel back a small section of the husk and squeeze a kernel with your fingernail, the liquid that comes out should be milky white. Clear liquid means the cob is not ready yet. Thick paste means you have left it too long.
Pick cobs by twisting them firmly downwards rather than pulling. They come away cleanly when ripe. Eat them as soon as possible after picking. Sweetcorn loses its sweetness within hours of being harvested as the sugars start converting to starch. The difference between a cob eaten 20 minutes after picking and one eaten the next day is genuinely dramatic.
Common Sweetcorn Growing Problems
Patchy or empty cobs. Almost always a pollination problem. Either the plants were grown in a single row instead of a block, or there were not enough plants for proper pollination. Plant in a grid of at least 3 by 3 next year.
Seedlings rotting in the soil. The compost was too cold or too wet at sowing time. Wait for warmer conditions or sow indoors first.
Plants growing slowly. Usually cold soil or unhardened plants planted out too early. Sweetcorn needs warmth and once it is checked early in the season it can take weeks to recover. Cold Soil Problems in UK Gardens covers exactly why this happens and how long it takes plants to recover.
Plants flopping over. Wind damage. Sweetcorn gets tall and a sheltered spot is important. If you garden somewhere exposed, planting in a block helps as the plants support each other to some extent. For more general protection advice, How to Protect Vegetables from Frost in the UK covers fleece and cloches that also help with wind.
Animals eating the cobs. Squirrels, badgers, and birds will all happily strip a sweetcorn patch just before you were planning to harvest. If this is a problem in your area, netting or fleece bags around individual cobs work well in the last few weeks before harvest.
FAQs
When is the best time to plant sweetcorn in the UK?
Sow seeds indoors from mid-April to early May. Plant out from late May to early June once frost risk has passed and overnight temperatures are reliably above 10°C.
Can I sow sweetcorn directly outside in the UK?
You can but it is much less reliable. Soil temperatures in April and early May are usually too low for proper germination. If you want to direct sow, wait until late May or early June in southern England, or even later further north. Starting indoors gives much better results.
Why do sweetcorn cobs come out empty or patchy?
Almost always poor pollination. Sweetcorn is wind-pollinated and needs to be planted in a block of at least 9 plants rather than a single row. A 3 by 3 grid of plants is the minimum for reliable cobs.
Can I grow sweetcorn in pots in the UK?
Yes, but only if you grow several pots grouped together. A single plant in a pot will not pollinate properly. Use pots at least 30cm wide and group at least 4 to 6 of them close together for proper pollination.
How long does sweetcorn take to grow from seed?
Around 3 to 4 months from sowing to harvest in the UK. Seeds sown indoors in mid-April are usually ready to harvest from late August onwards.
How do I know when sweetcorn is ready to harvest?
The tassels turn brown and dry, the cob feels full and firm, and a squeezed kernel produces milky white liquid. Clear liquid means it is not ready. Thick paste means it is past its best.
A Sensible Place to Start
Sweetcorn is one of the most rewarding crops you can grow in a UK garden, and it is genuinely worth the small amount of planning it takes. Sow indoors in mid-April, plant out in late May or early June in a block rather than a row, and harvest from late August onwards.
If you have not grown it before, start with around 9 plants in a 3 by 3 block in the sunniest spot you have. The pollination works properly, the cobs come out full, and the whole experience tends to convince people to grow it again the following year.
For everything else worth planting in May, What to Sow in May in the UK and May Gardening Jobs in the UK cover the full picture. And for general advice on growing veg from start to finish, Growing Vegetables Successfully in the UK is the practical year-round guide.