RHS Growing Guides
How to grow sweetcorn
Our detailed growing guide will help you with each step in successfully growing Sweetcorn.
Getting Started
Sweetcorn is an easy and rewarding crop, providing delicious cobs from mid-summer onwards, to enjoy fresh from the garden at the peak of sweetness. These tall, tender plants love warmth, shelter and full sun. You can also grow baby corn and popcorn, for added variety.
Sweetcorn is most successful in long hot summers, although many modern cultivars are better suited to our cooler climate. There are lots to choose from, including many ‘supersweet’ and ‘tendersweet’ varieties that have been bred for greater sweetness.
Sweetcorn is tender, so is best sown indoors, then planted out after the last frost. It grows rapidly in warm conditions to form tall plants, each usually bearing two cobs that ripen from mid-summer onwards. Expect to harvest six to nine cobs from one square metre/yard of ground.
Sweetcorn plants are tall, so not suited to growing in containers. If space is limited, you can also grow smaller, fast-growing crops, such as salads or spinach, in between sweetcorn plants – especially those that benefit from a little shade in mid-summer to prevent bolting.
Month by Month
| J | F | M | A | M | J | J | A | S | O | N | D | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Sow | ||||||||||||
| Plant Out | ||||||||||||
| Harvest |
Choosing
Sweetcorn is usually grown from seed and there is a vast array of varieties, with different ripening times, levels of sweetness and flavour, cob sizes and colours. Most are F1 hybrids, which produce reliably uniform plants with good vigour.
You can buy early, mid-season and late-ripening cultivars, allowing you to harvest over a long season if you grow several different types. In colder regions, early cultivars are recommended.
Most modern varieties are ‘supersweet’ types – the flavour is much sweeter than older varieties and cobs retain their sugar content for longer after picking, but plants are less vigorous and kernels may be chewier. Just take care not to grow supersweet varieties close to other varieties, as cross-pollination can mean you may not get the super sweetness you expect. You can also buy ‘tendersweet’ varieties, which are almost as sweet and less chewy.
More unusual options include baby corn varieties, for harvesting finger-sized immature cobs for eating whole, and varieties for using as popcorn.
Nine sweetcorn varieties have been awarded an RHS Award of Garden Merit (AGM), as they performed particularly well in trials – see our list of AGM fruit and veg for these and other recommended crop varieties.
For more veg-growing inspiration, visit any of the RHS gardens, as they all grow wide range of crops that you could try in your own garden.
Recommended Varieties
Late F1 variety, well adapted to indifferent summer weather.When well grown, averages 3 cobs per plant. Tendersweet...
Mid season supersweet type with 12 to 14 rows of kernels. Delicious suger sweet flavour. Vigorous grower, needs full sun to...
A mid season corn with well filled tender cobs. Bred as an 'extra tender' variety. Individual kernels slightly...
Preparing the Ground
Choose a warm, sheltered, sunny growing site, protected from strong winds, and with fertile soil. Sweetcorn is less successful in dry or heavy soil. Cobs are unlikely to ripen in shade.
Prepare the site by removing any weeds, then dig in two buckets of garden compost or well-rotted manure per square metre/yard. Also rake in a high potassium general fertiliser, such as Vitax Q4, at a rate of three handfuls per square metre/yard.
Sowing
Sweetcorn is tender and needs warmth to germinate – ‘supersweet’ varieties especially – so seeds are best sown indoors in spring. Plants can then be moved outside after the last frost. This gives them a head start and a longer growing season, which is especially useful in colder regions. In late spring you can also sow outdoors or buy young sweetcorn plants.
Sweetcorn is a type of grass, so is wind pollinated. To ensure good pollination, arrange the plants in a block or group rather than a long row. That way, each plant is surrounded by several near neighbours, so the chances of successful pollination, resulting in a cob packed with kernels, is much greater, whichever direction the wind is blowing. Poor pollination leads to sparsely filled cobs with missing kernels.
The exception is when growing baby corn varieties – with these, the female flowers should not be pollinated, otherwise the kernels will start to swell. So plant these in rows and reducing the spacing to only 20cm (8in) between plants.
Sowing indoors
Sweetcorn seeds need warm conditions to germinate, so sow indoors at 18–21°C (65–70°F) from mid-April to early May. A warm windowsill or heated propagator are ideal. Sow the large seeds 2.5cm (1in) deep, singly in toilet roll tubes, deep pots or modules to minimise root disturbance. Germination should take about two weeks.
For an extended cropping season, sow early, mid-season and late cultivars at the same time, or sow one early cultivar in several batches a few weeks apart.
In cold locations, choose an early ripening variety, and sow early to get a head start.
Once the seedlings are at least 8cm (3in) tall, they can be moved outside, as long as there is no longer any risk of frost. See Planting, below.
Sowing outdoors
Sowing indoors is generally more reliable, as temperatures can be kept consistently warm and the seeds and seedlings are protected from pests, such as mice, slugs and snails.
However, if you don’t have space indoors, you can sow outdoors in late spring, for a later crop. This is usually more successful in southern locations and in warm, light soils – germination may be poor in cold, wet ground, especially with ‘supersweet’ varieties. Make sure the soil temperature is consistently above 10°C (50°F). It is also best to warm the soil beforehand with cloches or plastic sheeting, and sow under cloches or fleece, keeping them in place for as long as possible after germination too.
Sow seeds 2cm deep, in a grid not a row, spacing them 34–45cm (14–18in) apart in each direction. With baby corn varieties, on the other hand, sow seeds 20cm (8in) apart in rows not grids, as pollination is not required.
It’s best to sow two or three seeds at each point, to allow for losses or failures, then thin out any extra seedlings to leave just the strongest one at each point.
Planting
It's important to harden off these tender plants first, to gradually acclimatise them to outdoor conditions – this will ensure they settle in well without a check to their growth.
Plant in a block rather than a row to maximise pollination, spacing plants 34–45cm (14–18in) apart. But to grow baby corn, plant 20cm (8in) apart in a row not a block, as these mustn’t be pollinated.
With ‘supersweet’ varieties, avoid planting standard sweetcorn varieties nearby, as cross-pollination can reduce their sweetness.
Plant Care
Watering
Water young plants regularly until well established and growing strongly.
After that, sweetcorn usually only needs watering in hot dry spells and especially while flowering and when the cobs are starting to swell.
Mulching
Mulch the soil with garden compost to hold in moisture and suppress weeds.
Weeding
Weed regularly, especially when the plants are young. But if hoeing, be carefully not to damage the shallow roots. Add more soil or mulch to bury any visible roots.
Supporting
Sweetcorn plants can grow up to 2m (6½ft) tall, depending on the variety, so are vulnerable to being rocked by wind – this can loosen the roots and hinder growth. So if plants are being buffeted around, either support each one individually with a tall bamboo cane or insert several canes around the outside of the block and link together with string.
You can also earth up the plants by mounding soil around the base of the stems to provide extra stability. Additional roots should grow in this extra soil, to help anchor plants more securely in place.
Flowering
Sweetcorn is wind pollinated, so plant closely in a block rather than a long row, so the plants are surrounded by others. This increases the chances of pollination whichever way the wind is blowing.
It’s also worth giving an extra helping hand once the male flowerheads open at the top of the plant – simply tap the stems to loosen the pollen so it’s more likely to reach the female flowers lower down the plants. Poor pollination results in sparsely filled cobs.
The only exception is when growing baby corn, whose female flowers must not be pollinated, to keep the cobs small and prevent the kernels swelling. Plant these in rows, not blocks, or position them individually around the veg plot.
Harvesting
Once the tassels at the end of a cob have turned chocolate brown, test for ripeness before picking – peel back a little of the leafy husk and pierce a kernel with your fingernail. If a watery liquid comes out, it’s not yet ripe, if the liquid is creamy, the cob is ready, but if it’s paste-like then it’s over-mature. Over-ripe cobs can be dried for use as popcorn (see below). Baby corn is harvested when the cobs are still immature and only about finger-sized.
To pick sweetcorn, twist the ripe cob and pull sharply from the stem.
Once picked, sweetcorn declines in sweetness, as the sugars turn to starch. This is one of the big advantages of growing your own – you can eat it within minutes of harvesting, at maximum sweetness. Standard varieties generally lose their sweetness more quickly than the newer ‘supersweet’ varieties.
Sweetcorn can be eaten raw or cooked. It is ideal for grilling on summer barbecues, freshly picked. Alternatively, steam or boil it – either whole cobs or after removing the kernels from the cob – and serve with butter.
If you can’t eat your ripe cobs straight away, pick them to prevent over-ripening and store in the fridge for up to a few days, with the outer leaves (husk) left in place. Sweetcorn also freezes well.
When growing sweetcorn for making popcorn, leave the cobs on the plants until over-ripe and starting to dry. Bring them indoors and hang up somewhere cool and well ventilated to dry fully. Kernels must be completely dry to pop successfully. To cook, either heat in a lidded saucepan with a little oil or in a microwave in a covered bowl. Fully dry kernels can be stored in an air-tight jar for several months.
Problems
Wildlife enjoy sweetcorn as much as we do, so you’ll probably need to protect the seeds and ripening cobs. Mice in particular like to eat the seeds, so it’s usually safest to sow indoors.
Seedlings and young plants are also vulnerable to slugs and snails.
Birds, mice, squirrels and others may eat the cobs, so netting the plants or covering individual cobs with bags may be necessary. See our guide to protecting crops from birds.
Cross-pollination between ‘supersweet’ varieties and standard varieties can result in starchy kernels with poor flavour, so avoid growing these nearby. A similar problem can happen when growing sweetcorn near maize crops or ornamental maize varieties.
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