RHS Growing Guides
How to grow sweetcorn
Our detailed growing guide will help you with each step in successfully growing Sweetcorn.
Getting Started
The great advantage of growing your own sweetcorn is that you can enjoy it freshly harvested, at its peak of flavour and sweetness. Best sown indoors or bought as young plants, sweetcorn needs in a warm, sunny site to ensure the cobs ripen over the summer. You can also grow baby corn and popcorn in a similar way, for added variety.
Long, hot summers are ideal for sweetcorn, allowing the cobs to develop well and ripen fully
Sweetcorn is not suited to growing in containers, as it’s so tall. If space is limited, you can sow 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
Depending on the variety, cobs can vary in sweetness, size, quantity and ripening time
F1 hybrids produce reliably uniform plants with good vigour. 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.
Sweetcorn varieties with an RHS Award of Garden Merit (AGM) make a good choice, as they performed particularly well in trials – see our list of AGM fruit and veg and our Recommended Varieties below. For more veg-growing inspiration, visit the RHS gardens, where you’ll see a wide range of crops that you could try in your own garden.
What and where to buy
Recommended Varieties
Supersweet – earliest supersweet so great for British summers. One or two 20cm cobs per plant.
Supersweet – early variety, really sweet and juicy. Cobs only 15cm long, but at least two per plant.
Supersweet – early to mid-season, with large cobs filled to the tip with sweet kernels.
Preparing the Ground
Choose a warm, sheltered, sunny growing site, protected from strong winds, and with fertile soil. Cobs are unlikely to ripen if they’re not in full sun, and sweetcorn is less successful in dry or heavy soil. Prepare the site by removing any weeds, then add two bucketfuls of garden compost or well-rotted manure per square metre/yard. You can also rake in a high potassium general fertiliser, such as Vitax Q4, at a rate of three handfuls per square metre/yard.
Sowing
Sweetcorn needs warmth to germinate – ‘supersweet’ varieties especially – so seeds are best sown indoors in spring. This gives them a head start and a longer growing season, which is particularly useful in colder regions. In late spring you can also sow outdoors in milder regions, but this is less reliable.
Sowing indoors
From mid-April to early May, sow sweetcorn seeds 2.5cm (1in) deep, singly in deep pots or modular trays, or in toilet roll tubes – see our easy sowing guides below. They germinate best at 18–21°C (65–70°F), so put them on a warm windowsill or in a heated propagator. To extend your cropping season, you could sow early, mid-season and late varieties at the same time, or sow one early variety in several batches a few weeks apart. Germination should take about two weeks. Keep the seedlings in bright light and water lightly every few days. In late spring or early summer, once they’re at least 8cm (3in) tall, transplant them into their final growing position outside – see Planting, below.
Sowing outdoors
Sowing sweetcorn seeds outdoors can be unreliable and is only really worth trying in southern areas and in warm, light soil, if you don’t have space indoors. Germination is likely to be poor in cold, wet ground, particularly with ‘supersweet’ varieties. Seeds and seedlings are at risk from mice, slugs and snails. The delayed start also means that cropping will be later and cobs may not have time to ripen in a cool summer.
Seeds need a soil temperature above 10°C (50°F) to germinate, so wait until late spring. Warm the soil with cloches or a plastic-free crop cover before sowing, and keep in place for as long as possible to protect the young plants. Sow seeds 2cm (1in) deep, in a grid not a row, spacing them 34–45cm (14–18in) apart in each direction. 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. With baby corn varieties, sow 20cm (8in) apart in rows not grids, as pollination is not required.
Planting
By late May or early June, after all risk of frost has passed, plant indoor-sown and newly bought plants into their final position outdoors. Plants should be 8–15cm (3–6in) tall at this point. It’s important to harden off these tender plants first, to gradually acclimatise them to outdoor conditions.
Sweetcorn is a type of grass, so is wind pollinated. To maximise pollination, plant 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 blows. Poor pollination leads to sparsely filled cobs with missing kernels. The exception is when growing baby corn varieties – with these, the female flowers shouldn’t be pollinated, otherwise the kernels will start to swell, so plant these in rows.
Space plants 34–45cm (14–18in) apart, depending on the variety, but with baby corn, plant 20cm (8in) apart. With ‘supersweet’ varieties, avoid planting standard sweetcorn varieties nearby, as cross-pollination can reduce their sweetness.
Plant sweetcorn in a grid rather than a row, to optimise wind-pollination
Plant Care
Watering
Water young plants regularly until well established and growing strongly. Sweetcorn may also benefit from watering in dry spells, especially when flowering and when the cobs are starting to swell.
Mulching
Mulch the soil surface with a thick layer of garden compost to hold in moisture and suppress weeds.
Weeding
Weed regularly, especially when the plants are young. But if hoeing, be careful not to damage the shallow roots. Add more mulch or soil 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 with a 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 stem to provide extra stability. Additional roots should grow in this extra soil, to help anchor plants more securely in place.
Flowering
As sweetcorn is wind-pollinated, it’s important to 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 plant. 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
Wait for the tassels to turn brown, then check for ripeness before picking the cob
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.
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.
Problems
Birds, mice and squirrels enjoy sweetcorn as much as we do – the seeds (if sown outdoors) as well as the ripening cobs. So it’s best to sow seeds indoors, and either net the plants once the cobs start to form or cover individual cobs with bags. Young plants are also vulnerable to slugs and snails. See Common problems, below.
Cross-pollination between ‘supersweet’ varieties and standard varieties can result in starchy kernels with poor flavour, so avoid growing both nearby. A similar problem can happen when growing sweetcorn near maize crops or ornamental maize varieties.
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