Overview
What sweet corn needs, how long it takes, and whether it suits your plot.
Sweet corn is wind pollinated: pollen falls from the tassels at the top onto the silks of the cobs below, one silk for every kernel. A block of short rows gives the pollen the best chance of reaching every silk; a single long row often gives gappy cobs. Give it warmth, sun, rich soil and water, then pick at the milky stage and cook it straight away, before the sugar turns to starch.
Quick facts
- Botanical name
- Zea mays var. saccharata
- Soil pH
- 6.0 – 6.5
- Feeding
- Heavy feeder
- Temperature
- 16 – 27 °C
- Hardiness
- Any zone, as an annual
- Time to harvest
- 60 – 100 days
- Height
- Up to 200 cm (79 in)
- Propagation
- Seed
Optimal growth
Killed by frost
Growing details
- Spacing
- 35 cm (14 in)
- Sowing depth
- 25 mm (1 in)
- Frost sensitivity
- Frost tender
Growing calendar
- Start indoors: April, May
- Sow outdoors: May, June
- Plant out: May, June
- Harvest: August, September
Mid- to late spring, one seed per deep pot at 18 to 21 °C (65 to 70 °F) (RHS). Once soil is above 10 °C (50 °F); supersweets at 16 to 18 °C (60 to 65 °F) (RHS, Iowa State, UMD). Late May or early June, after the last frost, at 8 to 15 cm (3 to 6 in) tall (RHS). About 17 to 24 days after the silks appear, from August in the UK (RHS, UMD).
Is this plant right for you?
- Needs warmth
- Frost tender
- Plant in blocks
- Needs space
- Hungry and thirsty
- Eat straight away
- Fun with children
Good to know
It is a hungry plant, so feed it regularly right through the growing season. Wait until the last frost has passed before it goes outside.
Before You Grow
Choose a sweetness type and season, and a warm, sunny, sheltered spot with rich soil.
Keep supersweets away from standard varieties: crossing can leave starchy kernels with poor flavour (RHS).
What you need
Fresh seed
Fresh seed of one type, or young plants in spring. See choose your sweet corn.
Deep pots
Sweet corn hates root disturbance. See sowing.
Space for a block
Three or more short rows, ideally four. See planting.
Compost
For a hungry crop. See soil.
Fleece or cloches
For direct sowings and cold nights. See when to sow.
Choose your sweet corn
Varieties differ in sweetness, colour, cob size and how early they ripen. In cool areas, choose an early variety (RHS, Utah State).
Standard (su) is the traditional flavour. It germinates best in cool soil, but its sugar turns to starch within a day or two of picking (Iowa State, Utah State).
Sugary enhanced (se) is sweeter and more tender and keeps a day or two longer (Iowa State).
Supersweet (sh2) is much sweeter and stays sweet for up to a week in the fridge, but kernels are crunchier and seed needs warmer soil (Iowa State, RHS). Most modern UK varieties are supersweet, such as ‘Earlibird’ and ‘Ovation’ (RHS AGM).
Tendersweet varieties such as ‘Lark’ and ‘Swift’ (RHS AGM) are nearly as sweet but less chewy (RHS). Gardeners’ World lists ‘Swift’ as an extra-tender supersweet.
Seed or young plants. Young plants are sold in spring and early summer, but the choice of varieties is limited (RHS).
Baby corn and popcorn varieties are grown the same way and picked at different stages (RHS).
Sweet corn types compared
-
Standard (su)
- Sweetness
- Traditional
- Keeps after picking
- 1 to 2 days
- Cold soil
- Tolerant
- Isolate?
- From se and supersweets
-
Sugary enhanced (se)
- Sweetness
- Sweeter, tender
- Keeps after picking
- 2 to 4 days
- Cold soil
- Fairly tolerant
- Isolate?
- From supersweets; Iowa State also says su
-
Supersweet (sh2)
- Sweetness
- Very sweet, crunchy
- Keeps after picking
- Up to a week chilled
- Cold soil
- Needs warm soil
- Isolate?
- From all other corn
From Iowa State, the RHS and UMD. Clemson says standard and se types need no isolation from each other. In a cool garden, start an early supersweet indoors. If you sow direct into cooler soil, choose a standard or se type (Utah State).
Where sweet corn grows best
Sweet corn is a tender, warm-season crop that does best in long, hot summers, though modern varieties crop in cooler climates (RHS).
Temperature. It grows best at 16 to 27 °C (60 to 80 °F) (Clemson). Frost injures it (UMD).
Sun. Full sun: at least six hours, better eight to ten (UMD). Cobs are unlikely to ripen out of full sun (RHS).
Shelter. Plants reach up to 2 m (6½ ft), so choose a spot out of strong wind (RHS). See support.
Space. Room for a block of at least three short rows, ideally four (UMD). Expect six to nine cobs per square metre (RHS).
- Best growth
- 16 to 27 °C (60 to 80 °F)
- Sow when soil is
- Above 10 °C (50 °F); supersweets 16 to 18 °C
- Sun
- At least 6 hours; 8 to 10 is better
Soil
Sweet corn needs fertile, well-drained soil (Iowa State, Utah State).
Preparation. Weed, then add two bucketfuls of well-rotted organic matter per square metre (RHS), ideally the autumn before (Gardeners’ World).
pH. 6.0 to 6.5 (Iowa State).
Nutrients. A hungry crop that needs plenty of nitrogen. Work in a general fertiliser before planting, guided by a soil test if you have one (Iowa State, Utah State).
Planting
Sow into warm soil or pots, and plant out in a block after the last frost.
Seed rots in cold, wet soil: wait until it reaches at least 10 °C (50 °F) (Clemson, RHS).
When to sow sweet corn
Seed needs warm soil: at least 10 °C (50 °F), and 16 to 18 °C (60 to 65 °F) for supersweets (RHS, Clemson, UMD). In cold, wet soil it rots.
In the UK, sow indoors in mid- to late spring (April or May) and plant out in late May or early June after the last frost (RHS, Gardeners’ World). Direct sowing is only reliable in the south, in warm, light soil, in late spring or early summer (RHS).
In the US, sow after the last frost. In central Iowa, standard types go in from late April and supersweets from mid-May (Iowa State). Clear plastic lets you sow three to four weeks earlier (Utah State).
Succession. Sow early, mid and late varieties together, or one variety every 10 to 14 days until early July (Utah State, Clemson).
- Soil above 10 °C (50 °F).
- Plant out after the last frost.
- Sow in batches for a longer season.
How to sow sweet corn
Starting indoors gives a longer season, especially in cold regions (RHS). Use fresh seed: old seed germinates poorly (Iowa State).
-
One seed per deep pot
2.5 cm (1 in) deep in deep pots or modules of seed compost; sweet corn hates root disturbance (RHS, Gardeners’ World).
-
Keep warm
At 18 to 21 °C (65 to 70 °F) on a warm windowsill or in a propagator (RHS).
-
Grow on
In bright light with the compost slightly moist until 8 to 15 cm (3 to 6 in) tall (RHS).
-
Or sow direct
Under fleece or cloches warmed in advance, 2.5 cm (1 in) deep, two or three seeds per spot in a grid 35 to 45 cm (14 to 18 in) apart; thin to the strongest (RHS).
Planting out in a block
Plant in a block of short rows rather than one long row; pollination explains why (RHS, Iowa State). Bought young plants go in the same way.
-
Harden off
For about a week, once frost is over (Gardeners’ World).
-
Plan a block
Three or more short rows side by side, ideally four, such as a 4 by 4 or 5 by 5 grid (UMD, Gardeners’ World).
-
Space the plants
35 to 45 cm (14 to 18 in) apart each way (RHS). US extensions use 20 to 30 cm (8 to 12 in) in rows 60 to 90 cm (2 to 3 ft) apart.
-
Plant gently
Without disturbing the roots, and water in.
-
Baby corn is different
Plant it 20 cm (8 in) apart in rows, not blocks: its cobs must not be pollinated (RHS).
Care
Water most at tasselling and silking, feed a hungry crop, and steady the stems.
Drought during tasselling and silking gives poorly filled cobs (Iowa State, Clemson).
Care by growth stage
Young plants
Pests: slugs and mice
Weed by hand or with shallow hoeing. See soil.
Knee high
Feed: 8 to 10 leaves
Side-dress with nitrogen. See feeding.
Tassels and silks
Water: critical now
Water well and tap the stems. See pollination.
Cobs swelling
Wind: earth up or support
Keep watering; protect cobs from animals. See pests.
Ripe
Pick: at milk stage
Test the kernels. See harvesting.
End of season
Clear: debris and smut galls
Clear the stalks and debris. See diseases.
Watering sweet corn
Sweet corn uses a lot of water because of its large leaf area (Gardeners’ World).
How much. At least 2.5 cm (1 in) a week from rain or watering (Clemson, Iowa State), soaking the soil 15 cm (6 in) deep. Light sprinklings only encourage shallow roots (Clemson).
Keep it even. Keep the soil evenly moist and water deeply in dry spells (UMD, Utah State).
When it matters most. Tasselling, silking and cob filling: drought then gives poorly filled cobs and poorer flavour (Utah State, Clemson). In the UK, water young plants until established, and in long dry spells, especially at flowering (RHS).
How. In the morning, so leaves dry before night (Clemson).
Mulch around the base keeps roots cool and moist (Gardeners’ World, RHS).
Feeding sweet corn
Sweet corn needs plenty of nitrogen and moderate phosphorus and potassium (Iowa State).
Before planting. Compost and a general fertiliser (Iowa State, UMD).
Side-dressing. Utah State and UMD add nitrogen twice: when plants are knee high, with eight to ten leaves, and again as the tassels or silks appear. Place it 15 cm (6 in) from the stems and water it in (Utah State). Clemson, in the warm South, side-dresses monthly, more often on sandy soil or after heavy rain.
Not too much. Excess nitrogen makes plants prone to falling over (Clemson).
Pollination
Sweet corn is wind pollinated. The male tassels at the top shed pollen onto the silks of the cobs lower down; each silk leads to one kernel (RHS, Iowa State).
Blocks, not rows. In a block, pollen is more likely to reach the silks whichever way the wind blows (RHS, Iowa State).
Help it along. Once the tassels open, tap the stems to loosen the pollen (RHS).
Hot, dry wind and dry soil at silking reduce pollination; water if the soil is dry (Iowa State).
Isolation. Different types, and popcorn or field corn, cross readily and spoil sweetness and colour. Iowa State suggests at least 76 m (250 ft) between types and UMD 366 m (400 yd); in most gardens it is more practical to sow so they tassel at least two weeks apart (Iowa State, UMD). Clemson suggests maturity dates a month apart, and says standard and se types need no isolation. White varieties pollinated by yellow corn get yellow kernels (Iowa State).
Support and side shoots
Earthing up. Mound soil around the base of the stems for stability: extra roots grow into it (RHS, Gardeners’ World).
Support. Tie each plant to a cane, or run string around canes at the edges of the block (RHS, UMD).
Side shoots (suckers). Leave them: removing them doesn’t help and can reduce yield (UMD, Clemson). Lots of suckers mean plants are too far apart (Utah State).
Weeding. Hand pull or hoe shallowly: roots are near the surface (Gardeners’ World, Utah State).
Sweet corn in containers
Sweet corn does best in the ground, but compact varieties can crop in large containers (RHS).
Container. As large as you can manage: plants need about the same spacing as in the ground, and several together so they pollinate (RHS).
Position. Sunny and very sheltered (RHS).
Compost and water. Use peat-free multi-purpose compost and water regularly in dry weather (RHS).
Problems
Seed that fails, cobs that fill badly or taste starchy, pests after the ripening cobs, and wind.
Common sweet corn problems
Match what you see to its likely cause.
Gappy cobs, unfilled tips
- Pollination
- Drought
A single row, hot dry weather at silking, crowding or low fertility (Utah State, Clemson). Plant in blocks and water well. See pollination.
Poor germination
- Cold soil
- Old seed
Planted too early, or old seed; supersweets suffer most (Utah State, Iowa State).
Starchy, less sweet cobs
- Cross-pollination
- Late picking
Crossed with another type (see pollination), or picked late (Iowa State, RHS).
Tough, doughy kernels
- Over-mature
Picked past the milk stage. In hot weather cobs stay prime for only a day or two (Iowa State).
Plants blown over
- Wind
- Too much nitrogen
Earth up and support; don’t overfeed (RHS, Clemson).
More problems with sweet corn
Common pests
Mice, squirrels and birds
Mice dig up seed; squirrels and birds take ripening cobs. Start plants indoors, and net plants or cover cobs with bags (RHS, UMD).
Raccoons
In North America, they strip cobs just before they’re ripe. A two-wire electric fence, put up about two weeks before the milk stage, keeps them out (Iowa State).
Corn earworm
Caterpillars eat the silks and cob tips. Choose varieties with tight husk tips and cut off damaged tips at harvest (UMD, Utah State).
Slugs and snails
Young plants are most at risk, so protect them after planting out (RHS).
Beetles and cutworms
Handpick Japanese beetles, which eat the silks. Sap beetles follow damaged cobs: pick on time and remove damaged or overripe cobs. Collars around seedlings stop cutworms severing them (Iowa State, Utah State).
Common diseases
Corn smut
Swollen, grey-white galls on cobs, tassels or stems. Remove them before they burst; most modern hybrids resist it (Iowa State, Clemson).
Seed rot and damping off
In cold, wet soil. Waiting until soil is above 13 °C (55 °F) reduces most seedling rots (Clemson); supersweets suffer most (Utah State).
Wilts and stalk rots
Streaked, drying leaves and collapsing stalks. Remove infected plants and keep the garden clean (Utah State). Clearing debris after harvest also reduces smut and rust (Clemson).
Harvest & Use
Pick at the milky stage and eat or chill it at once.
Harvesting sweet corn
Cobs are ready about 17 to 24 days after the silks appear, sooner in hot weather (UMD, Utah State): from late summer in the UK (RHS). Each plant gives one or two cobs.
Signs. Silks brown and dry, husk still green, cob plump to the tip (Iowa State, Clemson).
The thumbnail test. Peel back a little husk and press a kernel. Watery juice: not ready. Milky or creamy: ready. Paste or nothing: over-ripe (RHS, UMD).
How. Grip the cob and twist it sharply downwards (RHS, Iowa State). Pick in the cool of the morning (Clemson).
Baby corn is picked when finger-sized, 1 to 3 days after the silks appear. Popcorn is left on the plant until over-ripe and drying, then hung somewhere airy (RHS, Iowa State).
- Silks brown and dry.
- Kernel juice milky.
- Eat or chill at once.
Storing and using sweet corn
Sugar turns to starch from the moment of picking: standard varieties can lose half their sugar within 12 hours unless chilled (Iowa State). Cook within minutes if you can (RHS).
Fridge. Unhusked, as close to 0 °C (32 °F) as possible: four to eight days at best, less sweet each day (Iowa State, Clemson). Supersweets hold their sugar longer (RHS, Iowa State): UMD says newer types cooled in ice water keep for two to three weeks.
Freezing. Blanch whole cobs in boiling water for four minutes, drain, bag and freeze (Gardeners’ World).
Cooking. Steam, microwave or barbecue until just tender (Gardeners’ World).
More to Explore
Quick answers, guides that pair well with sweet corn, and the sources behind this one.
Frequently asked questions
Why plant sweet corn in blocks?
It’s wind pollinated, and in a block the pollen is more likely to reach every silk. See pollination.
When should I sow sweet corn?
In the UK, indoors in April or May. Outdoors, after the last frost, once soil is above 10 °C (50 °F). See when to sow.
How do I know when sweet corn is ripe?
The silks turn brown and a pressed kernel gives milky juice. See harvesting.
Why are my cobs not full?
Usually poor pollination or drought at silking. See common problems.
Can I grow different varieties together?
Keep supersweets away from other types and all sweet corn away from popcorn, or sow so they tassel at least two weeks apart. See pollination.
Should I remove side shoots?
No, removing them can reduce yield.
How many cobs does a plant give?
One or two, about six to nine per square metre.
Can I grow sweet corn in pots?
Compact varieties, several to a large pot, in a sheltered sunny spot. See containers.
Related guides
Sources
- How to grow sweetcorn (opens in a new tab) Royal Horticultural Society
- Growing sweet corn in the home garden (opens in a new tab) Iowa State University Extension and Outreach
- Sweet corn in the garden (opens in a new tab) Utah State University Extension
- Growing sweet corn in a home garden (opens in a new tab) University of Maryland Extension
- Sweet corn (opens in a new tab) Clemson Cooperative Extension
- How to grow sweetcorn (opens in a new tab) BBC Gardeners’ World