Allotment ProYour allotment, organised

Private by default. No third-party analytics.

Sweetcorn

This Allotment Pro editorial draft summarises the cited RHS guide. Check the source and adjust timings for local conditions.
RHS source guide
Sweetcorn crop illustration

Month by month

Sow

  1. Mid- to late spring

    sow indoors in warmth for the most reliable crop.

  2. Late spring to early summer

    direct-sow only in mild areas once soil is warmer than 10°C.

Plant out

  1. Late May to early June

    harden off and transplant after frost danger has passed.

Harvest

  1. August onward

    test cobs when their silks turn brown and pick as soon as the kernel juice is creamy.

1

1 - Getting Started

Sweetcorn, Zea mays, is a tender annual grass grown for fresh, sweet kernels. Long hot summers produce the best cobs, but modern cultivars can crop in cooler UK conditions. Sow indoors or buy young plants, then use a warm full-sun site so the crop ripens. Plants usually carry two cobs, with an expected six to nine cobs from one square metre or yard.

Home-grown cobs can be eaten within minutes of picking, before their sugars turn to starch. Baby corn and popcorn forms can be grown similarly but have different pollination and harvest requirements.

2

2 - Choosing What To Grow

Cultivars vary in sweetness, plant height, cob size and ripening time. Early types suit colder districts. F1 hybrids offer uniform vigorous crops, but saved seed will not reproduce the parent reliably.

Supersweet corn retains very high sugar levels after picking, although plants can be less vigorous and kernels chewier. Extra-tendersweet types are nearly as sweet with more tender kernels. Keep supersweet cultivars away from standard sweetcorn, maize and ornamental maize because cross-pollination can reduce eating quality.

Recommended varieties

3

3 - Preparing The Ground

Choose a warm sunny site sheltered from strong wind; cobs rarely ripen without full sun. Weed the ground and incorporate two bucketfuls of well-rotted organic matter per square metre or yard. In a no-dig system, apply organic mulch ahead of planting and set plants through it.

Allow enough width for a pollination block rather than planning one long row. A compact block lets wind-borne pollen reach silks from several directions.

4

4 - Sowing

Sowing indoors

From mid- to late spring, sow seeds singly 2.5cm (1in) deep in deep pots or modules of peat-free seed compost. Germinate at 18–21°C (65–70°F) on a warm windowsill or in a heated propagator. Emergence takes about two weeks. Keep seedlings in bright light and the compost slightly moist.

For an extended harvest, sow early, mid-season and late cultivars together, or sow an early cultivar in several batches a few weeks apart. Transplant once seedlings are at least 8cm (3in) tall.

Sowing outdoors

Direct sowing is less reliable and suits southern districts with warm light soil. Wait until soil is above 10°C (50°F), pre-warming it beneath cloches or plastic-free cover. Sow 2.5cm deep in a grid, 34–45cm (13–18in) apart each way. Put two or three seeds at each station, then retain the strongest seedling. For baby corn, use rows with 20cm (8in) spacing instead.

5

5 - Planting

By late May or early June, after frost danger has passed, harden off plants and transplant them when 8–15cm (3–6in) tall. Space ordinary sweetcorn 34–45cm apart in a block. This wind-pollinated crop produces fuller cobs when each plant has near neighbours in several directions.

Baby corn is the exception: plant in rows at 20cm so the female flowers are less likely to be pollinated and kernels remain small. Keep supersweet and standard types apart to prevent loss of sweetness.

Containers are possible but less reliable than open ground. Use the largest practical pots, peat-free multi-purpose compost and compact cultivars in a sunny, very sheltered place. Retain near-ground spacing and water frequently.

6

6 - Plant Care

Water young plants during dry weather until established. Later, prioritise irrigation when flowers open and cobs swell. Apply a thick organic mulch to retain water and suppress weeds. Weed carefully because roots are shallow.

Plants can reach 2m (6½ft) and may rock or break in wind. Support individuals with canes, or surround the whole block with tall canes or woody stems and twine. Earthing soil around each base encourages extra anchoring roots.

When male tassels open, tap the stems to release pollen toward female silks below. Sparse kernels usually indicate poor pollination, reinforcing the need for a block rather than one row.

7

7 - Harvesting

Cobs ripen from late summer. When silks turn brown, peel the husk back slightly and pierce one kernel. Watery juice means it is unripe, creamy juice means it is ready, and paste-like contents mean it is over-mature. Twist a ripe cob and pull it sharply from the stem, then cook promptly for maximum sweetness.

Harvest baby corn while finger-sized and immature. For popcorn, leave cobs until over-ripe and beginning to dry, then hang them indoors somewhere cool and well ventilated until the kernels are completely dry.

8

8 - Problem Solving

Birds, mice, squirrels, rats and badgers may take seed or cobs. Indoor sowing avoids some seed losses. Once cobs form, protect plants with wildlife-safe netting or cover individual cobs with suitable bags or cut-ended bottles. Young plants also need protection from slugs and snails.

Common problems

Grey squirrels

Squirrels may strip or remove developing cobs. Secure physical protection before kernels fill.

Problem source

Smuts

Smut fungi can create abnormal swollen growth on maize relatives. Remove affected material and avoid saving it for compost where diagnosis is uncertain.

Problem source

Mice and voles

Rodents eat seed and damage young plants or ripe cobs. Indoor sowing and physical protection reduce access.

Problem source

Slugs and snails

Young seedlings are most vulnerable. Inspect hiding places and combine barriers, habitat management and wildlife-friendly controls.

Problem source