Sweetcorn Growing Guide
Sweetcorn (Zea mays) is a tall plant that produces cobs of kernels, valued for its superior flavour and sweetness when freshly harvested. It is best suited for warm, sunny sites and is tender, requiring indoor sowing or starting as young plants before transplanting outdoors after the last frost. Each plant typically yields two cobs that ripen from mid-summer onwards. It is not suitable for growing in containers due to its height. Varieties include 'supersweet', 'tendersweet', baby corn, and popcorn types.
This guidance applies to the crop in general. Individual varieties and local conditions can differ.
Planting information
For indoor sowing (mid-April to early May): Sow seeds 2.5cm deep, singly in deep pots, modular trays, or toilet roll tubes. Germinate best at 18–21°C on a warm windowsill or in a heated propagator. Keep seedlings in bright light and water lightly every few days. Transplant outdoors when they are at least 8cm tall. For outdoor sowing (late spring, milder regions): Only in warm, light soil with a soil temperature above 10°C. Warm the soil with cloches before sowing. Sow seeds 2cm deep, in a grid (not a row) spacing 34–45cm apart in each direction. Sow two or three seeds at each point and thin to the strongest seedling. For baby corn varieties, sow 20cm apart in rows. Plant indoor-sown or newly bought plants outdoors in their final position by late May or early June, after all risk of frost has passed. Plants should be 8–15cm tall and hardened off first. Plant in a block or group (not a long row) to maximise wind pollination. Space plants 34–45cm apart, depending on the variety. For baby corn, plant 20cm apart. Avoid planting 'supersweet' varieties near standard sweetcorn to prevent cross-pollination.
Harvesting information
Cobs begin to ripen from mid-summer. Test ripeness when tassels turn chocolate brown: peel back a little husk and pierce a kernel. Ready if creamy liquid emerges; unripe if watery; over-mature if paste-like (can be dried for popcorn). To pick, twist the ripe cob and pull sharply from the stem. Harvest baby corn when immature and finger-sized. For popcorn, leave cobs on plants until over-ripe and starting to dry, then bring indoors and hang in a cool, well-ventilated place to dry fully until kernels are completely dry.
Storage information
Sweetness declines rapidly after picking as sugars turn to starch, especially in older varieties; eat as soon as possible. 'Supersweet' varieties retain sugar content longer. For popcorn, dry harvested cobs completely indoors in a cool, well-ventilated place.