When to plant sweetcorn

Sweetcorn needs warmth from start to finish. Seed sits and rots in cold soil below about 50–54°F, and frost kills the plants. The summer also has to be long enough to ripen the cobs, which is why most gardens in Central Europe and the UK grow early or mid-early varieties.

The safest start in both regions is indoors: sow one seed per deep pot or root trainer in late April or May, somewhere at 64–70°F. Seeds come up within two weeks. Harden the plants off and plant them out in late May or early June, after the last frost, when they are about 3–6 in tall. Use deep, individual pots or root trainers rather than seed trays.

In mild areas you can sow directly from mid-May to the end of May, or from the end of April under fleece. In the UK, direct sowing only really works in the south.

Choose the warmest, sunniest spot you have, sheltered from strong wind. Sweetcorn is a fairly hungry crop: work in a couple of buckets of compost or well-rotted manure per square metre. Cold, waterlogged soil suits it poorly.

The key to full cobs is planting in a block. Maize is pollinated by wind: pollen falls from the tassels at the top onto the silks of the cobs below. In a single long row much of the pollen blows away, and you get gappy cobs. Set plants 14–18 in apart each way in a block at least three or four rows deep.

Sow outdoor seed 1.2–2 in deep, a little deeper on light soil so birds find it harder. In pots, 0.8–1.2 in is enough.

A 120 × 80 cm raised bed fits 6–8 plants in two rows. That is a small block, so expect the odd gap in the cobs; a larger square block pollinates better.

Keep the block weeded while the plants are young and mulch to hold moisture. Water in dry spells, and above all when the tassels and silks appear and the cobs swell: dry soil at that stage means small, poorly filled cobs.

Draw some soil up around the base of the stems as they grow; it steadies the plants against wind. Tall varieties, up to about 79 in, may need a cane or string on exposed sites.

Early varieties ripen from August, later ones into September. Cobs are usually ready about 17–24 days after the silks first appear, when the silks have turned brown and dry but the husks are still green.

To be sure, peel back a little of the husk and push a thumbnail into a kernel. Watery juice means not yet; creamy, milky juice means ready; a thick paste means you've left it too long.

From the moment it's picked, the sugar starts turning to starch, so cook it the same day or keep it cold. Modern extra-sweet varieties hold their sweetness much longer than old ones.

Sweetcorn comes in standard sweet (su), sugar-enhanced (se) and supersweet or extra-sweet (sh2) types. Supersweet varieties are the sweetest and stay sweet longest after picking, but are a little less vigorous and want warmer soil to germinate.

Because maize is wind-pollinated, varieties cross. If a supersweet variety is pollinated by a standard one, by popcorn or by field maize, its kernels turn starchy and tough. Grow only one type, or sow two types so that they flower at least two weeks apart. Field maize nearby is a real risk: commercial growers keep several hundred metres away from it.

Mice and birds take freshly sown seed, which is one more reason to start in pots. Slugs go for young plants. Later, ripening cobs attract badgers, squirrels, rats and birds; netting or bags over the cobs help.

Corn smut (Ustilago maydis) turns kernels and other parts into large, swollen, silvery-grey galls full of black spores. It is worse on stressed plants after drought, heat or damage. Cut galls off before they burst, and choose resistant varieties if it keeps coming back.

In areas with field maize, the European corn borer lays eggs in June and July; its caterpillars tunnel into stems and cobs. Frit fly larvae can twist and stunt young plants; fleece over seedlings keeps it off.

Maize is a grass, unrelated to most vegetables, so it fits easily into a rotation. Don't grow it after maize: corn borer larvae overwinter in old stalks. Leave two to three years, and chop and dig in old stems after harvest.

The 'Three Sisters', maize with climbing beans and squash, is a traditional Indigenous American planting and a favourite in garden books. It can work on a generous plot, but we found no trials for home gardens in our climate showing it beats growing the crops separately. In a small bed, expect the three to compete for light and water.

What does matter: maize is tall and casts shade, so put the block on the north side of the plot, where it won't shade lower crops.