Why Aren't My Sweetcorn Cobs Filling Out?

Gappy cobs almost always mean poor pollination

Peel back a half-empty cob and you'll see the pattern: full kernels next to bare gaps. Each one of those gaps is a single silk — that fine thread coming out of the top of the cob — that never caught any pollen. No pollen, no kernel.

Sweetcorn is wind-pollinated. The feathery tassels at the top of the plant shed pollen, and it has to drift down onto the silks below. Every silk leads to one potential kernel, and every kernel needs its own grain of pollen. So a gappy cob isn't a disease or a feeding problem — it's a delivery problem.

The good news: it's one of the easiest things to fix once you know what's going wrong.

Ranked causes and fixes

1. You planted in a single row, not a block

This is the big one. A long, thin row lets the wind carry pollen sideways and away, so very little lands on the silks. The plants at the ends barely get pollinated at all.

Fix: grow sweetcorn in a square block — at least 4 plants by 4, never a single line. In a block, pollen drifting in any direction lands on a neighbour. This one change does more than anything else, so it's worth reading why sweetcorn is grown in blocks before you sow next year.

2. Too few plants

Even a tidy little block of half a dozen plants can struggle. With so few tassels there simply isn't much pollen in the air, and a still, calm UK day means even less of it moves.

Fix: grow more, not fewer. Aim for a minimum block of around 12 to 15 plants. More plants means a denser cloud of pollen and far better odds for every silk.

3. Dry roots while the cobs are filling

In a dry UK summer spell — often July into August, just as the cobs are swelling — plants under water stress can abort kernels or fill them unevenly. The pollination happened, but the plant couldn't carry the kernels through.

Fix: water deeply once the silks appear and keep going through cob-fill, especially on light or sandy soil and in containers. A good soak a couple of times a week beats a daily splash. A mulch around the base holds that moisture in.

4. You didn't give pollination a helping hand

UK summers can be still and damp, and a calm, overcast week right when the tassels are shedding means the pollen never travels. Left to the weather alone, pollination can be patchy.

Fix: hand-pollinate. On a warm, dry day when the tassels are dusty with pollen, walk through the block and gently tap or shake each plant so pollen showers down onto the silks below. For a more thorough job, snap off a tassel and brush it directly over the silks of each cob. Do this every couple of days while the tassels are active.

How to prevent gappy cobs next year

• Plant in a block, always. A square of at least 12 plants beats a long row every time. This is the single most important habit.
• Sow one variety at a time. Avoid having a supersweet and a normal variety flowering together nearby — cross-pollination can spoil the eating quality and lead to disappointing kernels.
• Don't sow too early. Sweetcorn hates cold soil. Sow indoors in April and plant out after the last frost — check your local frost date — or sow direct under a cloche from late May once the soil has warmed.
• Water through cob-fill. Keep the roots steadily moist from the moment silks appear right through to harvest.
• Hand-pollinate on still weeks. If the tassels are out and there's no breeze, give them a daily tap. It takes seconds.

Get the block right and lend the wind a hand, and you'll be pulling up cobs that are full from tip to base. For the full picture on sowing, spacing and when to harvest, head back to the sweetcorn guide.