Why Is My Sweetcorn Not Filling Out?
Sweetcorn cobs with gaps or only a few kernels are usually the result of incomplete pollination. Each silk needs pollen to make one kernel, so a single straight row often performs poorly. Wind, warm weather and the way you arrange your plants all make a real difference to a successful crop.
Check a cob at the right stage before deciding it has failed. Sweetcorn is usually ready when silks have turned brown and dry, and a gently pressed kernel releases a milky liquid rather than clear water. Peel back only a small section of husk to inspect, then cover it again if it needs longer. If kernels are missing in irregular patches, especially towards the tip, incomplete pollination is the most likely explanation. Poorly filled cobs are still edible, but you can use the diagnosis to improve future crops. Note whether all plants were flowering together, because staggered tassels and silks can miss their chance to exchange pollen.
Grow sweetcorn in a block, not a single row. A square or rectangular block of at least several short rows allows wind-blown pollen from the tassels to fall onto silks below and around them. Space plants according to the packet so they are close enough to share pollen but not competing heavily for light and water. On a small plot, four rows of a few plants are better than one long row. Avoid planting sweetcorn at the edge of a windy exposed allotment if you have a more sheltered, sunny position. A permeable windbreak can soften damaging gusts without stopping the gentle airflow needed for pollination.
Help pollination by hand if weather is still, wet or if you only have a small block. When tassels are shedding visible yellow pollen and fresh silks are emerging from cobs, gently shake each plant in late morning on a dry day. You can also tap tassels over the silks of neighbouring plants. Repeat this for several days as new silks appear. Do not pull at silks or remove husks, as they protect the developing cob. Hand pollination cannot entirely overcome a cold season, but it is a worthwhile extra step in a sheltered garden or polytunnel. Wash pollen from hands before touching other crops if you are moving around the plot.
Keep soil deeply and evenly moist from the time plants begin growing rapidly until cobs mature. Sweetcorn is a large, thirsty crop, and drought during flowering or kernel filling can reduce cob size and fullness. Water at the base in the morning, soaking the root zone rather than sprinkling leaves lightly. Mulch between plants with garden compost or well-rotted manure once soil has warmed, leaving a little space around stems. In dry regions or sandy soil, a soaker hose can make watering more even if you already use one. Do not allow plants to sit in waterlogged ground either, as roots need air to support their tall, heavy growth.
Give sweetcorn a sunny, fertile bed and avoid checking young plants with cold. Sow or plant after the general last spring frost, once the soil has warmed. In cooler northern gardens and Scotland, raising plants briefly in module trays can provide a head start, but harden them off and transplant gently to avoid root disturbance. Use fresh peat-free multipurpose compost for modules. Improve the planting area beforehand with garden compost or well-rotted manure, then keep weeds down with shallow hoeing or mulch. Sweetcorn feeds heavily, so use a suitable fertiliser only as directed if growth is pale or weak. Strong plants make more pollen and have greater capacity to fill their cobs.
For next season, choose an early-maturing variety if your summer is often short, and sow all plants of one variety together so flowering is synchronised. If you grow more than one sweetcorn variety, keep their flowering times in mind because pollen can cross between them; this matters most if you save seed. Harvest promptly once kernels are milky, because sweetness declines after picking. Poor filling is frustrating, but it is rarely permanent soil damage or a mysterious disease. Reorganising the crop into a block, watering reliably and helping pollen move on calm days normally provides the biggest improvement. Record your layout and weather, then adjust the block size next year.
Frequently asked questions
Why are kernels missing mainly from the tip of the cob?
The silks at the tip are often the last to emerge and can miss pollen if tassels have already finished shedding, or if weather is wet and still. A larger block of plants and several days of gentle hand pollination improve the odds. Drought during flowering can also make silks less receptive and worsen gaps.
Can sweetcorn grow in a polytunnel?
It can, but ventilation and pollination need extra attention. Open doors and vents during flowering, and shake plants or transfer pollen gently on dry days. Give each plant enough space and water generously. A sheltered outdoor block is often simpler, but a polytunnel can help in a cool area if light and airflow are good.
How many sweetcorn plants should I grow together?
Grow as many as your space allows, arranged in a block rather than one row. Even a modest group in four short rows usually pollinates more reliably than the same number in a line. More plants generally mean more pollen, provided they receive sun, water and sufficient spacing to remain healthy.
General guidance for UK home gardens and allotments. Conditions vary by region, soil and season; the RHS or a local garden centre can give site-specific advice. Page updated 19 Sep 2026.