How Do I Prevent Cross-Pollination Ruining My Sweetcorn?

Corn is unusual among vegetable crops in that cross-pollination affects the current season's harvest in a direct and visible way. When sweetcorn silks receive pollen from a different corn type — field corn, popcorn, flint corn or even a different sweetcorn variety — the kernels that develop from those crossed silks inherit the genetics of both parents. Starchy corn types dominate, and the resulting kernels are tough, starchy and unpleasant. This effect, called xenia, is unique to corn and can ruin an otherwise perfect harvest.

What types of corn cause cross-pollination problems

All corn types — sweetcorn, field corn, popcorn, ornamental corn, flint corn, flour corn — are the same species (Zea mays) and will freely cross-pollinate. The problem is worst when a sweet variety receives pollen from a starchy type. Super-sweet (sh2) varieties are particularly sensitive: even a few kernels pollinated by a starchy type will be visibly hard and tasteless in an otherwise milky cob. Standard sugary (su) varieties are somewhat more tolerant of crossed pollen but still affected.

Isolation by distance

Corn pollen is wind-dispersed and can travel hundreds of metres in open conditions. The recommended isolation distance between incompatible corn types is:

• Super-sweet (sh2) varieties from any other corn type: at least 400 metres
• Standard sugary (su) varieties from starchy types: 250–300 metres
• Two super-sweet varieties of different genetic backgrounds: 400 metres if you want to save seed, though flavour cross in the harvest is less extreme than starchy crossing

In a typical suburban or rural garden with neighbouring farms or allotments, distance isolation is often impractical. Timing isolation is usually more achievable.

Isolation by timing

If you cannot achieve distance isolation, you can "time-isolate" your corn so that your patch tassels and silks at a different time than neighbouring corn. Stagger sowings by three to four weeks or choose an early-maturing variety that will finish silking before late-planted neighbouring corn begins. Check the "days to maturity" figure on your seed packet and plan accordingly. If you know your neighbour's corn is planted in June, plant yours in early May to flower in July before theirs does.

Bag isolation for seed saving

If you are growing an open-pollinated sweetcorn variety and want to save seed that comes true to type, you can manually control pollination by bagging tassels and ear shoots before they open, hand-transferring pollen within your own variety only. This is standard seed-saving practice and ensures complete purity.