Why Did My Corn Have Poor Pollination This Year?

Corn pollination failure is the most common reason for disappointing harvests in the home garden. A sparse, gapped cob with half the rows of kernels missing, or a short underdeveloped cob where a fat one was expected, almost always traces back to something that interrupted the transfer of pollen from the tassels to the silks during the ten-day window when both were active. Fortunately the causes are well understood and largely preventable.

Single row or small isolated patch

The most common cause of poor pollination in the home garden is planting corn in a long single row rather than a block. Corn is wind-pollinated: pollen falls from the tassel at the top of each plant and must travel sideways to reach the silks on neighbouring plants. In a long single row, almost all pollen travels sideways — out of the row and into empty air — rather than landing on silks. In a block planting, pollen from each tassel is surrounded by silks from many neighbouring plants on all sides, and pollination is thorough. Always plant corn in a block at least four plants wide and four plants deep. Even a three-by-three block of nine plants is far superior to a single row of any length.

Prolonged wet weather during pollen shed

Pollen is rendered non-viable when wet, and it is washed from silks before fertilisation can occur during heavy rain. A week of overcast, wet weather during pollen shed means silks are continuously wetted and dried, and viable pollen in the air is minimal. There is little you can do about the weather, but hand-pollinating each morning during dry windows between showers compensates for wind-pollination failure. Snap a tassel and rub it directly on the silks of surrounding plants.

Heat killing pollen viability

Pollen viability drops rapidly above 35°C and is effectively zero at 40°C. During a heat wave, pollen is shed but is non-viable within an hour or two of release. Combined with the short daily window in which pollen shed occurs — mostly morning hours — a hot sunny morning can see an entire day's pollen shed wasted. Water deeply to maintain plant health; nothing else can compensate for heat-killed pollen except hand-pollinating in the early morning before temperatures peak.

Tassel-silk timing offset

In an ideal season, tassels shed pollen while silks are at peak receptivity. If plants are stressed or sown at different times, or if two different varieties with different flowering windows are grown together, tassels may have finished shedding before all silks emerged, or silks may emerge after pollen shed has ended. Grow a single variety sown all at once in a block to maximise synchrony.

How to hand-pollinate

If your block is small or conditions have been unfavourable, hand-pollination is simple and highly effective. Each morning during pollen shed, snap a fully open tassel from a plant and rub it directly across the silks of every plant in the block. The pollen you can see as yellow dust on your hands is enough to pollinate many cobs. Repeat for five to seven days and every silk in the block will be covered.