Sweet Corn Pollination: Why Cobs Have Missing Kernels

One silk, one kernel, no second chances

Each silk emerging from the top of a young cob is connected to a single ovule inside the husk. A grain of pollen has to land on that silk, grow a tube down its length, and fertilize the ovule. That ovule then becomes one kernel. A cob with 600 kernels needed 600 successful pollinations, and every gap you can see in a cob is one silk that never received pollen.

This is why corn problems look so specific. Half a cob filled and half empty, or kernels only at the base, or a scattering of kernels with bare patches between them, are all the same fault: pollen did not reach enough silks. Nothing you do after the silks turn brown will change it.

The timing window is about ten days

The tassel opens and starts shedding pollen two to three days before the silks appear on the same plant, and it keeps shedding for about a week. Silks are receptive for around ten days from the moment they emerge, growing an inch (2.5 cm) a day until they are pollinated, at which point they stop and start to dry. That overlap is the whole opportunity.

Pollen is shed mainly between mid morning and early afternoon on dry days, and an individual grain stays viable for only a few hours in warm weather. Rain during shedding washes pollen off the tassels and out of the air, and temperatures above 95F (35C) kill it outright, which is why a heatwave at exactly the wrong week produces a block of near empty cobs.

1. Watch for the tassel opening When the tassel spikes separate and you can see small dangling anthers, shedding has begun. Flick a tassel over a dark sleeve and you will see a dust of yellow pollen.
2. Check the silks the same week Silks push out of the tip of the husk a couple of days later. Fresh silks are pale, moist and slightly sticky. Brown, dry silks are past receiving pollen.
3. Write down both dates Note the day the tassel first shed and the day the silks first showed. If they were more than four days apart, that gap explains a poor cob, and it is the number to compare against next season.

Hand pollinating takes five minutes a morning

In a small block, hand pollination is the single most effective thing you can do, and it costs a few minutes on three or four mornings. Work on a dry, still morning between about 9 and 11, when the anthers are open and the pollen is loose. Wet tassels shed nothing, so skip a rainy morning and go out the next day.

The simplest method is to bend a tassel over the silks of a neighboring plant and tap it. The thorough method is to collect pollen in a paper bag from several plants, then sprinkle a pinch onto every set of silks in the block. Collecting from several different plants matters, because pollen from a different plant sets a fuller cob than a plant's own pollen.

1. Collect pollen into a paper bag Mid morning, bend three or four tassels into a dry paper bag and shake them. You want a teaspoon of fine yellow dust in the bottom. Use paper, not plastic, or condensation ruins it.
2. Sprinkle it onto every set of silks Tip a pinch straight onto the silk tuft at the top of each cob. Do not pull the husk back or strip the silks. The whole tuft needs coating, not just the outside.
3. Repeat for three mornings Silks keep emerging over several days, and the ones that appear last become the kernels at the tip of the cob. Three or four consecutive mornings is what fills a cob right to the end.
4. Or just shake the block If you have no time for any of that, walk through the block at mid morning and give every stem a firm shake for three days running. It is cruder but it is far better than nothing.

Silks stop growing within a day of being pollinated. If the silks on a cob have visibly stopped extending and are starting to dry from the tip, that cob is done and no further hand pollination will add kernels.

Keep different types apart

Sweet corn kernels are affected by the pollen that fertilized them in the same season, not the next one. If a supersweet block is pollinated by a standard sweet type, or by popcorn or ornamental corn growing nearby, the kernels you harvest turn starchy and tough. The plant looks perfectly healthy and the cob is well filled, but it tastes wrong.

Separate blocks of different types by at least 25 feet (8 m), and further if the prevailing wind runs between them. The easier solution in a small garden is to stagger the sowings by three weeks so the two blocks shed pollen at different times, or simply to grow one type.

Heat, wind and rain during the critical week

The weather during the shedding week has more effect on your crop than anything you did in spring. Sustained temperatures above 95F (35C) kill pollen within a couple of hours of release, heavy rain strips it from the air, and a completely still, humid morning keeps it clumped in the anthers instead of drifting.

You cannot change any of that, but you can work with it. Water the block heavily going into a forecast heatwave, because a well watered plant sheds and silks closer to schedule. Hand pollinate on the first dry morning after rain. And where summers are reliably brutal, shift the sowing earlier so tasselling happens before the worst of the heat arrives.

1. Water ahead of a heatwave Soak the block the day before high temperatures arrive. Drought and heat together during shedding is the combination that empties cobs.
2. Go out the morning after rain Rain washes out whatever pollen was airborne. The first dry mid morning afterwards is the moment to hand pollinate the remaining silks.
3. Log the weather against the dates Record what the weather did during the shedding week alongside how the cobs turned out. After two seasons you will know whether your local summer needs an earlier sowing.