Corn

Wait for warm soil, and sow in a block not a row

Once soil at 2 inches has reached 60F, sow seed 1 inch deep, 7-9 inches apart, in rows 24-30 inches apart. Hold supersweet (sh2) seed back until the soil hits 65F (UMN Extension, https://extension.umn.edu/vegetables/growing-sweet-corn). Plant at least three or four short rows side by side rather than one or two long ones. Corn is wind-pollinated and every kernel needs its own silk caught by a falling pollen grain, so a single long row sheds most of its pollen into the neighbours' yard and gives you gap-toothed ears. Shrunken sh2 seed carries a smaller starch reserve and rots in cold ground where standard su seed germinates fine.

Stagger the sowings, and keep the sweet corn types apart

Rather than planting the whole patch at once, sow again every 10 to 14 days, or when the previous planting has three to four leaves. If you are growing a supersweet alongside a standard or sugar-enhanced variety, separate them by about 400 yards or sow them two weeks apart so they do not shed pollen together. Each planting ripens in one week-long rush, so a single sowing means a glut and then nothing. The separation matters because pollen from a su or se plant landing on a supersweet silk turns that kernel starchy - the cross spoils this year's ears, not just next year's seed.

Do not let corn go dry at tasseling and silking

From the first tassel through silking and ear fill, keep the soil evenly moist. This is the stretch where corn must not dry out, whatever else in the garden has to wait. A silk only carries pollen down to its kernel while it is still turgid, so drought at this moment aborts kernels outright. Stress here cuts yield, kernel quality and flavour at once, and corn gets no second chance - the ear is made only once.

Sidedress nitrogen when the corn is knee high

When plants are knee high with 8 to 10 leaves, work about half a pound of 21-0-0 per 100 square feet into the soil alongside the row and water it in. Add another quarter pound when the first tassels or silks appear. Corn is a grass and a heavy nitrogen feeder, and it decides ear size during the fast growth just before tasseling. Starve it in that stretch and you get short ears that no amount of later feeding will lengthen.

Check the whorls for corn borer before the tassel pushes out

In the late whorl stage, before the tassel emerges, examine 20 stalks at each of five spots across the patch. Look down into the whorl for tiny shot holes with a little light powdery frass beside them. Treat with a Bt product such as Dipel or XenTari if 30 or more of those 100 stalks are infested. For very early, high-value corn you may drop that threshold to 15. Borer larvae feed in the open whorl only briefly before tunnelling into the stalk, and once they are inside no spray can reach them. The late whorl is the single window where treatment still works, which is why you scout on a stage and not a date.

⚠ Below the threshold, do not spray - the scouting count exists precisely so you skip applications the patch does not need. Bt kills only caterpillars that eat it, so it has to go on while the larvae are still feeding in the whorl. This is a pre-tassel treatment - no pollen is shedding yet, which keeps it clear of foraging bees.

Corn earworm: one oil drop per ear, once pollination is finished

Wait until the silks have reached full length and just begun to wilt and turn brown, about 5 or 6 days after half the patch has silked. Mix 1 part Btk with 20 parts vegetable oil and place 5 drops, about half a millilitre from an eyedropper, directly on the silk channel at the tip of each ear. The earworm moth lays on fresh silks and the larva crawls straight down the silk channel into the ear tip, feeding almost not at all on the outside of the plant - which is why broadcast sprays miss it. The oil sits in the channel the larva must pass through and carries the Btk to the one spot where it will actually be eaten.

⚠ Timing is the whole rule: oil on green silks interferes with pollination and gives you poorly filled ears, and once the silks are fully brown the worms are already inside and out of reach. Btk kills only caterpillars that ingest it, and this directed drop keeps it off the tassels that bees collect pollen from. Do not substitute a broadcast carbaryl or pyrethroid spray here - it would go on while pollen is still shedding and bees are working the tassels.

Pick at milk stage - the thumbnail decides, not the calendar

Start checking about 17 to 24 days after the first silks appeared. The ear is ready when the silks are dry and brown, the husk is still green, and the tip kernels are plump. Pull back a little husk and puncture a kernel with your thumbnail: clear juice means wait, milky juice means pick. The 17-to-24-day window swings with temperature, so the same variety ripens on a different date every year and in every zone - the calendar cannot tell you but the thumbnail can. Past milk stage the sugars turn to starch and the kernels go doughy.