How to Sow Sweet Corn: Block Planting, Depth and Spacing

Sow in a block, never in a single row

Sweet corn carries its male flowers in the tassel at the top of the plant and its female flowers halfway down as the silks on each young ear. Pollen falls from the tassel and has to land on the silks, and it travels on the wind. In a single long row most of that pollen blows straight past the plants and lands on the path, which is why a row of corn gives you cobs with scattered kernels and bald patches.

Plant in a square block at least four rows deep instead. Sixteen plants in a four by four block will pollinate far better than forty plants strung out in one line. If you only have room for a dozen plants, make it three rows of four rather than one row of twelve.

If you are growing more than one kind of sweet corn, keep the blocks at least 25 feet (8 m) apart or sow them three weeks apart. Cross pollination between a supersweet type and a standard one gives you starchy, tough kernels in the same season, not the next.

Sow by soil temperature, not by the calendar

Sweet corn seed rots in cold, wet ground. The seed is shriveled and starchy compared with field corn, it takes up water fast, and if the soil is below about 55F (13C) it sits there swelling until it goes moldy. Wait until the last frost has passed and the soil at seed depth has reached 60F (16C). Supersweet types are fussier still and want 65F (18C).

At 60F (16C) germination takes 10 to 12 days. At 70F (21C) it takes 5 to 7. Sowing two weeks early does not put you two weeks ahead, because the later sowing catches up and the early one gives you gaps in the block. Push a soil thermometer in to the depth of your knuckle in the morning and read it before you commit the seed.

1. Warm the bed first Lay black plastic or a cloche over the bed a week ahead. It lifts soil temperature by five to nine degrees F (three to five C) and lets you sow a fortnight sooner.
2. Read the soil in the morning Take the temperature at two inches (5 cm) deep, early in the day, not after an afternoon of sun. The morning figure is the one the seed experiences.
3. Sow a second block three weeks later A succession sowing three weeks after the first stretches the picking season, because a block of corn all ripens within about a week.

Depth, spacing and how many seeds per station

Sow one inch (2.5 cm) deep in a moist spring soil, or up to two inches (5 cm) if the surface dries fast. Corn pushes up a single strong shoot and copes with depth better than most vegetables, but seed sown shallow in dry ground simply never comes up. Firm the soil over each seed with your knuckles so it makes contact with damp earth.

Space stations 12 inches (30 cm) apart with 24 to 30 inches (60 to 75 cm) between rows. Crowding is the second most common cause of poor cobs after bad pollination: plants closer than 10 inches (25 cm) shade each other, run thin, and produce a small cob or none at all. Sow three seeds at each station and thin down later rather than trying to place single seeds perfectly.

Thin to one plant when they are four inches tall

Once the seedlings are three to four inches (8 to 10 cm) tall and have three leaves, thin each station to the single strongest plant. Leaving two is tempting when germination has been patchy, but two plants on one station compete for the same water and end up with two undersized cobs instead of one good one.

Cut the surplus off at soil level with scissors rather than pulling. Corn roots knit together fast, and pulling one seedling lifts and loosens the one you meant to keep. A plant whose roots are disturbed at this stage rarely catches up.

1. Pick the thickest stem, not the tallest The tallest seedling is often the one that was stretching for light. Keep the one with the broadest leaves and the stoutest base.
2. Cut, do not pull Snip the others off at ground level. The stumps rot away in a fortnight and the remaining plant never notices.
3. Fill gaps from the same block If a station failed entirely, you can lift a spare seedling from a neighboring station with a trowel and a good ball of soil, but only in the first week after emergence.

Protecting the seed and the seedlings

Birds pull newly sown corn seed out of the ground, and mice and crows will work along a whole row overnight. The seed is large, sweet and easy to find, and the fortnight between sowing and a leafy seedling is the vulnerable window. Cover the block with netting, row cover or a cloche until the plants are six inches (15 cm) tall.

Slugs and cutworms take the seedlings after that. A cutworm cuts a plant off at soil level overnight, and because each station now holds a single plant, one cutworm equals one lost cob. A collar cut from a plastic bottle pushed an inch into the soil around each seedling stops them completely.

1. Net the block from day one Lay netting or row cover over hoops immediately after sowing. Take it off once the plants are six inches (15 cm) tall and the birds have lost interest.
2. Record the sowing date on the spot Write down the date you sowed, the soil temperature you read, and the date the first shoots broke through. Next season those three numbers tell you exactly how early you can safely start, which is worth more than any general advice.
3. Note where each block sat Mark the position of the block on a plan. Corn is a heavy feeder and you want it on different ground next year, and knowing where it stood also tells you where to expect volunteer seedlings.