Preparing for sweet corn

Our first sweet corn of the season. Bodacious
Photo Pam Dawling

Preparing for sweet corn

It’s too early for us to be planting sweet corn, but not too soon to be planning, preparing and thinking about it!

Crop Rotations and Cover Crops

Rotation Pinwheel Sus Mark Farm

If you don’t already have a crop rotation plan, this is the time to put one into place, especially if a lot of your ground is taken up with sweet corn every year! Our sweet corn occupies a lot of our space, so we pay attention to crop rotation. Corn is the only crop in the grass family that we grow, so this is not hard. We keep two or three years without corn between the corn years. Here’s the crop and cover crop sequence for each of our six sowings:

Sowings 1 & 2: The winter before the corn, we have a winter-killed cover crop of oats, (or sometimes a clover patch which has been growing all the previous year). Half the patch of this early corn is followed by oats in August and then garlic in early November. The other half gets oats and soy in August, to be winter-killed. This area will be easy to work up in early spring for our broccoli and cabbage.

Sowings 3, 4, & 5: The previous winter cover crop is winter rye or wheat with crimson clover (if we sow before 10/14) or winter rye or wheat with Austrian winter peas if after 10/15. The corn is followed by more rye or wheat and crimson clover in October. The following year we will plant potatoes here in June. The clover will have plenty of time to reaching flowering and therefore have plenty of nitrogen nodules on the roots.

Cover crop of winter rye, hairy vetch and crimson clover.
Photo Kathryn Simmons

Sowing 6: The previous winter cover crop is winter rye and Austrian winter peas. We undersow this corn with oats and soy. We mow high after harvest, and leave the oats and soy to grow until winter-killed. This patch is easy to prepare for our potatoes in March.

Late sweet corn undersown with oats and soy beans for a winter-killed cover crop. Photo Kathryn Simmons

For the later sowings of sweet corn, a good stand of a preceding winter cover crop mix including legumes can provide all the nitrogen the corn needs (100-125 lbs/acre; 112-142 kg/ha). The nitrogen nodules on the roots contain the maximum nitrogen when the legume reaches its flowering point, so this method doesn’t work for early sowings.

 No-till planting into strips tilled in a white clover living mulch sounds good but has been found tricky. Jeanine Davis addresses this in NCSU’s Organic Sweet Corn Production. The clover can out-compete the corn, become invasive and hard to get rid of. Soil temperatures will be lower (a disadvantage in spring) and slugs and rodents may increase. Trials of sowing corn into rolled and crimped hairy vetch are underway.

Undersowing (interseeding) cover crops such as white or crimson clover into the corn at the V5 or V6 stage is more successful. Ensure good seed-to-soil contact. The clover grows after the corn dies.

Some growers undersow with forage brassicas at last cultivation. Research shows this does not reduce corn yields. The forage can be harvested after the sweet corn harvest finishes.

Caring for Sweet Corn

Young sweet corn plants after imperfect hoeing.
Photo Pam Dawling

Never allow soil in corn plantings to dry out, especially with close planting. You might need more than 1” (2.5cm) per week for maximum productivity, although corn is more drought-tolerant than some crops. The most important times for watering are silking (when the silks first become visible) and while the ears are filling out. We use overhead irrigation for corn, which works well to also water undersown cover crops.

Corn plants closer than 8” will compete with each other, so be sure to thin. People used to recommend removing the suckers that came from the base of the plant, thinking it led to higher yields. This has been tested, and in fact it can damage plants and possibly even reduce yields. (Reports from Clemson and Colorado State).

Sweet corn needs cultivating at least twice, at two weeks and four weeks after sowing. Even better are four rounds: at 7, 14, 21 days and finally one around 35 days when plants are 18-20” (45-50cm) high. Corn is shallow rooted so avoid deep cultivation. We use a walk-behind BCS tiller, followed by hoeing (and thinning at the first cultivation).

Young sweet corn plants. To the right are ropes to mark rows for the next corn sowing.
Photo Bridget Aleshire

Each time we sow sweet corn, we wheel hoe or till between the rows of the previous planting, hoe and thin the plants to 8-12” (20-30 cm). We also till between the rows of the corn planting before that, hoe, and sow soybeans. Although they don’t supply the highest amount of nitrogen compared to other legumes, they are cheap, quick, somewhat shade tolerant, hinder weeds, and withstand foot traffic during harvesting.

While harvesting the corn we pull out any pigweed that has somehow survived our earlier efforts. Pigweed puts out its seeds in one big burst, so pulling up enormous pigweed is worthwhile, if it hasn’t yet seeded. Our soil has improved over the years, so it is now possible to uproot the 5ft (1.5 m) pigweeds. Sometimes we have to hold the corn plant down with our feet, but we do almost always succeed in getting the weeds out.

Silver Queen sweet corn with wilting pulled pigweed amaranth. Photo Kathryn Simmons

Flame-weeding can be used after sowing before the corn emerges, or after the crop is 2” (5cm) tall, using a carefully directed flame. See ATTRA Flame Weeding for Vegetable Crops

Sweet Corn Growth Stages

It is helpful to understand corn growth stages so that you know when the plant is most susceptible to damage, and when to take action. Corn growth stages are divided into vegetative (V) and reproductive (R). Stages are determined when at least 50% of the plants have reached or are beyond a particular stage.

Vegetative stages begin at emergence (VE), and each leaf with a fully developed collar is a new stage. Leaves within the whorl, not fully expanded and with no visible leaf collar, are not counted. Most sweet corn hybrids produce 18-21 leaves. Vegetative growth takes about 55-60 days after emergence. V3 (three leaf collars) begins 2-4 weeks after VE, and plants switch from using kernel reserves to photosynthesis and roots for nutrients.

Around V4, broadleaf weeds will stunt yields and should be removed. By V5, the numbers of potential leaves and ears are determined. Plants are 8-12” (20-30 cm) tall and the growing point may still be underground. Tillers (suckers, branches that grow from the lower five to seven nodes) appear.

V6-V8 – Beginning 4 to 6 weeks after VE, the growing point emerges above the surface, making the crop susceptible to frost, hail and winds. Beginning at about V6, the lower leaves may fall off naturally. At V7, rapid growth begins, and the number of kernel rows is determined. The number of potential kernels per row begins is set between V7-V16. By V8, the plant reaches 24” (60 cm) tall.

V9-V11 – At V9, tassels (not yet visible) and ear shoots are developing. New leaves appear every 2-3 days

V12 – The plant is 4ft (1.2 m) tall or more. Nutrients and water are important from this stage until the start of the R stages. The leaves are full-sized and about half are exposed to sunlight. “Brace roots” develop and the number of kernels per ear and size of the ear are set. Kernels can be damaged by insects and hail.

V15 – The plant is two weeks away from silking. Tassels are almost full-sized, but still hidden. Moisture and nutrient shortages now result in shorter ears and lower yields.

VT – Beginning 9-10 weeks after emergence, good pollination is essential to develop the kernels. Tassels are fully visible and silks emerge in 2-3 more days. Pollen shed begins, continuing for 1-2 weeks. Hail can be very damaging at this stage. Vegetative stages end when the corn develops a tassel (male flower). It takes about 20 days from tasseling to ripe.

Sweet corn growth stages. University of Illinois Extension

R1 – The plant has silks outside of the husks and is at its most vulnerable. Environmental conditions can greatly affect pollination. The worst is drought, which dries the silk out, reducing its ability to collect pollen falling from the tassels.

R2 (“Blister”) stage – The kernels are filled with clear liquid. About 12 days after silking, the silks darken and dry out. Stress (especially drought) can cause kernels to abort.

R3 – Milk stage – About 20 days after silking, kernel fluid turns milky, as starch accumulates. The effects of stress are not as severe after this stage, but can still lead to shallow kernels.

R4 – Dough stage – About 26 days after silking, the kernels have a dough-like consistency. Stress can produce scrawny kernels.

The dent stage (R5) and full maturity (R6) will only happen if you leave the plant to produce seed.

Corn growth stages can be estimated using corn growing degree days (GDD), accumulated daily from the date of planting. GDD are calculated by averaging the max and min temperatures in a 24-hour period. 50°F (10°C) is subtracted from that average temperature to give the GDD for the day. A Corn GDD Tool is available at the High Plains Research Climate Center. Stress, especially drought, can affect growth and GDD alone may not provide an accurate estimate of growth stage.

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