Start planting sweet corn early. When to plant sweet corn? Rowcover can be used to pre-warm soils (and then keep cold temperatures and birds off the germinating seeds). Clear plastic mulch can increase soil temperature and germination rate, and conserve moisture, producing earlier harvests. Spread the plastic over the seeded beds and slit it when the seedlings emerge. Cut and remove it thirty days after emergence. Weed-free seedbeds are needed for this method to work organically, and plastics disposal is an issue.
Transplanting Sweet Corn
Direct sowing is the usual way of planting sweet corn, but transplanting can also be successful. It is important to transplant before the plant gets too big and the taproot takes off. 2-3” (5–7.5-cm) plants seem OK. Corn has no tolerance to frost. Escape from a late spring frost is possible if the seedlings are less than two weeks old and not yet very tall, as the growing point may still be underground. Thus, in a spring that promises to be warm and dry, you can risk an early planting as much as 2–3 weeks before the last frost date. Having some transplant plugs for a backup helps reduce the risk level. We usually prepare some plugs the same day we sow our first corn outdoors and use these to fill gaps at the first cultivation. We use 200-cell Styrofoam Speedling flats (1", 2.5 cm cells). We float these in a tank of water until we set them out. Some vegetable seedlings would drown if continuously in water, but corn does not. The plugs transplant easily using butter knives.
But don’t plant corn too early! Sweet corn needs warm soil. 50°F (10°C) is the absolute minimum, and applies to treated seed and OP or (su) varieties only. 60°F (15.5°C) is better for most, and 65°F (18°C) or higher is required by some varieties. Common phenology signs for the season being advanced enough to sow corn are that oak leaves are the size of squirrels’ ears and that ragweed is germinating. For us the first corn sowing date is usually around April 26, which is also our average last frost date. Either pre-warm the soil, use transplants, or wait for warm soil.
Avoid mixing types of corn. There’s a confusing aspect of hybrid corn varieties: There are several genotypes, and if you inadvertently plant a mixture of different types, it can lead to starchy unpleasant-flavored corn. Also don’t plant Indian corn, popcorn or any kind of flint or dent corn within 600′ (180 m) of your sweet corn. For this reason we grow only sweet corn in our garden. Ignore those cryptic catalog notes at your peril!
Normal sugary (su or ns) types have old-fashioned corn flavor but are sweeter than open pollinated varieties, although the sweetness disappears fairly rapidly after harvest. Not a problem for home gardeners who can cook the corn they harvested earlier that day. Most can germinate well in cool soil. Sugary-enhanced (se) and sugary enhanced homozygous (se+ or se-se) types are more tender, and usually sweeter, than (su), and slower to become starchy after harvest. Triplesweet sugary enhanced (se-se-se) were created to be sweeter than se-se. We grow these first three types, and avoid the newer types below – sweet and simple!
Synergistic (se-se-se-sh2) types are combinations of genetics from several genotypes. Each ear has 75 percent (se) kernels and 25 percent (sh2) kernels. They do not require strict isolation from other corn types. They are flavorful, tender and sweet, but only when they are ripe. If picked too soon, they are a watery disappointment.
The Super Sweet (sh2) varieties, also known as shrunken, are very sweet and slow to become starchy. They have very poor cold soil germination. The kernels are smaller than other corns, giving this type its name. “Augmented shrunken” types contain the sh2 gene and some of the tenderness from the se types. Cross-pollination with other corn groups will produce the dominant genetics of field corn, that is, starch not sugar. Don’t mix Super Sweet sh2 types with any other corn.
Plant corn in blocks, not single rows. Corn is wind pollinated (although you will find plenty of bees collecting pollen). For well-filled ears, plant in patches at least four rows wide. Inadequate pollination leads to ears with flat undeveloped patches among the kernels.
Water corn wisely and remove weeds. Corn seed must have moisture to germinate. If you use a push seeder, irrigate after sowing. Because we sow small areas of many different varieties, and because people love to plant corn, we sow by hand. We measure and make furrows (drills). Then we flood the drills with water from a hose, then hand sow into the mud. After covering the seed and tamping the soil, we ignore the patch until the seed germinates. The watering in the furrow reliably provides enough moisture to get the plants up out of the ground. Our method delivers water right where the seed needs it. If you use drip tape, you might set out the tape, turn on the water for long enough to mark the soil with damp spots, then sow those spots with a jab planter.
The most important times for watering are silking (when the silks first become visible outside the husks) and ear-filling. Generally, corn needs cultivating at least twice: once two weeks after sowing and once at four weeks. Corn plants closer than 8” will compete with each other, so be sure to thin.
Plant several corn varieties each time. Here’s some we like:
Bodacious, 77-day (se) yellow, great flavor for one this early
Kandy Korn, 89-day (se) yellow workhorse
Silver Queen (see image), 96-day (su) white longtime favorite with some drought tolerance and insect resistance
Luscious, 77-day (se-se-se) bicolor (organically grown, good cold soil emergence)
Tuxedo, 80-day (se) yellow (tightly-wrapped, earworm resistant)
Sugar Pearl, 72-day (se+) white (very early, on short plants)
Argent, 86-day (se) white (tasty with tight earworm-defeating husks)
Spring Treat, 66-day (se+) yellow, one of the earliest yellow sweet corns with good cold soil tolerance
Sparkler, 78 day synergistic (se-se-se-sh2?) bicolor, high-yielding, big ears, good husk protection, strong plants.
For an even supply, sow several different varieties, with differing days to maturity, on the same date. We often use Bodacious, Kandy Korn and Silver Queen together, to get over two weeks of harvests.
To have a continuous supply of sweet corn all summer, a bit of planning and record keeping is called for. The easy and approximate method of getting a good supply is to sow more corn when the previous sowing has three or four leaves or is one to two inches (2.5–5 cm) tall.
To fine-tune to get the most continuous supply, nothing beats real information about what happened, written at the time it happened. We write down actual sowing dates as well as harvest start and finish dates. Having graphs of sowing and harvest dates for each crop has been very useful for planning effective planting dates. You can read more about this in my book Sustainable Market Farming, or see my slideshow on SlideShare.net – search for my name and then click on Succession Planting for Continuous Vegetable Harvests. The slide show is also on my blog.
We make six plantings to provide fresh eating every two weeks: April 26, May 19, June 6, June 24, July 7 and July 16. The planting intervals are 23, 18, 18, 13 and 9 days. Because we plant three varieties, new corn comes in three times during each two weeks.
Don't Stop Too Soon
To calculate the last worthwhile sweet corn sowing date, add the number of days to maturity and the length of the harvest window (7–14 days) and subtract this number from your average first frost date. For our October 14 frost date, using an 80-day corn as an example, 80 + 7 = 87 days, which brings us back to July 19 for our final sowing date.
We’re looking forward to plenty of sweet corn this year – we’re off to a good start!