How to Grow Sweet Corn

Choose the type before the variety

Sweet corn comes in three main genetic types, and the type matters as much as the variety name. Standard sugary (su) corn has the classic creamy texture but turns its sugar to starch quickly. University of Georgia Extension says su corn keeps only a few days. Sugary-enhanced (se) corn is noticeably sweeter but keeps the tender, creamy kernels. Supersweet (sh2) corn holds its sugar longest after picking, but its kernels are crisper and tougher-skinned. UGA rates se types better for eating unless the corn has to be stored or shipped. Many newer varieties combine two or three of these genes.

Supersweets have two drawbacks. Their seed germinates poorly in cold, wet soil, and their pollen is weak. If pollen from standard, sugary-enhanced or field corn reaches supersweet silks, the kernels turn starchy and tough. Maryland and Clemson both recommend keeping supersweet and standard corn about 400 yards apart. If you lack the space, time the plantings so they tassel at different times. Maryland suggests a two-week gap, while Clemson and UGA suggest maturity dates a month apart. UGA treats multi-gene varieties the same way as sh2.

Early varieties have smaller, less sweet ears but suit short, cool seasons, while later ones do best in long, warm summers (Utah State).

Extension services publish their own variety lists. Clemson recommends Early Sunglow, Illini Gold, Merit, Sugar Buns, Kandy Korn, Silver King, Silver Princess, Silver Queen, Ambrosia, Chubby Checkers and Peaches and Cream for South Carolina. UGA’s list for Georgia includes Bodacious, Silver Queen, Ambrosia and Providence, among others. For Florida, UF/IFAS recommends Silver Queen, How Sweet It Is, Sweet Ice, Sweet Riser and Early Sunglow.

Wait for warm soil

Seed sown in cold, wet soil rots. Clemson says not to plant until the soil at 4 inches is at least 50°F, and to wait for 65°F for supersweets. Clemson and UGA both suggest waiting for 55°F soil to avoid most seedling rots. UGA advises waiting at least two weeks after the average last frost. Utah State and Maryland plant once frost danger is past. UC Cooperative Extension’s germination table explains the caution: sweet corn takes about 22 days to emerge at 50°F, 12 days at 59°F and 4 days at 77°F.

To get an earlier crop, Utah State notes that sowing under or through clear plastic tunnels allows planting 3-4 weeks earlier, and that some gardeners set out transplants after the frost-free date. If you plant early, use su types, which handle cool soil best. The dates for your zone above show the window.

In warm-winter areas, sweet corn goes in during late winter or early spring so it matures before peak heat. Frost-free South Florida grows it from fall through winter, and the Phoenix area has a second window in mid to late summer.

For a longer season, either sow the same variety every 10-14 days (Utah State, Clemson) or every two weeks (UGA, Maryland), or sow early, mid and late varieties of the same type on one day. Utah State stops succession sowings in early July.

Plant in blocks

Corn sheds pollen from the tassels at the top of the plant, and the wind carries it to the silks on the ears. When too little pollen reaches the silks, kernels are missing and the ear tips stay bare. A block of several short rows keeps more pollen within reach of the silks than one or two long rows. Maryland recommends at least three rows side by side and prefers four.

Plant seed about 1 inch deep in full sun. UGA and Maryland want 8-10 hours of sun. Spacing recommendations vary. Clemson uses 10 inches between seeds in rows 3 feet apart, and Utah State uses 7-9 inches in rows 24-30 inches apart. Maryland matches spacing to plant size: 8-9 inches in 30-inch rows for small early varieties, and 12 inches in 36-inch rows for larger ones. Plants spaced too far apart produce more suckers (Utah State).

Feed, water and support

Corn needs plenty of nutrients. UGA gives pH 6.0-6.5 as optimum, and a soil test is the best guide to lime and fertilizer. Beyond what goes in at planting, side-dress with nitrogen twice. Utah State applies ½ pound of 21-0-0 per 100 square feet when plants are about knee high (8-10 leaves) and another ¼ pound when the first tassels or silks appear, placed 6 inches from the plants and watered in. Maryland times the first feeding for when plants are 12-18 inches tall. Too much nitrogen, though, makes plants fall over.

Give at least 1 inch of water a week from rain or irrigation, enough to wet the soil 6 inches down. The critical periods are tasseling, silking and ear fill. Heat or drought then leads to poor pollination and ears with missing kernels. Clemson suggests watering in the morning and UGA in the early morning or early evening, so the leaves dry before dark.

Leave the suckers (side shoots) alone. To keep plants upright in summer storms, UGA suggests hilling soil 2-3 inches up around the stalk bases after cultivating, and Maryland suggests running one or two strands of heavy string around the outside of the block.

UF/IFAS notes that pole beans can be grown with corn, using the stalks as support. Beyond that, companion pairings for corn are garden tradition rather than tested advice.

Common problems

• Corn earworm: caterpillars feed on silks and ear tips. Choose varieties with husks that close tightly over the tip, apply Bt or mineral oil to the silks, and cut off damaged tips at harvest.
• Poorly filled ears: heat or drought at silking, crowding, low fertility or too few rows. Plant in blocks and water at silking.
• Common smut: white fungal galls on ears, tassels or stalks. Remove and destroy galls and badly infected plants, choose resistant varieties and clear plant debris after harvest.
• Seed rot and damping off: seedlings darken and die after sowing in cold, wet soil. Wait for warm soil, especially for sh2 types.
• Raccoons: they raid ears at the early milk stage. Utah State suggests a two- or three-wire electric fence, with wires 4 inches apart starting 4 inches above the ground, switched on before the corn ripens.

Harvest and storage

Ears are ready about 17-24 days after the first silks appear, sooner in hot weather. Pick when the silks are brown and dry, the husks are still green and the kernels are plump to the tip. The thumbnail test confirms it: milky juice means ready. Each planting ripens over about 5-7 days.

Sweet corn starts converting sugar to starch the moment it is picked. Clemson recommends picking in the cool of early morning and chilling the ears right away. UGA warns that many varieties lose half their flavor within 12-18 hours if left unrefrigerated. Keep the husks on until you cook them. Clemson gives about five days in ideal cold, moist storage, though Maryland says the newer supersweet types can stay sweet for 2-3 weeks.

Utah State suggests 10-15 feet of row per person for fresh eating, plus 30-40 feet more for freezing or canning.