Growing popcorn in home gardens

Popcorn (Zea mays var. everta) is a type of corn with kernels that burst when heated. A small amount of moisture within the hard kernel turns to steam, causing the kernel to explode. Homegrown popcorn is a special treat and a fun challenge for gardeners.

Soil pH and fertility

Soil testing and fertilizer

• Have your soil tested. 
• Popcorn grows best in soil that is well-drained and well-supplied with organic matter, with a pH of 5.8 to 7.0.
  • Apply phosphorus (P) and potassium (K) according to soil test recommendations.
  • Many Minnesota soils have enough phosphorus. Unless your soil test report specifically recommends additional phosphorus, use a low- or no-phosphorus fertilizer.
• Improve your soil by adding well-rotted manure or compost in spring or fall. 
• Do not use fresh manure as it may contain harmful bacteria and may increase weed problems.
• Side-dress when the plants are one foot tall with 0.15 pounds of actual nitrogen per 100 feet of row. Spread the fertilizer evenly in a band alongside the row, four to six inches away from the plants. Scratch it into the soil and then water the garden.

Planting

Isolate popcorn to prevent cross-pollination

Corn must self-pollinate or pollinate by a similar variety, for best quality. Gravity and wind moves corn pollen from the tassel at the top of the plant to the silks of the ears, lower down the plant.

Always plant corn in blocks of at least four rows. Corn planted in a single row will have much of its pollen blown out of the row, and probably produce ears that have blank areas where kernels did not form.

If field corn or sweet corn pollinate popcorn, it may not pop well after harvest. Likewise, if wind blows popcorn pollen onto sweet corn silk, the sweet corn will have poor eating quality. You must isolate different types of corn from one another.

Isolate by space and time

The simplest form of isolation is putting distance between the plants. If you garden in an agricultural area, plant popcorn 300 feet from the nearest cornfield. Large acreages of field corn will produce so much pollen that they could spoil your crops unless you plant your popcorn far enough away.

The home garden may not be big enough to plant different corn types far enough apart. To isolate popcorn from sweet corn by time, plant the popcorn first, once the soil has warmed. Wait three weeks before planting the sweet corn. The popcorn should produce tassels and silks before the sweet corn. Because popcorn takes a month or longer than sweet corn to mature, it is best to plant the popcorn first.

You could also choose to grow only popcorn, and not worry about isolation.

Direct seeding

• You must direct seed popcorn.
• Seeds germinate best when soil temperatures are close to 60°F.  Use a soil thermometer to be sure the soil is warm enough.
• Diseases are more likely to infect seeds in cooler soil, so most gardeners plant fungicide-treated seed. Even treated seed can fail to germinate under poor conditions.
• If you use untreated seed, be sure that your garden soil has warmed to 60°F.
• Some gardeners use plastic mulch to speed soil warming. 
• Plant seeds one inch deep in heavier soils, and up to two inches deep in sandy soil. Space the seeds about 8 inches apart.
• Plant in at least four rows, with 18 to 24 inches between rows.
• Popcorn germination takes longer than sweet corn, and most varieties require 90 to 120 days to reach full maturity.

How to keep your popcorn plants healthy and productive 

Watering

• Proper watering will help popcorn grow well.
• All types of corn will continue to grow in very hot weather. They can become drought-stressed in hot dry spells unless you irrigate them.
• Popcorn has a relatively shallow rooting depth. Although you may see new roots developing just above the soil at the base of the plant, these roots play very little part in absorbing water or nutrients. They stabilize the plant.
• If the planting does not receive one inch of rain each week, soak the soil thoroughly at least once a week. If your soil is sandy, it is important to water more often than once a week.
• An inch of water will wet a sandy soil to a depth of ten inches, a heavy clay soil to six inches.
• Use a trowel to see how far down the soil is wet. If it is only an inch or two, keep the water running.
• Once the ears have filled and matured, you can let the plants dry. There is no need to continue watering.

Controlling weeds

• Frequent, shallow cultivation with a hoe or hand tool will kill weeds before they become a problem.
• Do not hoe or chop too deeply. Cultivate just deeply enough to cut the weeds off below the surface of the soil.
• Once the corn plants have established, they will form a canopy of leaves that can discourage new weeds from growing.
• Mulching with herbicide-free grass clippings, weed-free straw or other organic material to a depth of three to four inches can help prevent weed growth, decreasing the need for frequent cultivation.
• If you use the herbicide trifluralin (sold as Treflan, Preen, and many other trade names) in your vegetable garden, note that you cannot apply it near corn.

Insects

Two insect pests that feed on the developing ears of corn are corn earworm and European corn borer. It is common to husk an ear of popcorn and find chewed-up area of kernels. If this happens, strip the good kernels from the cob and discard the rest. 

You should base your need to manage these pests on their activity in your garden. If you find they are damaging your corn more years than not, anticipate that they will be a problem. If you do not see them attacking your corn, you can assume they will not be a problem.

If you have had trouble with either or both of these pests in the past, you will want to take steps to control them. Take action just as the corn begins to produce silk. Observe the ears as they form, and watch for the first silks to emerge from the husk leaves.

Treat the leaves and silk with a labeled insecticide, such as products that contain Bacillus thurngiensis, permethrin or carbaryl. Make applications every four to seven days, using the shorter interval during hotter temperatures, to make sure you treat all silk as it emerges. Silk can emerge one to two inches per day under August temperatures. Roughly seven to ten days from the beginning of silking on the first plant, all silk emergence and elongation should be complete.

Once European corn borer or corn earworm enters the ear, there are few control options except to pull the husk back on the ear tip and hand pick the larvae.

Vertebrate Pests

Crows and other birds will eat corn seeds before they emerge from the soil. A floating row cover left in place until the plants emerge will discourage them.

Diseases

Diseases that affect popcorn include common smut and leaf rust.

Use good cultural control practices to reduce disease problems to a tolerable level and allow for a successful harvest. 

Smut causes unusual, firm, tumor-like growth on leaves, stems, ears and tassels. These start out light green but become purplish gray with age and eventually explode and release many black powdery fungal spores.

Look for smut galls throughout the season and cut them out before they produce spores. Remove these galls from the garden, and bury them in a location where you will not grow corn in the future. Do not compost them.

Leaf rust appears as rusty orange streaks on leaves that release many powdery orange spores. Rust resistant varieties are available and are the best form of control. 

For assistance in diagnosing unknown problems, visit the University of Minnesota Extension diagnostic site “What’s wrong with my plant?”

Harvest and storage

Harvest

You can achieve maximum popping potential only if the corn reaches full maturity. Mature ears of popcorn have plump, hard, shiny kernels and dry husks. Let popcorn remain in the garden as long as possible, to allow the kernels to dry on the stalks.

If the weather turns rainy while the plants are drying in the garden, harvest the ears and bring them inside to continue drying. Remove the husks and put the cobs in mesh bags. Hang the bags in a warm, well-ventilated area.

The ideal moisture content for popcorn is between 13% and 14%. If the kernels get too dry, they will not pop as well.

Most gardeners have no way to measure kernel moisture except by popping performance. At least once a week, try popping 20 kernels. While the corn is still too moist, the kernels will pop, but may not be fluffy or crisp. If the popped kernels are chewy or tough, with hard edges, they are still too moist. With each sample, the corn should produce better quality popcorn. When most of the kernels in the sample pop up crisp and fluffy, it is time to store the whole crop.

Storage

You can store popcorn on the cob, but most gardeners will want to shell the kernels off the cob and store just the seeds. Wear a heavy glove and rub the kernels off the cob. Store popcorn in airtight containers to prevent it from drying out further.