5 Causes of Underdeveloped Corn Cobs in the Garden

Sweet corn, with its juicy, sugary kernels and (usually) easy-to-grow nature, makes it a rewarding addition to the home garden. With early and late season varieties, and by sowing successional rounds, we can enjoy the harvest throughout the summer.

Sometimes, corn runs into problems in its various development stages. Underdeveloped corn cobs relate to genetics, with a propensity for malformed ears or increased vulnerability to environmental stress, pests, and diseases. Often, it’s a combination of cultural factors, like weather conditions or management inconsistencies.

Starting with strong varieties, such as heirlooms and those with good disease resistance, sets the foundation for sturdy ears. By meeting the best growing requirements for corn, from balanced watering to ample nutrition to boosting pollination, we can increase our yields of plump, juicy ears. Here’s how to assess and fix underdeveloped corn cobs.

About Sweet Corn

Corn appreciates similar growing conditions to many of our favorite crops and is an easy addition to the summer vegetable garden with a few specific nuances for best production. The historic selection has ancestors in wild grasses dating back 9,000 years. As early as 4,000 years ago, Zea mays became a staple in the diet of ancient peoples in Central and South America, who cultivated and shared the kernels. Today, it’s one of the most widely grown crops for its versatile uses.

Harvest times for sweet corn vary by variety and climate, usually maturing within 60 to 100 days. The key indicator for corn readiness is the silks, the threadlike tassels that appear on the tips of the ears. They emerge and transition from green to brown as the corn ripens. With a small window for peak harvest, keeping an eye on days to maturity and appearance is important in gauging when to pick.

Each corn stalk produces one upper ear of corn and smaller secondary ears. Enjoy them close to picking time, as the starches that create the sweetness begin to break down as time lengthens from stalk to table.

Not Enough Sunlight

Corn relies on full sun and warmth to develop, needing six or more hours of sunlight daily. With too much shade, stalks show less vigor, failing to produce or flopping over. With prolonged cloudy days, the effect is similar, and ears may be small or filled with fewer kernels due to less sunlight for fruit and seed production.

Hot or Cold Temperatures

Hot, dry weather can stress the crop and result in energy and resource conservation for survival. Especially in the final phase of development, cobs can show stunting or unfilled kernels. Heat waves and drought also slow pollinator activity, which can result in poor pollination during the silk phase.

Alternatively, cold, wet weather, especially during silking, results in a lack of full kernels or full cobs. Cool spring conditions after sowing can also hinder growth and result in fewer cobs. Wait for warm temperatures to sow kernels, as seeds need new warm days and nights to germinate and develop.

Sow kernels one to two weeks after the final frost date. Corn germinates best in soil temperatures above 60°F (16°C) and ideally between 65-90°F (18-32°C). The warmer, the better for these heat-loving crops.

Direct sow kernels for the least stress on young seedlings. Roots are sensitive to transplant disturbance, and seeding in place prevents disruption. But in areas with short growing seasons, start corn indoors for transplanting outside as the weather warms. Try to time it so the kernels are only in cell packs for a few weeks before moving outside (to lessen extensive root development), or opt for soil blocks or biodegradable pots.

A round of seeding every few weeks ensures plenty of cobs to enjoy throughout the summer and into early fall. Since stalks produce all at once, or nearly so, staggering the plantings yields multiple harvests. Sow the same variety successively or use different cultivars suited to early, mid, and late-season harvests. 

Poor Pollination

A corn cob with poor pollination is underdeveloped, with missing rows of kernels. Before the silks form, tassels that hold pollen emerge from the tops of the ears. The pollen granules are carried by the wind or travel with insects. Each silky strand needs pollination for all kernels to develop. 

To promote pollination, plant the stalks in blocks rather than long, single rows. Sow them in groups of two to three seeds with a spacing of 9 to 12 inches apart (or according to varietal guidelines). Thin the seedlings to the strongest one when they reach several inches tall. Sweet corn also grows well in raised beds. 

A gap in kernel infill occurs when there are too few plants for cross-pollination or there’s a lack of pollinator activity. Increase the number of plants in a block to encourage cross-pollination. But, avoid planting the stalks too closely. Tightly packed, overcrowding leads to competition for resources like water and nutrients. Poor air circulation can cause pest and disease issues.

Attract pollinators by interplanting companion plants and blooming annuals and perennials with nectar and pollen resources. The blooms draw beneficial insects that prey on sweet corn pests like corn earworm. Ladybugs, green lacewings, and parasitic wasps feed on the eggs and larvae to reduce populations. 

An additional way to boost pollination is to hand-pollinate the stalks by shaking them so that tassels disperse the granules, dropping them onto the silks. Or, snap off a tassel and brush the silks on up to 10 plants.

Water Fluctuations

Sweet corn needs regular water throughout the growing season, and a lack of consistent moisture can lead to underdeveloped corn cobs. Even moisture is especially critical a couple of weeks before the silks form and as kernels develop. 

An inch or two of water per week is usually sufficient to support the growing stalks, leafy growth, and cob development. Supplement with irrigation if seasonal rainfall isn’t enough, especially during dry spells.

Lacking Nutrition

Corn is a heavy feeder, as the productive annual does all its growing and reproducing in a single season. Organically rich, well-draining soils are best for vigorous stalks and fully-formed cobs. A neutral to slightly acidic soil with a pH near 6.0-6.5 is ideal, though corn isn’t overly picky as long as there is fertility and good drainage. Amend with compost to add nutrition, aeration, moisture retention, and drainage, especially in clay or sandy types. 

Nitrogen and phosphorus are essential in promoting healthy growth, silks, and fully developed cobs. With a lack of nitrogen, ears may underdevelop.

Topdress with a balanced organic, slow-release granular fertilizer like a 10-10-10 at sowing to supplement nitrogen, phosphorus, and potassium. Sidedress with an additional application four to six weeks after seedlings emerge.

Pests and Diseases

Pests such as corn earworm and corn rootworm cause kernel damage and missing infill. For these two common pests, planting early varieties helps miss the peak active season when larvae emerge to feed.

For both pests and diseases, weed management helps reduce migration and overwintering sites, and planting a diversity of selections to draw beneficial predators can help. Insects are vectors for diseases like maize dwarf mosaic virus, which shows stunted growth, weak cobs, or no ears at all.

Pests

Earworms are common corn pests that beat us to the goodness by entering the silks and ears to feed in the larval stage. Early on, they feed in the whorls (tightly packed, newly emerging leaves), potentially stunting development and damaging tissues needed for photosynthesis. Most likely, though, we won’t know they’re there until we unwrap the ear.

Corn earworms (Helicoverpa zea) are a type of cutworm that rely on corn, tomatoes, and other crops for feeding in the larval stage before pupating into moths. The caterpillars feed on leafy whorls and kernels, settling into the silks to begin feeding at the top of the cob. Preventative measures include planting pest-resistant varieties. Vegetable and mineral oil may prevent the caterpillars from progressing into the husks. Apply oil with a medicine dropper or brush the silks and tips of the ears within a week after silks appear. 

Corn rootworms feed on the silks as adult beetles and on the roots in the larval phase. Silk-feeding results in missing kernels, or few at all. Root damage results in fewer yields, weak growth, and can lodge the entire stalk. Crop rotation is the most beneficial way to prevent the insect in subsequent seasons. 

Diseases

Dwarf mosaic virus is spread by several species of aphids. Infected leaves show green and yellow mottling and striations, and growth may be stunted. Yields are low or not at all in severe cases, and weakened plants are susceptible to secondary diseases like root rot.