Saving Seeds from Sweet Corn

Crop Types

Corn is primarily grown for its kernels, which have different culinary uses depending on the type of corn. Varieties are commonly categorized as flint corn, flour corn, dent corn, popcorn, or sweet corn. Flint, flour, and dent corn are differentiated from one another by the character­istics of their endosperm, the part of a seed that provides the embryo with nutrition as it develops. Flint corn has a hard endosperm, flour corn has a soft endosperm, and dent corn has an endo­sperm comprised of hard and soft starches, which results in a small impression–or dent–that is visible on the top of the dry kernel. Dent corn is commonly used for making cornmeal and tortillas and in the production of high-fructose corn syrup. Popcorn is a form of flint corn whose seed coat retains moisture; when the seeds are heated, the steam created causes the seed coats to burst open, popping the kernels. Sweet corn, which is the only type eaten in an immature state, contains one or more mutations that delay or prevent sugar in the kernels from converting into starch, giving the crop its sweet taste. Sweet corn kernels shrink as they dry, giving the mature seeds a wrinkled, translucent appearance. Corn is also grown as animal feed, and a few varieties have been bred so that the whole plant can be used as silage, or fermented animal fodder.

Corn kernels range in color from yellow, white, orange, and red to blue, green, and brown. Some varieties have striped, or variegated, kernels, and many varieties are known for their multicolored ears. Varieties can differ in ear size and shape, tassel color, cob color, plant height, the number of ears produced per plant, and even the arrangement of the kernels on the cob.

History

Corn was most likely domesticated from teo­sinte, its wild relative, 9,000 years ago in the lowlands of southwestern Mexico. During the next several millennia, corn spread through­out Central America and had spread across the Americas by about 5,000 years ago. By the sixteenth century CE, corn was being grown as an agricultural crop from present-day Argentina to present-day Canada. Around this time, corn was brought from the Americas to the rest of the world by European explorers. South American corn migrated to Africa and southern Europe. Corn from the Caribbean was brought to the coast of North Africa and later to India, and corn from Mexico was introduced to Indonesia and Southeast Asia. Most of the corn grown in Europe was brought from North America.

Growing Corn for Seed

Zea mays is a wind-pollinated annual crop that is widely adapted and successfully grown in most parts of the world. Cultivating corn plants is the same whether saving seeds or producing a grain crop because the mature seeds are the part of the plant typically harvested. The ears of sweet corn, which are harvested and eaten while the kernels are still immature, need to remain on the plants past market maturity until they reach botanical maturity for seed saving, but in all other ways sweet corn is cultivated in the same manner for seed as for eating.

Corn germinates and grows best when the soil temperature has reached at least 50°F (10°C); sweet corn generally germinates better at a slightly warmer soil temperature. Plants are spaced 6 to 12 inches (15 to 30 cm) apart with 24 to 36 inches (61 to 91 cm) between rows, the same as when grown for eating. It is better to plant corn in blocks at least six rows wide than in one long row to help ensure better pollination and good seed set.

Flowering, Pollination, and Seed Set

Corn is a monoecious species, producing male and female flowers on the same plant. The tassel that appears at the top of the main stalk is the male inflorescence. The anthers on the tassels dehisce and shed their pollen over several days. The shoot, the part of the plant that will develop into an ear of corn–is the female inflorescence. Corn silks, which are typically cleaned off of the ears of sweet corn during husking, are the stigmas, each silk attaches to a single ovary that will develop into a kernel if successfully fertilized. Corn kernels are actually single-seeded, indehiscent fruits, which are most often referred to as seeds.

Corn is an outcrossing, wind-pollinated species. During flowering, the silks protrude from the husks are fertilized by pollen grains disseminated on wind currents. On any one plant, it is possible that male flowers will shed their pollen before the female flowers have emerged from their husks. Because wind is a haphazard carrier, pollination is most successful when many plants are grown together.

Flowering in corn is determined by a combi­nation of decreasing day length and accumulated warm temperatures. When growing corn to seed maturity, it is important to know whether there will be a sufficient number of warm days to both induce flowering and allow enough time for the seeds to mature.

Variety Maintenance

Because corn is wind-pollinated, varieties should be separated by a minimum of 800 feet to half a mile (244 to 805 m). Corn pollen is heavy and generally does not travel as far as that of other wind-pollinated crops. However, in areas where large amounts of corn are produced, or when genetic preservation is a gardener’s goal, an isolation distance of 1 to 2 miles (1.6 to 3.2 km) between varieties is recommended. If isolation by distance is not feasible, which is often the case with corn because of its pollination method and its popularity as a garden and agricultural crop, hand-pollination is the most practical method for seed savers who wish to save true-to-type seeds.

Due to corn’s propensity to suffer from inbreeding depression, a population size of 50 to 120 plants is recom­mended for seed saving. While a population of just 10 plants or fewer could produce viable seeds, it is always best to save seeds from more plants, especially if seeds will be shared with others. A population of 200 plants or more is recommended if long-term preservation or stew­ardship of a rare variety is the gardener’s goal.

When roguing or selecting corn, seed savers should consider traits such as tassel color, plant size, ear shape, kernel arrangement, kernel color, kernel endosperm type, and cob color.

In most crops, visible signs of outcrossing do not become apparent until the next generation of plants is grown, but this is not always so with corn. While fruit traits are always maternally derived, the transparent pericarp (fruit wall) of some corn kernels allows a gardener to see the endosperm of the seed. The endosperm is a prod­uct of fertilization, and endosperm qualities are distinct to each type of corn; thus an individual kernel can show evidence of outcrossing both in its color and in its shape.

Seed Maturity and Harvest

As corn matures, the kernel endosperm will slowly turn from liquid to solid, forming a distinct milk line that progresses to the base of each kernel. A change in husk color, from green to yellow to brown, is an outward sign of seed matu­rity. If weather conditions permit, ears should be left to dry in the field. When mature, ears can be handpicked.

To protect the kernels, corn is typically harvested with the husks still attached. Once the ears are brought in from the field, they can be hung in mesh bags or set out on screens to con­tinue drying; removing the husks can facilitate drying. Corn seeds should be dried until they are hard to the touch and easy to shell off the cob.

Cleaning and Storage

Individual kernels can be shelled by hand, with a hand corn sheller, or with a mechanical sheller. If using a sheller, seed savers must take care to avoid damaging the kernels, especially if saving a sweet corn variety, as sweet corn seeds are espe­cially fragile. Seeds savers often shell sweet corn by hand to prevent damage to the seeds. The large, heavy seeds separate easily from the chaff through screening and winnowing.

When stored under cool, dry conditions, sweet corn seeds can be expected to remain viable for two to three years. The expected lon­gevity for field corn seeds is at least five years.

Zea mays

FAMILY: Poaceae

LIFE CYCLE: Annual

SUGGESTED SPACING: Same as when grown as a grain crop or as sweet corn

OTHER REQUIREMENTS: None

FLOWER TYPE: Corn is monoecious (male and female flowers on the same plant) and self-compatible.

POLLINATION: Wind-pollinated

MATING SYSTEM: Cross-pollination, though corn can self-pollinate if wind moves pollen between male and female flowers on the same plant

ADDITIONAL CROSS-POLLINATION CONCERNS: None

FRUIT TYPE: Dry, indehiscent, single-seeded fruits (caryopses) are commonly referred to as grains. The fruit acts as a seed for propagation purposes.

SEED MATURITY: Seeds are mature when the husks are dry and the kernels are hard.

SCREEN SIZE: 10⁄64-26⁄64 inch (4-10.5 mm)

EXPECTED SEED LIFE: 2-3 years

ISOLATION DISTANCE: 800 feet-1⁄2 mile (244-805 m)

Population Size

For Viable Seeds: 10 plants

For Variety Maintenance: 50-120 plants

For Genetic Preservation: 200 plants

For more on seed saving, see our Seed Saving Guide.

Reprinted with permission from The Seed Garden, by Micaela Colley & Jared Zystro, edited by Lee Buttala & Shanyn Siegel and published by Seed Savers Exchange, 2015.Buy this book from our store: The Seed Garden.