Article and photos by Penny Woodward

A mixed planting of vegetables

Marigolds, garlic chives and basil with tomatoes and sweet corn

I’m getting my garden bed ready to plant sweet corn. I know its early but it is so much warmer this year that I am putting everything in a bit earlier.
In brief Sweet corn (Zea mays) is a native of Central and Southern America. There are three main types: standard, sugar-enhanced and super sweet. They are all annuals that can be grown in most climates and should be planted in late spring in temperate regions, September to January in the sub-tropics and all year in the tropics. The prefer soils pH of 5.5 to 7, like nutrient rich soil and copious water during cob production. Space plants 25cm apart in rows 50 cm apart, harvest 3 months after sowing, and you will know they are ready to pick when the silk turns brown and the cop angles away from the stem. If you want to know more then keep reading!

Sweet corn seedlings can be sown into biodegradable pots to get an early start in cooler regions

Sweet corn seedlings can be sown into biodegradable pots to get an early start in cooler regions

Sweet corn and maize are types of Zea mays. Both are annual grasses that can grow up to 4 m with strong, upright stems. Kernels can vary in colour from white, yellow, red, purple and blue or combinations of all of these. Maize typically comes in a variety of colours and has much higher starch levels in its kernels making it suitable for corn flour (for making bread, cakes, polenta, tacos and more), stock feed and a range of starchy dishes like cachupa a traditional meal from North West Africa. Sweet corn has higher sugar levels that vary depending on the variety grown, and the length of time since harvest. New varieties being developed through breeding and genetic modification have much higher sugar levels and will keep for longer before the sugars start to break down. These are known as sugar enhanced and super sweet types.

I prefer and only grow open-pollinated types (see the table below) but those who love the intense sweetness may want to grow hybrid types. But why grow your own corn? Because you haven’t tasted true sweet corn until you have put a pot on to boil, once boiling run out to the garden, picked picked the corn and cooked it for a few minutes, then served and eaten. In fact some corn can be eaten fresh from the garden without cooking.

Soil preparation is really important for good sweet corn. It needs deeply dug soil with added compost, well-rotted manures and blood and bone, good drainage and as much sunlight as possible. Dig the manures in well, as much as a wheel barrow full for every square metre, water and leave to stand for a week or so before planting.

Seed can be sown directly where it is to grow, or into biodegradable pots for later transplanting. Corn does not like it’s roots disturbed so planting into punnets and later transplanting really sets back growth.  Seed must be fresh, preferably only 12months old, and corn plants need to be close together to enhance fertilization and cob production. So plant in blocks rather than single rows, spacing seed 25cm apart, in rows about 50cm apart (this will vary a little depending on the type grown). Sow seeds between  2 and 5cm deep, the warmer it is the deeper the seed should be planted. Always sow into moist soil and once the seed is planted and well-watered don’t water again until the green shoots appear as too much water can cause them to rot. Corn needs warm temperatures to germinate, so in cold and temperate regions plant early into biodegradable pots or into the ground from early November to mid-January when temperatures are consistently over 15ºC; in the sub-tropics sow from September to early March; and in the tropics sow seed all year, although fewer cobs are produced in wet, sultry summer weather.
Plant only one variety at a time as cross-pollination between different varieties (especially if one is an sh2 variety) can result in starchy, rather than sweet and juicy, cobs.

Corn planted in pots

Corn doesn’t like its roots disturbed so plant the pot into damp soil and water well

When plants reach 50cm mulch with well rotted manure and a thick layer of pea or lucerne straw.

When plants reach 50cm mulch with well rotted manure and a thick layer of pea or lucerne straw.

Growing Once your corn plants are up and growing strongly water fortnightly with fish emulsion and seaweed extract. When plants are over 50 cm tall, mulch with more well-rotted manure and a good thick layer of pea or lucerne straw, 20 to 30 cm up the plant stems. This encourages the growth of stem roots that increase the ability of the plants to extract nutrients from the soil. Mulching also prevents weeds growing. If you do have a weed problem, don’t hoe them out as corn is shallow rooted and doesn’t like its roots disturbed. Just pull weeds by hand.

As plants approach their full height you will notice swellings in the stem, these indicate that flowers are about to appear and from this point on consistent watering is essential for good cob development. By following this feeding and watering routine, not only will the individual cobs be larger but you should get more cobs per plant.

Sweet corn with squash planted underneath

Sweet corn with squash planted underneath

Sweet corn with beans climbing up the stem

Sweet corn with beans climbing up the stem

Corn cob ready to pick

The cob is ready to pick

Companions Corn is polygamous and is quite happy sharing its garden bed with other plants, especially other vegetables. It also benefits from sowing a winter green manure crop that is dug back into the soil before seed is planted.  Alternatively you can plant clover when the corn seedlings are about 20cm high. This not only protects the soil but provides nitrogen to the corn. After the corn is harvested, leave the clover, digging it in the following spring. The bed is now ready for the next crop. You can’t grow clover and mulch heavily as described above, so if you decide to follow the mulching path there are other plants that grow well with corn. Try climbing beans, cucumbers, pumpkins and other plants in the squash family. If you sow climbing beans beside each corn plant, the corn provides support for the beans, and the bean’s fix nitrogen for the corn. Once the cobs are harvested the corn plants are left in place until the beans are finished. Pumpkin and cucumbers with their sprawling habit and big leaves provide shade for the soil and also grow up the corn plants.

Pollination Corn is wind and gravity pollinated with the pollen occurring in the spikey male flowers at the top of the plant. This pollen drops to the female flower silks below. Each fine silk thread that is fertilized represents one kernel in the cob. So if pollination is erratic, cobs will have patches where there are no kernels. You can improve pollination especially on very still days by gently shaking the male flowers.

Harvesting When the silk shrinks and turns brown (about 18 days after the silk first appears) and the cob angles out more from the stem, the cob is now ready to harvest. If you want to check for ripeness, carefully peel back a bit of the leafy sheath, and puncture a kernel, a milky liquid will come out. If it is watery it is too early, if there is no liquid it is too late. Once harvested the sugars in the cob start changing to starch, so eat as soon as possible. Traditionally you put the water on to boil, once it is boiling run to the garden and pick the corn, run back pulling off the leaves, put the corn straight in to the pot, cook and eat immediately! If you do need to store them for a while put them in the refrigerator.

Pests and diseases Corn earworm moth can be a serious problem for corn. The pupae of these moths live in the soil so by digging deeply before planting you will disrupt the life cycle and so minimise their effect. The adult moths lay eggs on the cob that hatch and the small caterpillars burrow into the cob. Look for the eggs and caterpillars and squash them. Encourage beneficial insects by planting things like parsley, dill and coriander and allowing them to flower. Or spray with a molasses and water mixture to kill the caterpillars.

Seed saving As more and more F1 hybrids of corn are developed, the open-pollinated types are becoming harder to find, so it is important to grow and save seed from open-pollinated forms. But the number of plants needed (up to 100) and the techniques necessary to keep strains pure are probably beyond the space available and the ability of the average gardener. If you want to have a go then consult Michel and Jude Fanton’s Seed Saver’s Manual

Golden Bantam corn

Golden Bantam corn

Balinese corn

Balinese corn

Sweet corn freshly picked

Sweet corn freshly picked

Some sweet corn varieties
Standard (su): Eate this corn soon after harvest as otherwise the sugars convert to starch and they lose their sweetness. Open-pollinated corns are all this type.
Sugar enhanced (se): Sugar content is higher and so cobs are sweeter and more stable after harvesting
Supersweet (sh2) very sweet types that maintain their sweetness for up to 10 days but the skin of each kernel is tougher than other types and some of the true ‘corn’ taste is lost

A selection of commonly available types

Variety Characteristics
Baby – Pop F1 Baby corn, harvest continuously before silks appear or leave to mature for popcorn. 106 days, 2m.
Balinese OP Su type, pale yellow kernels, with at least 2 cobs per plant, 2m.
Breakthrough F1 Sh2 type. Very sweet and keeps sweetness for up to 10 days. 78 days.
Dwarf F1 Se type. Cold tolerant and early fruiting. 1.7m tall. 86 days.
Golden Bantam OP Su type. Golden kernels with two or more cobs per plant, early cropping, 1.5m, 86 days.
Hawaiian OP Su type with orange-yellow kernels and at least 2 cobs per plant. Good for humid conditions.180 cm
Honeysweet F1 Sh2 type. Very sweet, 90-100 days.
Honey and Cream F1 Su type. Very sweet with plump yellow and white kernels. Needs warmer temperatures, over 20º C for sowing. 78 days.
Legacy F1 Su type, with large yellow cobs, good germination. 235 cm. 85 days to harvest
Max F1 Sh2 type, large golden cobs, high sugar, retains sweetness. Good germination. 196 cm.
Ontos Oval OP Su type, oval shaped white kernels used for popcorn. 2m, 120 days
True Gold OP Su type, true corn flavour.100 days.
White F1 Sh2 type. Shiny white super sweet kernels. 97 days

OP = open pollinated   F1 = hybrid, seed cannot be saved

Multi-coloured corn

Maize, often grown here as ornamental corn, typically comes in a variety of colours and has much higher starch levels in its kernels