How to Grow Corn in Australia

Corn (Zea mays) is a frost-tender annual grass in the family Poaceae. It is grown for sweet cobs picked green, and for popcorn and flour maize harvested dry. It thrives in long, hot Australian summers, from the Queensland coast to the NSW tablelands. A common mistake is sowing a single row.

Three mistakes that matter

1. Planting a single row. Wind carries the pollen, and from a lone row most of it blows away. Sow a block at least four rows deep and four plants long.
2. Growing supersweet beside other corn. Pollen from any other type turns supersweet kernels starchy and tough, and sweet corn crossed with popcorn or maize suffers too. Keep types apart, as the table in Pollination shows.
3. Sowing into cold soil. Seed rots in cold, wet ground, and the shrunken seed of supersweet types is the least forgiving. Wait for the soil temperature in When to plant, not the date on the calendar.

When to plant

Sow once the soil holds at least 16°C at sowing depth; supersweet seed does best at 18°C or more. Standard and heirloom types germinate from about 12°C, but slowly. Sow a fresh block every two weeks or so for a longer run of cobs.

At the cold edge, a light frost usually spares young plants, since the growing point stays below ground for the first few weeks. At the hot edge, aim for silking to miss the worst heat, since days over 35°C with dry wind leave cobs half filled.

Site, soil and setup

Corn is a hungry, thirsty grass with roots that can reach 1.2 m, so deep, rich soil pays.

Sowing Seeds

Sow corn where it will grow, because it resents root disturbance and the large seed is easy to space by hand.

Sowing direct

• Use fresh seed, ideally under a year old, since old seed comes up patchy.
• Sow 25 to 30 mm deep; standard and dry types can go to 40 mm in warm, dry soil.
• Drop two seeds every 25 to 30 cm, in rows 60 to 75 cm apart. A four by four block at two seeds a station takes 32 seeds, so check the packet count.
• Water the bed well once, then hold off until shoots appear unless the soil dries out, because soggy soil rots the seed.
• Seedlings show in 5 to 15 days; about a week later, thin each pair to the stronger plant.

Due to quarantine restrictions and our shipping policy, we cannot send corn seed to the Northern Territory, Tasmania or Western Australia. Treated seed, such as the F1 hybrids and 'Legacy', carries a coloured fungicide coating. Wear gloves, wash your hands after sowing, keep it from children and animals, and never eat it.

Starting in pots

• Where spring is slow, sow one seed per biodegradable pot and keep it warm.
• Harden off (accustom seedlings to sun and wind over about a week), then plant the pot whole at the spacing in Sowing direct.

Pollination

Each plant carries both sexes: the tassel on top sheds pollen, and every silk on the cob below leads to one kernel. A plant sheds pollen for 7 to 10 days, mostly in the late morning and early evening.

Whether sugary enhanced (se) types need keeping from standard (su) sweet corn is unsettled; the table holds the rules that are. Few gardens can spare the 400 m that NSW DPI gives, so separate types in time, tasselling 14 to 21 days apart.

On still days or in a small block, pollinate by hand on several mornings, once the dew has dried.

• Bend a shedding tassel over a paper bag and tap it to collect the pollen.
• Sprinkle the pollen over fresh, moist silks on several plants.
• Or shake the stalks gently while the tassels are shedding.

Care by growth stage

Nitrogen and water decide this crop, and the four weeks around silking matter most. Weeks count from sowing.

Once plants are knee high, hill 15 to 20 cm of soil or compost around the stems against wind, then mulch with straw. Pull weeds by hand, because the roots run shallow.

If you prefer a conventional feed, mix a controlled release granular (NPK around 14-5-10) into the bed at sowing. Water on an all-purpose soluble (NPK around 23-4-18) fortnightly while plants grow. From tasselling, switch to a fruiting and flowering soluble (NPK around 15-4-26). Use all three at their label rates.

Companion planting

Corn, climbing beans and pumpkin are the classic three sisters.

Grows well with

• Climbing beans, sown once the corn is about 40 cm tall, so they climb the stalks without smothering them.
• Pumpkin or squash rambling underneath as a living mulch that shades out weeds.
• Cucumber, which covers the ground between rows and suppresses weeds.

Keep apart

• Tomato, which hosts the same corn earworm, known on tomatoes as tomato grub.
• Sorghum, which shares fall armyworm and the soil-borne diseases of corn.

Harvesting Cobs

Sweet corn is usually ready about three weeks after the silks appear, sooner in heat.

• Peel back a strip of husk and press a kernel with a thumbnail: milky juice means ready, clear juice means wait, paste means late.
• Pick in the cool of the morning, twisting the cob down and away from the stalk.
• Check every day or two: standard types stay prime for 2 to 5 days, supersweets for 5 to 12.
• Leave popcorn and maize on the plant until the husks are brown and dry and the kernels hard and shiny. In wet weather, pick mature cobs and dry them under cover.

Chill sweet corn in its husk straight away and eat it within a day or two, because warmth turns its sugar to starch within hours. Cook cobs briefly, pop the popcorn, and grind flour maize for tortillas, polenta and meal.

For dry types, husk the cobs and hang them in mesh bags somewhere warm, dry, airy and safe from rodents. After two to three weeks, test pop a few kernels; chewy, ragged popcorn needs another week. Once they pop well, shell the kernels and store them in airtight jars.

Common problems

Gaps and bare tips on the cob. Silks that caught no pollen, from a single row, hot dry wind or rain at silking, or grubs chewing the silks; see Pollination.

Starchy, tough kernels. Pollen from another corn type, a cob picked past the milk stage, or cobs left warm after picking; see Pollination and Harvesting Cobs.

Plants blown over. Heavy rain with wind flattens tall plants, especially in loose soil; hill soil around the stems early.

Side shoots at the base. Many varieties grow suckers (tillers), and removing them takes time without improving the crop, so leave them be.

Seed dug up or cobs gnawed. Mice take seed and cobs, and rats and corellas raid ripening cobs; taut white netting with 5 mm mesh or finer keeps them out. Read more

Pests and diseases

Caterpillars do most of the damage, and warm, wet weather brings the leaf and cob diseases.

Corn earworm (Helicoverpa armigera). The moth lays pearly white eggs singly on the silks, and the caterpillars follow them into the cob tip, usually one per cob. Native budworm (Helicoverpa punctigera) looks alike but feeds only on broadleaf plants, so it leaves corn alone. Once inside the husk it is beyond any spray, so clip a peg over the husk tip once silks show. Or spray fresh silks with a Bacillus thuringiensis product approved for the pest. Cut off damaged tips at picking.

Fall armyworm (Spodoptera frugiperda). Young caterpillars leave papery windows on leaves, then larger ones feed deep in the whorl. Grown caterpillars reach 3 to 4 cm, with a pale inverted Y on the head and four black spots in a square near the tail. Squash the felted egg masses under leaves. Treat small caterpillars with a Bacillus thuringiensis or spinosad spray approved for fall armyworm on sweet corn, since pyrethroids fail. It rarely persists where frosts occur. Read more

Corn aphid (Rhopalosiphum maidis). Colonies gather inside the husks but rarely harm the plant by feeding. The risk is the virus they can carry, Johnson grass mosaic virus, so pull grassy weeds nearby, which can harbour it. Read more

Common rust (Puccinia sorghi). Reddish-brown powdery pustules up to 2 mm long on both leaf surfaces, usually after tasselling and worst in warm, humid weather. Late sowings and some supersweets suffer most. Pull volunteer corn and weed out oxalis, which carries part of the rust's life cycle.

Northern leaf blight (Exserohilum turcicum). Long, grey-green lesions spread up from the lower leaves in warm, wet weather, and severe blight cuts cob fill. The fungus survives on old stalks and volunteer plants.

Boil smut (Ustilago maydis). Grey-white galls swell on cobs, stems and tassels, then burst into black, sooty spores that survive in soil for many years. It occurs in Queensland and New South Wales, and sweet corn and popcorn are more susceptible than maize. Cut galls off while firm and bin them. It has not been found in South Australia or Western Australia, so report a suspect gall there to the state biosecurity agency.

Prevention first

• Pull out old stalks after harvest and dig over the bed, which exposes earworm pupae in the soil.
• Where rust or leaf blight is common, choose a variety that tolerates them.
• Check leaf whorls and silks two or three times a week from a fortnight before tasselling.
• Sow early in your window, since late crops meet more earworm, rust and leaf blight.

Tips

1. For baby corn, grow a block of popcorn and pick cobs at 5 to 10 cm long, 1 to 3 days after the silks show. Picking early can prompt the plant to set more.
2. To save seed, grow 100 or more plants of one open-pollinated variety, since F1 seed does not come true and corn weakens when inbred. Pollen can travel 400 m or more, so if other corn grows nearby, bag the young cobs before the silks show, hand-pollinate and rebag. Dry the best cobs on the plant.
3. Stored popcorn that will not pop may be too dry. Add about 15 mL of water to a litre jar of kernels, shake often, and test after 3 or 4 days.