How to Grow Pumpkins in Australia

Pumpkin (Cucurbita maxima, C. moschata and C. pepo) is a frost-tender annual vine in the gourd family, Cucurbitaceae. It is grown for hard-skinned fruit that keeps for months, in any garden with four to five frost-free months and room to run. The usual disappointment is young fruit that yellows and drops, which is a pollination problem far more often than a disease.

Three mistakes that matter

1. Sowing outside the warm window. Seed fails in cold soil, and a late sowing runs into autumn cold before the fruit matures. Sow inside the window for your zone, once the soil is warm enough: see When to plant.
2. Letting vines go dry at flowering. Stressed plants shed flowers and young fruit, so the weeks of flowering, fruit set and fruit fill decide the crop. Water without a miss through those weeks, under straw mulch: see Care by growth stage.
3. Picking early and cutting the stalk short. Immature fruit has less flavour and stores poorly, and a stalk cut close lets rot into the fruit. Wait for the signs in Harvesting Fruit before you cut.

When to plant

Seed needs soil above 16°C, where it can take up to 14 days to emerge. At 20°C it is up within a week, and at 25°C in about four days. One sowing a season is usual, because the harvest keeps.

At the cold edge of the window, start seed in pots on a seedling heat mat and plant out after the last frost. At the late edge, choose a quicker type, because long-season pumpkins sown late can fail to mature before the cold.

Site, soil and setup

Pumpkin roots can reach a metre deep, so deep, free-draining soil with plenty of compost suits them. Plant at the edge of the vegie patch, where vines can run away from smaller crops.

Fresh manure burns young plants and encourages rhizoctonia rot, so use only aged manure. Cucurbits grown in the same soil year after year build up disease and nematodes in the soil.

Sowing Seeds

Sow direct into a mound where the season allows, and start in pots only where the warm months are short.

Sowing direct

• Sow two or three seeds per mound, 20 to 25 mm deep.
• Keep the mound moist; seedlings show in 5 to 10 days in warm soil.
• Thin each mound to the strongest seedling.

Times shift with climate: warm gardens are often quicker and cool gardens slower.

Starting in pots

• Sow two seeds to a pot at least 5 cm wide and deep. Biodegradable pots planted whole spare the roots.
• Grow seedlings on for about four weeks, until they carry two or three true leaves.
• Harden off (accustom seedlings to sun and wind over about a week), then plant out after the last frost.

Pollination

Each vine carries separate male and female flowers, and bees move the pollen between them. The first flowers are all male, on long thin stalks. Females follow, each sitting on a small swelling that becomes the fruit.

A female flower that misses out yellows and drops, often at golf-ball size. Hot, dry wind kills pollen, and cold, wet or cloudy weather keeps bees at home. Where fruit keeps failing, pollinate by hand early in the morning, soon after the flowers open.

• Pick a freshly opened male flower and strip its petals to expose the pollen-covered centre.
• Rub it gently onto the sticky centre of each open female flower.
• Use one male flower for several females, or carry the pollen on a small paintbrush.

Care by growth stage

Water at flowering and restraint with nitrogen decide this crop. Too much nitrogen early grows leaf at the expense of fruit, and female flowers can fail to set. Weeks count from sowing and suit the large types; quick types run ahead.

That 25 to 40 mm is 25 to 40 L per square metre. Where summers are humid, water in the morning and keep the leaves dry. In the dry heat of a South Australian summer, wet leaves dry fast and matter less.

If you prefer a conventional feed, mix a controlled release granular (NPK around 14-5-10) into the mound at planting. Water on an all-purpose soluble (NPK around 23-4-18) fortnightly while vines run, then a fruiting and flowering soluble (NPK around 15-4-26) from the first flowers.

Companion planting

Pumpkin is the ground layer of the old three sisters planting of corn, beans and squash.

Grows well with

• Sweet corn, which stands above the vines while pumpkin leaves shade out weeds below.
• Climbing beans, which use the corn as a pole and add nitrogen to the soil.
• Nasturtium, which can reduce pest pressure beside pumpkin and squash, according to university studies.
• Alyssum, borage and cosmos, whose flowers draw bees and beneficial insects to the patch.

Keep apart

• Potato, which hosts the same 28-spotted ladybird.
• Zucchini, cucumber and melons, which share both mildews, the mosaic viruses and cucumber fly.

Harvesting Fruit

Pick when the vine dies back and the stalk turns dry, brown and corky, usually 15 to 20 weeks from sowing.

• Tap the fruit: a ripe pumpkin sounds hollow.
• Press a thumbnail into the skin: ripe skin resists it.
• Cut with secateurs, leaving 5 to 10 cm of stalk.
• Lift fruit from underneath, never by the stalk, which can tear out and let rot in.
• Bring fruit in before a hard frost; a light frost that kills the vine rarely harms it.
• Set aside marked or stalkless fruit to eat first, since only sound fruit keeps.

Curing

Curing hardens the skin and dries the stalk, and it is what makes a pumpkin keep.

Store cured fruit on its side somewhere dry and airy at 12 to 15°C, such as a shed or back verandah. Below 10°C whole fruit suffers chilling injury, so keep it out of the fridge. Expect 2 to 6 months, and longer from good keepers: 'Jarrahdale' outlasts Jap and the small-fruited types. Check the pile often and remove any fruit that softens.

Roast it, turn it into soup, and toast the rinsed seeds with oil and salt as a snack.

Common problems

Young fruit yellows and drops. Usually the flower was not pollinated; see Pollination. Vines that dried out at flowering also shed fruit: see Care by growth stage.

All leaf and male flowers. Heavy nitrogen, high temperatures, dry air and long days all favour male flowers over females. Stop the fish emulsion and wait for milder weather.

Blossom end rot. A dark, sunken, leathery patch at the flower end of the fruit. Calcium reaches a swelling fruit only with steady water, so soil moisture that swings is the cause. Water evenly and mulch. More on blossom end rot

Fruit rotting underneath. Damp soil beneath a developing fruit starts the rot; slide straw or cardboard under each one.

Gnawed fruit. Rats and possums chew ripening pumpkins, and a broken skin will not store. Possums are protected, so rely on exclusion.

Pests and diseases

Seedlings are the vulnerable stage; a vine that is running outgrows most leaf damage.

Pumpkin beetle (Aulacophora hilaris). Yellow-orange, 6 to 7 mm long, with four large black spots; the plain pumpkin beetle (A. abdominalis) has none. Beetles feed in groups and chew leaves back to the veins, and a few can kill a seedling. Cover young plants with insect mesh until they are established.

28-spotted ladybird (Epilachna vigintioctopunctata). Dull orange with 28 black spots and a fine downy coat, unlike the glossy ladybirds that eat aphids. Adults chew the tops of leaves and the spiny, creamy-yellow larvae the undersides, leaving a skeleton of veins. Squash adults, larvae and the yellow egg clusters under leaves.

Cucumber fly (Zeugodacus cucumis) and Queensland fruit fly (Bactrocera tryoni). Cucumber fly is a major pest of cucurbits in Queensland, and occurs in the Northern Territory and north-east New South Wales. Pumpkin is one of its main hosts, and Queensland fruit fly has field records on C. pepo pumpkins. Maggots tunnel in stung fruit and rot follows. Bag young fruit once it has set, and freeze or boil stung fruit rather than composting it. Cucumber fly ignores the male lures in Queensland fruit fly traps. More on Queensland fruit fly

Melon aphid (Aphis gossypii). Pale green to yellow aphids cluster under leaves and on shoot tips, curling them and leaving sticky honeydew. Their worst damage is spreading papaya ringspot, zucchini yellow mosaic and watermelon mosaic viruses, which distort fruit, grey types most of all. Use insecticidal soap at the label rate on heavy colonies. Read more

Powdery mildew (Podosphaera xanthii). White powdery spots on the shaded, older leaves first. Spores travel on the wind and need humid air, not wet leaves. It matters most on young vines; late in autumn the crop is already made. Remove the worst leaves and spray 1 part full-cream milk to 10 parts water. More on powdery mildew

Downy mildew (Pseudoperonospora cubensis). Angular yellow patches boxed in by the leaf veins, in warm, moist weather early in the season. It needs 6 to 12 hours of wet leaf, so it follows rain and evening sprinklers. Read more

Prevention first

• Clear weeds and spent cucurbit vines before sowing, since both carry mildew, aphids and virus over.
• Give vines full sun and their full spacing, so air moves through the leaves.
• Check seedlings twice a week for beetles, and turn leaves for ladybird eggs and aphids.
• Pick up fallen and stung fruit promptly; mildewed vines can go into a hot compost heap.

Tips

1. Let vines root where they touch the ground. The extra roots feed and anchor the plant, so avoid shifting established runners.
2. Nip out the tip once the main vine is 3 to 4 m long. Late in the season, remove flowers and small fruit that cannot ripen before the cold.
3. Small-fruited types such as 'Golden Nugget' can climb a tripod or low trellis. Large fruit is too heavy.
4. Varieties cross readily within a species and rarely, if at all, between species, and a cross shows only in the saved seed. C. pepo pumpkins also cross with zucchini. For seed that comes true, tape a female flower shut before it opens. Next morning hand-pollinate it with pollen of the same variety, then retape it.
5. Pumpkin seedlings that volunteer from the compost heap are often the strongest vines in the garden. What they will fruit is a lottery.