How to Grow Zucchini in Australia

Zucchini (Cucurbita pepo), also sold as courgette, is a summer squash in the gourd family, Cucurbitaceae, grown for fruit cut young and tender. The usual failure is crowding, because a plant that spreads 1.5 m each way needs room and airflow it is rarely given.

Three mistakes that matter

1. Sowing into cold soil. Seed sown before the soil reaches the temperature in When to plant rots or sits. Seedlings set out into cold ground stall for weeks. Wait for warm soil, or start seed in pots four to six weeks before planting out.
2. Planting too close. A mature plant covers 1.5 m by 1.5 m, so plants at lettuce spacing shade each other, stay damp and catch powdery mildew early. Space plants as Site, soil and setup gives and give the bed morning sun.
3. Missing the fruit. A zucchini hides under the leaves and doubles in size in a day or two. A fruit left to swell into a marrow tells the plant to stop setting. Check plants daily and cut fruit at the size given in Harvesting Fruit.

When to plant

Zucchini seed germinates in soil between 21°C and 35°C, so sow when the soil, not the air, is warm. One or two plants feed a household, and a second sowing four to six weeks later keeps fruit coming after the first plants tire.

At the cold edge of the window, sow in pots on a seedling heat mat and plant out once nights stay mild. At the hot edge, shadecloth over the bed carries plants through an Alice Springs or Perth heatwave.

Site, soil and setup

Zucchini wants full sun, shelter from wind and heavy rain, and soil rich enough to drive a plant that grows a leaf a day. Where drainage is poor, a raised bed or a mound keeps the crown dry. It also spares seedlings from damping off (a fungal collapse at soil level).

Old cucurbit vines carry powdery mildew and virus into the next planting, which is why the bed rotates.

Container growing

One bush plant needs a pot at least 45 cm across, holding about 40 L or more, filled with premium potting mix blended with compost. Bush types such as 'Black Beauty', 'Cocozelle' and 'Grey Lebanese' suit pots.

• Water: daily in summer heat, at the base until it runs from the drainage holes.
• Feed: fish and seaweed blend at 25 mL per 9 L fortnightly, then liquid potash per label fortnightly from the first flowers.
• Position: full sun out of the wind, with room for leaves to spread a metre past the rim.

Sowing Seeds

Sow direct once the soil is warm, or start seed in pots where spring is slow or pumpkin beetles strip seedlings.

Sowing direct

• Sow 20 to 25 mm deep, two or three seeds to a station, at the spacing in Site, soil and setup.
• Keep the surface moist; seedlings show in 7 to 14 days, then thin each station to the strongest plant.
• Mulch with straw once the soil is warm, kept clear of the stems.

Starting in pots

• Sow one seed per 75 mm pot in seed-raising mix and keep it as warm as When to plant asks.
• Plant out four to six weeks after sowing, when the seedling has several true leaves, without breaking the root ball.
• Harden off (accustom seedlings to sun and wind over about a week) before planting at the spacing above.

Pollination

Zucchini carries separate male and female flowers on one plant, and bees move the pollen. Male flowers open first on long thin stalks. The females, each sitting on a tiny fruit, follow as the plant grows and the weather warms. In very hot or wet weather a plant can open only males, so flowers without fruit early on are normal.

A small fruit that yellows, shrivels and drops was never fertilised. Where bees are scarce, or the plant sits under insect netting, pollinate by hand in the morning before the female flowers close.

• Pick a freshly opened male flower and strip its petals to expose the pollen-coated stamen.
• Press the stamen onto the stigma in the centre of each open female flower; one male can serve several females.

Care by growth stage

Weeks count from sowing direct or from planting out.

That 25 mm is 25 L per square metre; sandy soils take less at a time, more often. If you prefer a conventional feed, mix a controlled release granular (NPK around 14-5-10) into the bed at planting. Water on an all-purpose soluble (NPK around 23-4-18) fortnightly while leaves grow, then a fruiting and flowering soluble (NPK around 15-4-26) from the first flowers.

Companion planting

Good partners bring the bees and hoverflies a zucchini depends on, or use the vertical space it leaves empty.

Grows well with

• Nasturtium, whose flowers draw the bees that set the fruit.
• Dill, coriander and parsley left to flower, since their umbels feed hoverflies whose larvae eat aphids.
• Climbing beans on a frame at the southern edge, which use height where the zucchini uses ground.

Keep apart

• Potato, which hosts the same 28-spotted ladybird.
• Pumpkin and cucumber, which share cucumber fly, powdery mildew and mosaic virus, and swamp a bed together.
• Tomato, another host of cucumber fly and the 28-spotted ladybird.

Harvesting Fruit

Pick from 6 to 9 weeks after sowing, while the skin is glossy and the fruit 15 to 20 cm long. Round types are good from golf-ball size and are at their best for stuffing at tennis-ball size.

• Cut the stalk with a knife or secateurs; snapping tears the plant and leaves a ragged end that rots.
• Wear gloves, because the leaf stalks carry fine glassy spines.
• Look under every leaf, and remove any fruit that has grown too large, even if you will not use it.

Zucchini keeps about a week in a plastic bag in the warmest part of the fridge, at 5 to 10°C. Colder than that for more than a few days, the skin pits and the flesh turns watery.

Eat it grilled, grated raw into salads, or baked into fritters and slices. Male flowers picked the morning they open, which costs the plant nothing, are for stuffing and frying.

Common problems

Flowers but no fruit. The first flush is male, and hot, wet or cloudy weather keeps bees home or brings males only; see Pollination.

Fruit rotting at the blossom end. Browning at the tip of a grown fruit is blossom end rot, from uneven watering; a tiny fruit that shrivels was unfertilised, see Pollination. Water steadily and mulch, since lime belongs in bed preparation and a calcium spray on the fruit does not fix it.

Bitter fruit. Mild bitterness follows heat and dry spells and fades once watering is steady. Strong bitterness is an inherited fault that has poisoned people in Australia: do not eat the fruit, pull the plant and never save its seed.

Leaves wilting on hot afternoons. Heat, not thirst, if the soil is moist; leaves recover by evening, and shadecloth helps in a heatwave.

Silver or yellow markings on the leaves. Many varieties carry silver mottling, and golden types often show yellow patches; both are colour, not disease. Twisted, blistered, small leaves signal virus; see Pests and diseases.

Pests and diseases

Bees set every fruit, so keep any spray, pyrethrum included, to late in the day and away from open flowers.

Cucumber fly (Zeugodacus cucumis). The cucurbit fruit fly of Queensland, the Northern Territory and north-east New South Wales. Maggots tunnel through young fruit, which rots or drops. Bag or net fruit once pollinated, pick daily, and freeze or boil stung fruit rather than composting it. Queensland fruit fly (Bactrocera tryoni) has validated field records on Cucurbita pepo, so gardeners in its range net or bait as well. Read more

Pumpkin beetle (Aulacophora hilaris). Yellow-orange, 6 to 7 mm, with four black spots, feeding in groups that chew large holes and leave only the veins. A few beetles can strip and kill a seedling, so check young plants twice a week. Catch beetles by hand early in the morning, and plant a spare seedling or two.

28-spotted ladybird (Epilachna vigintioctopunctata). Orange, 6 to 10 mm, with 28 black spots and a fine downy coat, unlike the glossy beneficial ladybirds. Adults chew holes and the yellow, spiny larvae graze leaf undersides; squash both, and the upright clusters of yellow eggs under the leaves.

Melon aphid (Aphis gossypii). This aphid and green peach aphid (Myzus persicae) carry zucchini yellow mosaic virus, which leaves foliage small, twisted and blistered and fruit lumpy and distorted. No spray cures it: pull and bin infected plants and wash secateurs. Hose small aphid colonies off or use insecticidal soap, and choose a variety sold with resistance where the virus is common. Read more

Powdery mildew (Podosphaera xanthii). White powder on the older, shaded leaves first, spreading in warm weather whether leaves are wet or not. It is usually the end of a zucchini plant. Cut out the worst leaves and spray 1 part full-cream milk to 10 parts water in the morning, repeating after rain. Read more

Prevention first

• Once mildew takes an old plant, sow a fresh one in a clean bed rather than nursing it.
• Keep netting or bags clear of the fruit, since fruit fly can sting through mesh that touches it.
• Turn leaves every few days for ladybird eggs, beetles, aphids and the first mildew spots.
• Bin spent vines, stung fruit and virus-struck plants rather than composting them.

Tips

1. In Brisbane and along the humid coast, grow a golden type beside a dark green one. The golden plants tend to keep cropping with mildew on their leaves.
2. Sink a 15 cm pot into the soil beside each plant and water into it. Water then reaches the roots without pooling around the neck, which can rot.
3. Save seed only from an open-pollinated variety grown away from other Cucurbita pepo such as pumpkin or pattypan. The parent's fruit is unaffected, but seedlings from crossed seed may not come true. Bag a female flower, hand-pollinate it and mark the fruit.