How to Grow Cauliflowers in Australia

Cauliflower (Brassica oleracea Botrytis Group) is a brassica in the cabbage family, Brassicaceae. It is grown for its curd, a tight head of undeveloped flowering shoots in white, orange, purple or green. It is a cool-season crop that rewards good timing, a rich bed and steady water. Many failures trace back to one thing: a check in growth that leaves a small, early button where a full head should be.

Three mistakes that matter

1. Sowing so heads form in heat or deep cold. Curds forming above about 26°C can turn loose and discoloured, and long spells below 10°C can trigger buttoning (a small, premature head). Count back from the cool months with the table in When to plant.
2. Planting out old or stressed seedlings. Overgrown transplants often stall after planting and button early. Plant out at 4 to 6 weeks, water in well, and never let young plants dry out.
3. Leaving young plants uncovered. Cabbage white butterfly (Pieris rapae) and diamondback moth (Plutella xylostella) caterpillars can strip seedlings in days, and diamondback caterpillars even pupate inside curds. Cover plants with fine insect mesh on hoops from the day of planting.

When to plant

Seed comes up in soil from about 10°C to 30°C, often within 4 to 10 days at 18°C to 24°C. The curd sets the timing, though: it forms best in mild, cool weather. Most types need 12 to 22 weeks from sowing, so sow while the soil is still warm and let the plants head as days cool. Sow a few plants at a time, several weeks apart, for a run of heads rather than a glut.

Sow the slow types early in a window and save quick types for the last sowings, so fewer heads mature in heat.

Site, soil and setup

Cauliflower is a heavy feeder, yet too much quick nitrogen grows leaves at the expense of the curd. Build the bed with slow organic matter instead.

Where clubroot has shown before, lime to lift the pH above 7.0. Liming also frees molybdenum, which cauliflower is prone to lack, but heavy liming can leave it short of boron (see Common problems). Test the soil first.

Container growing

One standard cauliflower needs a pot at least 30 cm wide and 30 cm deep, holding about 18 to 20 L. A compact type fits three to a 40 cm pot of about 28 L. Fill with premium potting mix blended with compost, and choose 'Mini White', 'Bunching F1' or the small-headed 'Alpha'.

• Water: never let the mix dry out, which means daily checks in warm weather.
• Feed: half a handful of pelletised chicken manure on the surface at planting, then fish and seaweed blend at 25 mL per 9 L fortnightly.
• Position: full sun, out of strong wind.

Sowing Seeds

Raise cauliflower in trays, where seedlings can be shaded and moved about, then plant them out young.

Starting in trays

• Sow 5 to 10 mm deep in seed-raising mix, two seeds to a cell.
• Keep the mix moist but not wet; seedlings show in 4 to 14 days.
• Keep pelleted seed, such as 'Orange F1', evenly moist until it emerges, since a drying coat draws water from the seed.
• On hot days move trays into dappled light, never full shade, which makes seedlings leggy.

Sowing direct

Direct sowing works too: sow a clump of seeds 5 to 10 mm deep at each spot and thin to the strongest. In hot months the seedbed may need water twice a day until seedlings emerge.

Transplanting

Space rows 60 to 75 cm apart.

• Plant out at 4 to 6 weeks, when seedlings stand 8 to 12 cm tall with two or three true leaves.
• Harden off (accustom seedlings to sun and wind over about a week) first.
• Plant into moist soil, bury a leggy stem up to its first leaves, firm the soil and water in with seaweed solution.
• Mulch with straw, clear of the stems.

Care by growth stage

Weeks count from planting out for a mid-season type; quick types move faster.

Sun can turn a white curd yellow or purple. Once it is 5 to 8 cm across, fold two or three outer leaves over it and tie them loosely with twine or a peg. Purple, orange and green types need no cover.

If you prefer a conventional feed, use each at its label rate. Mix a controlled release granular (NPK around 14-5-10) into the bed at planting. From week three, water on an all-purpose soluble (NPK around 23-4-18) fortnightly until curds show. No flowering feed is needed, since the curd is cut before it flowers.

Keep the mesh on, but lift it once a week to check under the leaves. It also shuts out the birds and beneficial insects that eat caterpillars and aphids, so pests that slip inside can build up unseen.

Thinning

Cauliflower needs no pruning, only thinning and a little tidying, so every plant grows on without a check.

• Snip the weaker seedling in each cell or clump once the first true leaf shows, so the keeper's roots stay undisturbed.
• Plant out only the strongest seedlings and compost the rest.
• Remove yellowing lower leaves, and bin any with white fuzz or black speckling.
• Weed while plants are young, then leave the soil around heading plants undisturbed.
• Cut the main curd of 'Multi Head F1' promptly, since the side curds follow it.

Companion planting

Good partners either confuse the cabbage white butterfly or host none of cauliflower's brassica-only pests and diseases.

Grows well with

• Dill, sage and coriander, whose strong scent the cabbage white butterfly avoids.
• Onions and dwarf beans, which host neither clubroot nor the brassica caterpillars.
• Land cress at the far end of the garden, a decoy for the butterflies' eggs; bin it once caterpillars appear.

Keep apart

• Cabbage, broccoli, kale and other brassicas in the same bed, which share clubroot, downy mildew (Hyaloperonospora parasitica) and diamondback moth.
• Mustard grown as a cover crop just before cauliflower, since it hosts the same pests.
• Wild radish, wild turnip and other cruciferous weeds, which carry diamondback moth and downy mildew between crops.

Harvesting Heads

Cut when the curd is full size, firm and tight, before the florets begin to separate.

• Check covered heads every few days, as a curd passes its best quickly in warm weather.
• Cut the stem 2 to 3 cm below the head with a sharp knife.
• Expect full-size white heads of about 15 to 20 cm across, and 10 to 15 cm on 'Mini White'.
• Cut an overdue head anyway: a loose curd is still good to eat.
• Leave the stump of any type that shoots, for a few small side heads.

Eat cauliflower soon after cutting for the best flavour. Even very cold, humid storage holds a head for only 2 to 4 weeks, so cut heads as you need them.

Roast or steam the florets, add them to a curry, or grate the head as a rice substitute.

Common problems

Small early buttons. A coin-sized curd that stops growing follows a check from old transplants, drought or a long cold spell. Cut and eat it, and see Three mistakes.

Loose, grainy or discoloured curds. Heat at heading can loosen the curd, sun can discolour an uncovered white head, and an overdue head turns grainy. See Three mistakes, and the leaf folding under Care by growth stage.

Whiptail. Leaves grow narrow, twisted and strap-like with yellow, dying edges, and no head forms. The cause is a lack of molybdenum, most often in soil below pH 6.5. Lime the bed before planting. A soluble trace element spray with molybdenum, used per label at the seedling stage, can prevent it, but sprays fail once symptoms show.

Hollow stem, bronzed curd. Both often mean boron deficiency, most likely on sandy soil or after heavy liming. It cannot be cured once seen. Where a soil test shows a shortfall, add a soluble trace element mix with boron before planting, per label: too much is toxic.

Running to seed. The head opens into a leafy, branching flower stalk, most often when heads form in hot weather or spring sowings meet rising heat. Cut and use it at once.

Seedlings chewed overnight. Ragged holes and silvery trails point to slugs and snails, which take newly planted cauliflowers first. Read more

Pests and diseases

Use only sprays labelled for the pest on brassicas, and keep to any withholding period before picking.

Cabbage white butterfly (Pieris rapae). A white butterfly with grey-black wing spots lays eggs under leaves, and its well-camouflaged green caterpillars chew ragged holes and leave green droppings. Squash eggs and caterpillars, and use a Bacillus thuringiensis (Bt) caterpillar spray for heavy numbers. Read more

Diamondback moth (Plutella xylostella). Pale yellowish-green caterpillars, up to 12 mm, chew see-through windows in leaves and tunnel into the plant. It resists many insecticides, so rely on mesh and weekly checks, when small dark-headed caterpillars are easiest to control. Read more

Cabbage aphid (Brevicoryne brassicae). Blue-grey colonies dusted with whitish powder cluster on young growth and spread plant viruses. Hose off small colonies or use insecticidal soap, and leave the puffed-up aphid mummies, which mean tiny wasps are at work. Read more

Clubroot (Plasmodiophora brassicae). Roots swell into deformed clubs, and plants stunt, yellow and wilt in warm weather. Its spores persist in soil for many years. Bin affected plants with their roots, never compost them, then lime and rotate as in Site, soil and setup.

Downy mildew (Hyaloperonospora parasitica). White, fluffy spores form under seedling leaves, with black speckling above, and on cauliflower it can blacken the curd inside and out. It thrives at 8°C to 16°C with wet leaves, so keep seedlings as dry as you can. Read more

Prevention first

• Plant clean, healthy seedlings into a limed, rotated bed.
• Water at the base, and not over seedlings in the morning, when downy mildew releases its spores.
• Clean soil off tools and boots after working a clubroot bed.
• Pull cruciferous weeds, and bin diseased plants rather than composting them.

Tips

1. Sow a quick type such as 'Alpha' beside the slower 'Selection 174' on the same day for heads over several weeks.
2. In warm districts, try the types described for warmer conditions: 'Mini White', 'Graffiti F1' and 'Macerata Green'.
3. Seed saved from F1 hybrids such as 'Orange F1' does not come true, so buy fresh seed for them.