How to Grow Broccoli in Australia

Broccoli (Brassica oleracea var. italica) belongs to the cabbage family, Brassicaceae. It is grown for heads of tight green flower buds and the side shoots that follow. It suits a cool-season bed in every Australian zone, and a well-grown plant keeps cropping for weeks after the first cut. What most gardeners get wrong is timing, because a head must form in cool weather to be worth picking.

Three mistakes that matter

1. Sowing into the heat. Heads that form in hot weather grow loose and bitter, then burst into flower within days. Count back from the cool months: most types take 10 to 16 weeks from seed to a cutting head, purple sprouting about 130 days.
2. Leaving plants uncovered. Cabbage white butterfly (Pieris rapae) and diamondback moth (Plutella xylostella) lay eggs on the leaves, and their caterpillars can strip a young plant in days. Cover seedlings with insect mesh on hoops the day they go in.
3. Letting growth stall. Seedlings held too long in punnets, or checked by drought, hunger or cold, form a tiny early head called a button and go no further. Plant out at four to six true leaves and keep water and feed steady.

When to plant

Broccoli seed germinates in soil from 7°C to 30°C, best below about 22°C, and the plant grows best when days stay under about 24°C. Time each sowing so heads form in the coolest months, and sow a few seeds every two weeks through the window to spread the picking.

At the warm edge of a window, raise seed in shade and choose a quick type such as 'Heading F1' or 'Ramoso Santana'. In cool gardens, plant out by early autumn so plants are established before winter.

Site, soil and setup

Broccoli is a heavy feeder, so build the bed up well before planting.

Clubroot spores can live in soil for up to 20 years, and warm, acid, waterlogged ground suits them. That is why the pH, drainage and rotation rows matter more for broccoli than for most crops.

Container growing

One plant needs a pot at least 33 cm across, holding 18 to 20 L of premium potting mix blended with compost. Compact types suit pots: 'Umpqua', 'Waltham' at about 52 cm tall, and 'Fusion F1' with its compact frame.

• Water: pots dry out faster than beds, so check them daily and water until it runs from the base.
• Feed: fish and seaweed blend at 25 mL per 9 L, fortnightly from planting out.
• Position: full sun, with insect mesh over a hoop fixed to the pot.

Sowing Seeds

Start broccoli in cells and plant out sturdy seedlings, or sow a few seeds direct where each plant will stand.

Choosing a type

Starting under cover

• 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.
• Plant out 4 to 6 weeks after sowing, when seedlings stand 7 to 10 cm tall.
• Harden off (accustom seedlings to outdoor sun and wind) over one to two weeks first.

Some hybrid seed, including 'Fusion F1' and 'Marathon F1', carries a fungicide seed treatment. Never eat it or feed it to animals. Handle it with gloves, wash your hands afterwards, and keep it away from children and animals.

Sowing direct and planting out

• Sow direct by dropping two or three seeds into each station, 10 mm deep.
• Space plants 40 to 50 cm apart in rows 50 cm apart; sprouting types can take 60 cm for more side shoots.
• Plant seedlings and firm the soil well around each stem.
• Water in with seaweed solution at 30 mL per 9 L.

Care by growth stage

Steady water and steady nitrogen decide the size of the head. Weeks count from planting out, for a mid-season type; quick types move through faster.

Keep the mesh on from planting to the last pick, stretched over hoops with the edges weighed down. Lift it only in the early morning or late afternoon, when butterflies are not flying. Mulch with 5 cm of compost once plants are established, and pull it aside when watering so water reaches the roots.

If you prefer a conventional feed, use each of these at its label rate. Mix a controlled release granular (NPK around 14-5-10) through the bed at planting. Water on an all-purpose soluble (NPK around 23-4-18) fortnightly while leaves build, then a fruiting and flowering soluble (NPK around 15-4-26) fortnightly once heads show.

Thinning

Crowded plants make small heads and few side shoots, and very wide spacing brings its own trouble (see Hollow stem in Common problems).

• Thin direct-sown stations to the strongest seedling at three to four true leaves.
• Snip unwanted extras at soil level rather than pulling them, so the keeper's roots stay undisturbed.
• To keep a good thinning instead, lift it gently with a ball of soil and replant it elsewhere.
• In cells, keep one seedling per cell once both are up.

Companion planting

Good neighbours either mask broccoli from egg-laying butterflies or fill space without shading it.

Grows well with

• Dill and coriander, strong-scented herbs the cabbage white is said to avoid.
• Sage and other aromatic herbs, for the same reason.
• Onions, upright and narrow, filling gaps between plants without shading them.
• Dwarf beans, which stay low enough not to shade the row.

Keep apart

• Cabbage, kale, mustard and other brassicas, which share cabbage aphid, diamondback moth and clubroot.
• Wild radish and wild turnip weeds, which carry diamondback moth and aphids into the bed.
• Tomato, another heavy feeder competing for the same nitrogen.
• Climbing beans, which grow tall enough to shade a low brassica row.

Harvesting Heads

Cut the central head while its buds are tight and dark green, before any yellow shows. Once flowers open, the flavour turns strong and the stem woody.

• Cut the stem on a slant about 15 cm below the head, so water runs off the stump.
• Leave the plant standing; side shoots grow from the leaf joints below the cut.
• Pick side shoots small and tight, cutting them the same way.

Chill heads straight after cutting. Broccoli keeps one to two weeks at 0°C in very humid air. A home fridge runs warmer, so bag it in the crisper and eat it first. The peeled stem and young leaves are as good as the head, steamed, stir-fried or sliced raw.

Common problems

Tiny heads soon after planting. This is buttoning (see Three mistakes); early varieties and overgrown transplants are the most prone.

Hollow stem. Cracks inside the stem join into a cavity, most often in large, fast varieties given wide spacing and plenty of nitrogen. Boron is rarely the cause, so keep to the spacing in Sowing Seeds instead.

No head at all. Through long spells of days over about 30°C and nights over 25°C, plants can stay leafy because they lack the cool they need to head.

Seedlings collapsing at soil level. Damping off, a fungal rot at the stem base, strikes in wet, crowded seed mix. Read more

Plants chewed overnight. Rats often eat broccoli, a problem Canberra's organic growers know well. Read more

Seedlings grazed, with silvery trails. Snails and slugs, including the garden snail (Cornu aspersum), favour young brassicas. Collect them by torchlight and clear nearby hiding places. Read more

Pests and diseases

Check under the leaves and into the heads every week, because most of these start small and hidden.

Cabbage white butterfly (Pieris rapae). White butterflies with black wing spots lay eggs under leaves, and velvety green caterpillars chew ragged holes. Pick caterpillars off into a bucket for the chooks, or spray a Bacillus thuringiensis subsp. kurstaki product at its label rate. Cover leaf undersides while caterpillars are small; it acts only on caterpillars that eat it. Read more

Diamondback moth (Plutella xylostella). Pale green caterpillars up to 12 mm long wriggle backwards when touched, leave see-through windows in leaves and can pupate inside florets. It resists many insecticides, so rely on mesh, and act while caterpillars are small and dark-headed, when they are easiest to control. Read more

Cabbage centre grub (Hellula hydralis). Cream caterpillars with a dark head and red-brown stripes, up to 15 mm long, web leaves together and bore into seedling centres. Worst in summer and autumn, they shelter from contact sprays, so check centres weekly for three weeks after planting out and remove any grubs.

Cabbage aphid (Brevicoryne brassicae). Dense blue-grey colonies coated in whitish powder build under leaves and into heads, worst in autumn and spring. Hose them off early, before they reach the head. Read more

Downy mildew (Hyaloperonospora parasitica). White fluffy spores under seedling leaves are followed by black speckling and yellowing lower leaves, in cool, damp weather around 8°C to 16°C. Water less often but more deeply, keep air moving, and remove infected seedlings; 'Marathon F1' carries intermediate resistance. Read more

White blister (Albugo candida). White, dusty blisters form under the leaves, below yellow to brown spots, and heads can distort. Spores need wet leaves to infect, so water with one short, heavy soak, avoid watering at night, and clear crucifer weeds.

Black rot (Xanthomonas campestris pv. campestris). Yellow V-shaped patches with dark veins spread in from the leaf edges, worst in wet weather. Control weeds, work among young plantings before older ones, and lift finished plants promptly.

Clubroot (Plasmodiophora brassicae). Plants yellow, wilt and stunt, and a lifted root shows swollen, knobbly galls. There is no cure once it is in the soil, so lime, drain and rotate as Site, soil and setup describes. Clean soil off tools and boots after working an infected bed.

Prevention first

• Start with clean, healthy seedlings, whether raised at home or bought.
• Clear wild radish, wild turnip and shepherd's purse from around the bed.
• Lift spent plants and their roots straight after the last pick.
• Let a few herbs flower nearby, since parasitic wasps and hoverflies need nectar.

Tips

1. Sow a quick type and a slow one together, such as 'Heading F1' and 'Purple Sprouting Early', to spread the picking over months.
2. Skip paper butterfly decoys: there is no evidence female cabbage whites avoid one another, and mesh does the job.
3. Plunge lightly cooked broccoli into cold water to keep it bright green and crisp.