How to Grow Brussels Sprouts in Australia

Brussels sprouts (Brassica oleracea Gemmifera Group) are the slow, stately member of the cabbage family, Brassicaceae. They are grown for the firm buttons that form in the leaf joints up a tall stem as winter sets in. Frost sweetens them, so they reward gardeners with cold nights. The usual mistake is sowing them in autumn beside the broccoli, when they need the whole summer to build a plant.

Three mistakes that matter

1. Sowing too late. Sprouts take about five months to form, and they must be forming as the cold arrives. Plants sown in autumn meet winter small, crop poorly and can bolt (run to flower) in spring. Sow in late spring or early summer, inside your zone's window.
2. Leaving summer seedlings uncovered. Young plants face the peak of the cabbage white butterfly season, and seedlings suffer the worst of the caterpillar damage. Raise and plant them under a frame covered with light shadecloth or fine insect mesh from day one.
3. Planting shallow in an open, windy bed. A plant carrying a winter of sprouts grows tall and top heavy, and a gusty westerly can flatten it. Plant deep, up to the first leaves, in a sheltered spot, and draw soil up the stem as it grows.

When to plant

Timing decides this crop more than soil warmth does. The seed germinates in soil from 7°C to 30°C, so summer soil suits it. The aim is plants big enough to set sprouts as nights turn cold. In most temperate and cool gardens, that means sowing by the end of December. One sowing is enough; a second tray a few weeks later spreads the picking.

Where a window opens in early summer heat, keep trays shaded and check them morning and evening.

Site, soil and setup

Give Brussels sprouts your sunniest, most sheltered bed, since plants can stand over a metre tall and hold it for most of the year.

A home pH kit shows whether the bed needs lime; the soil pH article covers testing.

Container growing

One plant needs a pot at least 30 cm across and 30 cm deep, about 20 L, with drainage holes and premium potting mix. A wide, heavy pot stands steadier than a tall, narrow one. The variety in the range, 'Long Island Improved', is compact and suits a large pot.

• Water: check daily and water when the top 2 to 3 cm of mix feels dry.
• Feed: fish and seaweed blend at 25 mL per 9 L, fortnightly, from two weeks after planting.
• Position: full sun, out of the wind, with the pot against a wall or tied to a railing on an exposed balcony.

Sowing Seeds

Raise seedlings in punnets or cells and plant them out, so summer seedlings stay where you can shade and watch them.

Starting under cover

• Sow 5 to 10 mm deep in seed-raising mix, two seeds per cell.
• Keep the mix moist; seedlings usually come up in about a week.
• Stand trays under light shadecloth or fine mesh on a frame, out of the hottest sun and away from butterflies.
• Snip out the weaker seedling once true leaves show, and pot on into 10 cm pots if planting is delayed.
• Harden off (move seedlings outdoors for a little longer each day over about a week) before planting.

Direct sowing also works in cool districts, but seedlings in the open bed need the same shade and netting.

Transplanting

• Plant out 4 to 7 weeks after sowing, before roots fill the cell.
• Space plants 60 cm apart, or up to 90 cm to walk between them at picking time; wide spacing also lets sprouts dry after rain.
• Set each plant deep, up to its first leaves, in a shallow dip so soil can be drawn up the stem later.
• Firm the soil and water in with seaweed solution at half strength, 15 mL per 9 L.

Care by growth stage

These plants must grow without a check. Steady water and a nitrogen-led feed through summer build the leafy frame, then feeding shifts towards potassium once the buttons start. Weeks count from planting out.

If you prefer a conventional feed, mix a controlled release all-purpose granular (NPK 12-3-8 to 14-5-10) into the bed at planting. Through leafy growth, use an all-purpose soluble (NPK 23-4-18 to 27-5-9) fortnightly. Once sprouts start, switch to a fruiting and flowering soluble (NPK 15-4-26 to 15-5-20), also fortnightly.

Removing Lower Leaves

Stripping lower leaves is optional in a home garden. It lets air reach the lowest sprouts, cuts the wind load and eases picking. Leave a crown of large, healthy leaves on top, because they feed the plant.

• Yellow or damaged leaves: snap them off at the stem as soon as they appear.
• Once sprouts form: remove the lowest six to eight leaves, then two or three more each week.
• At each pick: take the two or three leaves just above the sprouts you picked, so the next ones dry quickly after rain.
• Windy sites: draw soil up around the stem as it grows, and push in a stake beside any plant that leans.
• Topping: about three weeks before you want the last sprouts, cut out the growing tip so the upper buttons fill together.

Companion planting

A crop that stands this long gains most from neighbours that feed the insects hunting its pests.

Grows well with

• Dill and other herbs left to flower, whose blooms feed hoverflies; their larvae eat aphids.
• Onions, which grow low and upright between plants without shading them.
• Beetroot and dwarf beans, low crops that fill the gaps while the sprouts are young.
• Nasturtiums at the bed's edge, where aphids often gather first, in plain view for removal.

Keep apart

• Cabbage, broccoli, cauliflower and kale massed in one block, since they share caterpillars, aphids and clubroot.
• Mustard, bok choy and Chinese cabbage in the same row, which draw the same aphids and caterpillars.
• Tomatoes and climbing beans, tall summer crops that shade young plants in the same bed.

Harvesting Heads

The lowest sprouts are usually ready 11 to 16 weeks after planting out, once they are firm, bright green and about walnut size.

• Pick from the bottom up, a few from each plant at each visit.
• Twist each sprout downwards to snap it off, or cut it flush with a sharp knife.
• Pick while buttons are tight; loose or yellowing ones are past their best.
• Where frosts come, start picking after the first one.
• Expect to keep picking for 2 to 3 months as the crop ripens up the stem.
• At the end, cut the top knot of leaves and cook it like a small cabbage.

Store sprouts unwashed in a plastic bag in the fridge crisper. Flavour is best within a day or two, although very cold, moist storage near 0°C holds them for 3 to 5 weeks. Roast them, stir-fry them or shave them raw into salads, and take them off the heat while still bright green.

Common problems

Loose, leafy or bitter sprouts. Buttons that stay open, often called blown sprouts, form in warm weather, in dry soil or after too much nitrogen. Sprouts that mature in hot, dry weather can also turn bitter. Late sowing is the usual cause, covered in Three mistakes that matter.

Patchy emergence in summer. Trays that dry out in the heat, or seed sown deeper than 10 mm, are the usual causes. Read more

Seedlings collapsing at soil level. Damping off thrives in wet, crowded, airless trays. Read more

Tall, floppy seedlings. Seedlings raised in too much shade stretch towards the light and topple when planted out. Read more

Ragged holes and silvery trails. Snails and slugs graze seedlings on damp nights; hunt them by torchlight after dark. Read more

Pests and diseases

Caterpillars and aphids do most of the damage; the diseases follow damp weather or acid, wet soil.

Cabbage white butterfly (Pieris rapae). Its velvety green caterpillars chew ragged holes, and one female can lay up to 800 eggs. Pick caterpillars off, or spray Bacillus thuringiensis, a bacterial insecticide that affects only caterpillars, while they are small. Read more

Diamondback moth (Plutella xylostella). Plump, bright green caterpillars about 10 mm long graze leaf undersides into windows and can tunnel into sprouts, beyond the reach of sprays. They abound from spring to autumn. Mesh and Bacillus thuringiensis control them as for cabbage white. Read more

Cabbage aphid (Brevicoryne brassicae). Dull grey-green aphids up to 3 mm long form powdery, blue-grey colonies under leaves and between buttons, worst in warm districts. Hose them off, then spray insecticidal soap under the leaves as the label directs. Read more

Downy mildew (Hyaloperonospora parasitica). Yellow patches on the upper leaf carry a greyish white growth beneath, worst on crowded seedlings in dew and drizzle. Give plants their full spacing so air moves between them. Read more

Black rot (Xanthomonas campestris pv. campestris). This bacterium enters through leaf margins, making V-shaped yellow areas with dark veins, and spreads in wet weather. Remove affected leaves as soon as they show, and keep brassica weeds out, as in Prevention first.

Clubroot (Plasmodiophora brassicae). Roots swell into knobbly galls, so plants yellow and wilt in the warm part of the day. Its spores can survive in soil for up to 20 years. Bin affected plants with their roots, and clean soil off tools and boots.

Prevention first

• Every few days, lift the netting and turn a few leaves to look for eggs, caterpillars and aphids.
• Pull brassica weeds such as shepherd's purse and wild mustard, which harbour clubroot, downy mildew and aphids.
• Water at soil level early in the day so leaves dry quickly.
• Dig out the stumps and roots straight after the last pick rather than leaving them standing.

Tips

1. Spare any caterpillar wrapped in a bundle of tiny white cocoons. They belong to Cotesia wasps, which parasitise cabbage white caterpillars.
2. 'Long Island Improved' is an heirloom, so saved seed can come true. Plants left in the ground flower in spring after winter cold. They cross with cabbage, kale, broccoli, cauliflower and kohlrabi, all Brassica oleracea. Let only one of them flower at a time, and leave at least five plants to flower together.
3. Blanch surplus sprouts and freeze them; frozen, they keep for up to a year.