How to Grow Mustard in Australia

Mustard (Brassica juncea), also called mustard greens, leaf mustard or Indian mustard, is a fast leafy annual in the cabbage family, Brassicaceae. It is grown for its peppery leaves, thick midribs and, in one group, a swollen edible stem. It crops within weeks through the cool months in every Australian zone, in forms from frilled green salad leaves to deep purple giants.

Three mistakes that matter

1. Sowing into summer heat. Hot weather sends mustard quickly to seed, and the leaves turn fierce and bitter. Sow from autumn to spring at the times in When to plant.
2. Watering only now and then. Water stress makes the leaves harsh and unpleasantly hot, and it hastens flowering. Water before the surface dries, mulch with straw, and keep growth fast.
3. Leaving the leaves to grow old. Pungency builds as leaves age, so a row left too long turns tough and harsh. Start picking outer leaves from about four weeks, and sow a short row every two to three weeks rather than one big row.

When to plant

Mustard seed germinates in soil from about 10°C to 35°C, but plants grow best in mild weather of 15°C to 20°C. Above about 25°C they often turn bitter or bolt (flower early). That makes mustard an autumn to spring crop in the south and a dry-season crop in the tropics.

For late sowings that run into warming weather, 'Greenwave' and 'Broad Leaf' are slow to bolt. For early sowings in frosty districts, choose a curled type, which handles frost better than smooth-leaved ones. A hard freeze can still damage the leaves, so cover rows with frost cloth on the coldest nights.

Site, soil and setup

Mustard wants fertile soil that holds moisture yet drains freely, because fast, unchecked growth keeps the leaves mild.

Test before liming: see soil pH explained for how to read and adjust it.

Container growing

A pot at least 25 cm wide and 23 cm deep, about 8 L, grows a steady supply of baby leaf. Use a premium potting mix rather than garden soil. Sow a type that is good cut young, such as 'Wasabina', 'Red Giant' or 'Lime Streaks'.

• Water: feel the mix each day and water once the top turns dry.
• Feed: fish and seaweed blend at 25 mL per 9 L, fortnightly from the first true leaves.
• Position: full sun in the cool months; move the pot into afternoon shade as the weather warms.

Sowing Seeds

Most mustard is sown straight into the bed, with punnets as the fallback when the space is still occupied.

Sowing direct

• Sow about 5 mm deep into damp, raked soil, in rows 30 to 45 cm apart.
• For baby leaf, scatter seed thickly along a wide band and skip thinning.
• For full plants, sow a seed every few centimetres and thin later: see Thinning.
• Seedlings usually emerge in 7 to 14 days if the surface stays moist.
• Once seedlings stand, tuck a thin straw mulch between the rows.

Starting in punnets

Cells suit the big types such as gai choy, which go in at wide spacing anyway.

• Sow two or three seeds per cell of seed-raising mix, at the same depth, and keep the strongest.
• Plant out when seedlings are 5 to 10 cm tall, usually 4 to 6 weeks after sowing.
• Harden off (give seedlings a week of gradually longer spells outdoors) before they go in.

Mustard types

Care by growth stage

Week numbers run from sowing, and picking begins before plants reach full size.

If growth stalls in poor soil, a light dose of all-purpose controlled release granular (NPK around 14-5-10) scratched into the soil can help. Morning watering lets foliage dry by nightfall, which helps keep white rust away.

Thinning

Crowded rows hold damp air around the leaves, which invites disease; thin in stages and put the pulled seedlings in the salad bowl.

• Start thinning as soon as you can grip a seedling, and pull the whole plant.
• Finish curled and red types at 20 to 30 cm, and broadleaf types and 'Japanese Giant Garnet' at 30 to 45 cm.
• Where roots tangle with a keeper, snip the surplus with scissors rather than pulling it.
• Pull off older yellowing leaves as they appear.
• Pinch out a rising flower stalk to gain a little more picking time.

Companion planting

Grows well with

• Dill, whose flowers feed the parasitic wasps that attack cabbage white caterpillars.
• Onions, whose narrow leaves take little light from the row.
• Dwarf beans, low plants that leave light for the row.
• Beetroot, a low root crop that shares the space without crowding the leaves.

Keep apart

• Cabbage, broccoli, kale and other cabbage-family crops, which share caterpillars, aphids and white rust.
• Tomatoes and climbing beans, tall warm-season crops that throw shade across mustard sown beside them.

Harvesting Leaves

Start picking outer leaves about four weeks after sowing, once plants stand 10 to 15 cm tall. Baby leaf can be cut from about three weeks.

• Snip the outer leaves and let the inner ones grow on.
• For baby leaf, cut the whole row just above the soil.
• Once a flower stalk rises, cut the tender shoots and buds and cook them like broccolini.
• Lift stem mustard once the stem base has swollen, then peel it and cook or pickle it.

Refrigerate mustard greens as soon as they are picked. In cold, humid storage near 0°C they last one to two weeks.

Young leaves bring heat to a salad, and older ones suit stir-fries, soups and curries, where cooking softens the bite. Many types are also pickled. Pickle only by a tested method, such as CSIRO's preserving advice, because vegetables kept without enough acid can carry botulism.

Common problems

Early flowering. Plants throw up a leafy stalk topped with yellow buds, and new leaves come small and sharp. Heat, dry soil and age set it off, and the fix is in Three mistakes.

Leaves too hot to eat raw. Heat, dry soil and age raise the bite: see Three mistakes, and cook the older leaves.

Holes in the leaves. Green droppings point to caterpillars, and a silver slime trail to snails or slugs: see Pests and diseases.

Seedlings everywhere next season. Plants left to ripen pods self-sow freely, 'Black' most of all, so cut flower stalks before the pods dry.

Pests and diseases

Mustard is bothered by few pests, and those below are the ones that do real damage. Use only sprays and baits labelled for the pest on leafy or brassica vegetables, and keep to any withholding period before picking.

Cabbage white butterfly (Pieris rapae) and diamondback moth (Plutella xylostella). Their green caterpillars chew holes in the leaves, and diamondback caterpillars drop on a silk thread when touched. Check leaf undersides, squash eggs and caterpillars, and spray a product based on Bacillus thuringiensis if numbers build. Fine insect mesh keeps both off where they are bad. Read more

Turnip aphid (Lipaphis pseudobrassicae) and cabbage aphid (Brevicoryne brassicae). Dense colonies under the leaves suck sap, curling and yellowing them, and cabbage aphids wear a whitish, powdery coat. A firm jet of water or insecticidal soap clears most colonies, and ladybirds and lacewings help keep numbers down. Read more

Common garden snail (Cornu aspersum) and grey field slug (Deroceras reticulatum). They graze seedlings and tender leaves overnight, most of all after rain. Go out with a torch after dark when it is damp, and use only an iron-based bait if one is needed. Read more

White rust (Albugo candida). Shiny white blisters form under the leaves, with yellow patches above, in cool weather with heavy dew; Indian mustard is among its worst-hit hosts. Remove affected leaves, water in the morning and rotate beds.

Downy mildew (Hyaloperonospora parasitica). A fluffy white mass of spores grows under seed leaves and young leaves, with black speckling and puckering above, but no blisters. Seedlings in the cooler months suffer most. Keep air moving between plants and pull out badly infected seedlings. Read more

Clubroot (Plasmodiophora brassicae). Plants yellow, wilt and stunt, and a lifted root shows swollen galls. Spores can last up to 20 years in soil, and warm, acid, wet ground suits them. Where it is known, lime to pH 7.0 to 7.5, improve drainage and keep brassicas out of that bed for at least five years.

Prevention first

• Winter rows in the south usually see few caterpillars; in the tropics and arid zone, cool-season rows still need close checks.
• Check leaf undersides weekly for eggs, caterpillars and aphids.
• Clear cabbage-family weeds such as wild radish and turnip weed, which shelter caterpillars and aphids.
• Rotate beds away from cabbage-family crops, as set out in Site, soil and setup.

Tips

1. Indian mustard is the most studied biofumigant (a crop dug in to suppress soil disease). Grow a bed to early flowering, 8 to 12 weeks, then chop and dig it in: see brassica biofumigation. Skip it in soil with clubroot, which brassica cover crops can worsen.
2. Rows sown around Anzac Day in southern gardens grow into cooling weather and usually stand well through winter.
3. Leave a plant or two to flower at the end of the season, because bees and hoverflies visit the yellow flowers.