How to Grow Komatsuna in Australia

Komatsuna, or Japanese mustard spinach (Brassica rapa Perviridis Group), is a leafy green of the cabbage family, Brassicaceae. Its glossy leaves are eaten raw as baby leaf or cooked like spinach. A close cousin of turnip and bok choy, it copes with both heat and frost, so it suits much of the year in most gardens. The common mistake is waiting for a heart that never comes: komatsuna does not form a true head, and old leaves turn strong and hot.

Three mistakes that matter

1. Waiting for a head. The plant grows as a loose, upright bunch of leaves, and their flavour sharpens and heats up with age. Start picking outer leaves at about three weeks, and cut whole plants before a flower stem rises.
2. Letting the bed dry out. Komatsuna roots near the surface, so a dry spell makes leaves bitter and pungent and can push plants into flower. Mulch the bed and water whenever the top few centimetres feel dry.
3. Sowing without a cover. Caterpillars of cabbage white butterfly (Pieris rapae) and diamondback moth (Plutella xylostella) chew young leaves to lace. Stretch fine insect mesh over hoops on sowing day and bury or weigh down its edges.

When to plant

Komatsuna seed germinates in soil from about 10°C to 30°C, so heat and day length set its calendar rather than cold soil. For a steady supply, sow a short row about every four weeks through your window.

At the hot end of a window, sow where afternoon shade falls or stretch shadecloth over the row. Where frosts are hard, a clear plastic tunnel on hoops shelters spring seedlings and lengthens the season.

Site, soil and setup

Pick a sunny bed of rich, free-draining soil. Choose a spot where no brassica has grown for four years, so clubroot (Plasmodiophora brassicae) cannot build up.

Check with a home pH kit before adding any lime; the soil pH article walks through testing and adjusting.

Container growing

Like other Asian greens, komatsuna thrives in pots on a sunny balcony. A pot at least 25 cm across and 20 cm deep holds about 8 L. That is room for three plants picked leaf by leaf, or a thick sowing cut as baby leaf. Use premium potting mix in a pot with drainage holes; the upright habit of 'Red Leaf' suits troughs too.

• Water: check daily and water when the top of the mix feels dry; shallow roots in a pot dry out fast in sun and wind.
• Feed: fish and seaweed blend at 25 mL per 9 L, fortnightly, from the first true leaves. Or mix a controlled release fertiliser for containers into the mix at sowing.
• Position: full sun in the cool months; move the pot into afternoon shade once the weather heats up.

Sowing Seeds

Sow komatsuna straight into the bed, or raise it in cell trays and plant it out.

Sowing direct

• Sow 5 mm deep in rows 25 cm apart, trickling the seed thinly along the drill.
• Firm the soil lightly and water with a fine rose.
• Keep the surface damp; seedlings usually appear in 4 to 10 days.
• Thin in stages, as set out in Thinning.

'Red Leaf' seed is coated with a fungicide, thiram. Never eat the seed or feed it to animals, and keep packets away from children and pets. Wear gloves to sow and wash your hands afterwards. The crop grown from treated seed is edible.

Starting under cover

• Sow two or three seeds per cell of seed-raising mix, 5 mm deep.
• Snip out all but the strongest seedling once true leaves appear.
• Plant out at three or four true leaves, usually 3 to 4 weeks from sowing, or longer in cool weather.
• Harden off first (give seedlings a little longer outdoors each day for about a week).

Water transplants in with seaweed solution at half strength, 15 mL per 9 L, then mulch around them.

Care by growth stage

Quick, unchecked growth is what keeps komatsuna tender and mild, and that comes down to even moisture and a steady supply of nitrogen. Weeks count from sowing.

Mesh stops butterflies, but it also keeps out the birds and wasps that hunt sap suckers, so aphids, thrips and whitefly can multiply underneath. Lift it to weed in the early morning or late afternoon, when butterflies are not flying. Soft, nitrogen-heavy growth draws sap suckers too, so feed at the rates above and no harder.

Thinning

How far apart you leave the plants decides what you pick. Space also lets air move between leaves, which slows downy mildew and white blister.

• First pass: once true leaves show, thin seedlings to about 5 cm apart.
• Second pass: at about 10 cm tall, pull every second plant and eat it.
• Yellowing lower leaves: strip them off and bin them, so the base of each plant stays dry.

Companion planting

Dotting komatsuna among other crops makes it harder for caterpillars to crawl from plant to plant, and nearby flowers feed the wasps that parasitise them.

Grows well with

• Lettuce and radish, quick crops that want the same steady watering.
• Beetroot and carrots, low crops that leave the leaves in full light.
• Dill and coriander left to flower, whose blooms feed parasitic wasps and hoverflies.
• Sweet corn or climbing beans on the sunny side in hot districts, as living shade for a late sowing.

Keep apart

• Bok choy, Chinese cabbage, mustard, kale and broccoli massed in one block, as they share caterpillars, aphids and clubroot.
• Shepherd's purse, wild mustard and wild radish, brassica weeds that harbour aphids, downy mildew and clubroot.

Harvesting Leaves

Leaves can be picked at any size. Start as soon as they are worth eating, from about three weeks, and finish before a flower stem rises.

• Snip outer leaves at the base and leave the centre growing.
• For whole plants, cut at soil level with a sharp knife.
• Pick in the cool of the morning, when leaves are crisp and mildest.
• Harvest everything once a flower stem appears, as leaves toughen from then on.

In the fridge, picked komatsuna keeps for up to a week. Toss baby leaves raw through salads, cook young leaves like spinach, and treat older ones more like cabbage in stir-fries, soups and pickles.

Common problems

Plants flowering while still small. Heat and dry soil are covered in Three mistakes that matter. Fast-lengthening spring days and a week or more of cold on young plants can also set them flowering, so sow little and often in spring.

Gaps along the row. The seed is tiny, so a drill that is too deep or a surface allowed to dry before emergence are the usual culprits. Read more

Bitter, fiery or stringy leaves. Leaves left too long, dry soil and picking in the heat of the day all sharpen the taste, so water steadily and pick young.

Seedlings falling over at the base. Damping off takes hold in cold, wet, crowded trays with little air movement. Read more

Chewed seedlings and shiny slime trails. Snails, and slugs to a lesser extent, are the worst pest of Asian greens. Go out with a torch about two hours after sunset, especially after rain, or set a beer trap. Read more

Pests and diseases

Caterpillars and sap suckers cause most of the damage, while the diseases follow cool, dewy weather or wet, acid soil.

Cabbage white butterfly (Pieris rapae). Its velvety green caterpillars eat irregular holes and foul the leaves with droppings, and one female can lay up to 800 eggs. Squash caterpillars by hand, or spray small ones with Bacillus thuringiensis, a bacterial insecticide that affects only caterpillars, as the label directs. Read more

Diamondback moth (Plutella xylostella). Plump, bright green caterpillars about 10 mm long feed under the leaves, leaving thin windows that later tear into holes. Numbers build from spring to autumn, most of all in dry summers, and around Stanthorpe mid to late summer crops suffer most. Mesh and Bacillus thuringiensis work here as they do for cabbage white. Read more

Cabbage aphid (Brevicoryne brassicae). Dull grey-green aphids up to 3 mm long crowd into blue-grey, powdery colonies and can spread turnip mosaic virus. Hose them off, then spray insecticidal soap under the leaves as the label directs. Read more

White blister (Albugo candida). Small white blisters on the undersides of older leaves burst into powdery spores, mostly in cool weather with heavy dew. Blistered leaves are unsightly, so pick them off and bin them rather than composting them.

Downy mildew (Hyaloperonospora parasitica). Yellow to pale brown patches on the upper leaf carry a greyish-white growth beneath, and crowded seedlings suffer most. Thin on time, as set out in Thinning. Read more

Clubroot (Plasmodiophora brassicae). Roots swell into knobbly galls, so plants yellow, wilt on warm days and stop growing. Its spores can survive in soil for up to 20 years. Pull affected plants with their roots and bin them, never compost them.

Prevention first

• Lift the mesh once a week and turn a few leaves to look for eggs, caterpillars and aphids.
• Water at soil level early in the day, so leaves are wet for only a few hours.
• Clear crop remains straight after the last pick, as white blister and downy mildew carry over on debris.
• Clean soil off tools and boots after working a bed that has grown clubroot.

Tips

1. 'Red Leaf' is an F1 hybrid, so seed saved from it will not come true. Komatsuna also crosses with bok choy, turnip, mizuna and Chinese cabbage, which share its species.
2. Expect the odd green-leaved plant in a sowing of 'Red Leaf'; it needs exactly the same care.
3. Plant red komatsuna as a living edge or dividing line in an ornamental vegie bed.
4. Leave one unfed brassica outside the mesh as a sacrificial plant. It draws egg-laying butterflies away, and in time the wasps that parasitise their caterpillars.