Why Are My Mizuna Leaves Curling?

Flea beetle

Flea beetles (Phyllotreta species) attack mizuna as they do all Asian greens and other brassica family crops. The beetles chew small, round holes in the leaves from the moment seedlings emerge, and in warm, dry conditions can significantly damage a crop of mizuna in a matter of days. Mizuna's finely divided leaves mean that flea beetle holes cause the individual leaf segments to curl and die around the damage, giving a distinctive ragged, withered appearance. Established, mature mizuna plants with thick, well-developed clumps tolerate flea beetle damage far better than seedlings and young plants.

What to do

• Cover mizuna sowings with fine insect mesh from sowing through to harvest. This is the primary and most effective flea beetle control for mizuna, particularly for spring and early summer sowings when flea beetle activity is highest.
• Water consistently: flea beetles are most active and damaging on dry, stressed plants. Mizuna growing in consistently moist, well-composted soil suffers significantly less damage than plants in dry conditions.
• Sow from August onward for autumn and winter crops: flea beetle populations decline sharply as autumn temperatures fall, and late-summer sowings produce clean, undamaged mizuna with minimal pest management.

Downy mildew

Downy mildew (Hyaloperonospora parasitica) is the most significant disease of mizuna in UK growing conditions, particularly for crops grown under cover in autumn and winter. It produces pale yellow patches on the upper surface of the leaves and a white-grey, powdery fungal growth on the underside. Affected leaves curl, yellow, and die. Downy mildew spreads rapidly in cool, humid, poorly ventilated conditions; a single affected plant in a covered growing space can infect an entire mizuna crop within a week.

What to do

• Ventilate covered growing spaces thoroughly when growing mizuna in autumn and winter: open cold frame lids and greenhouse vents on dry days to reduce the humidity that promotes downy mildew. Even in cold weather, ventilation is more important than heat retention for mizuna health.
• Space mizuna plants at adequate distances (15 to 20 centimetres for cut-and-come-again; closer for baby-leaf harvesting) to ensure airflow between plants.
• Remove and destroy affected leaves promptly. Do not compost them: the spores remain viable in compost and can re-infect crops.
• Rotate mizuna and other Asian greens to a fresh growing position each season.

Aphids

Cabbage aphid (Brevicoryne brassicae) and peach-potato aphid (Myzus persicae) both attack mizuna, particularly in spring and early summer. Dense colonies on the underside of the leaves cause them to curl inward. On mizuna grown as a salad leaf, aphids within the fine, divided leaves are difficult to wash clean and are particularly problematic.

What to do

• Inspect mizuna plants weekly from May for aphid colonies. Apply insecticidal soap spray to affected leaves, covering the underside thoroughly. Repeat every 5 to 7 days. For severely infested crops, cut the plants to the base, remove and destroy the cut material, and allow the plants to regrow from the base.
• Insect mesh protection against flea beetles also provides aphid control simultaneously.

Caterpillars

Cabbage white caterpillars attack mizuna from June to September, rasping through the leaves and leaving characteristic ragged holes. The thin, divided leaves of mizuna are consumed rapidly by caterpillars, and a small white (Pieris rapae) caterpillar feeding at the base of the leaf cluster can cause substantial damage before it is noticed.

What to do

• Inspect mizuna plants weekly for eggs (yellow clusters on the underside of leaves) and caterpillars. Remove both by hand. Insect mesh from sowing is the most reliable prevention against all flying brassica pests including flea beetles, aphids, and cabbage whites simultaneously.

Bolting

Mizuna bolts (runs to flower) in response to high temperatures, long days, and drought, but it is notably more bolt-resistant than many other Asian salad greens and continues to produce usable leaves for longer before the plant becomes predominantly floral. Once mizuna bolts, the fine leaves become smaller and the plant produces a mass of yellow flowers. The flowers are edible and can be used in salads.

What to do

• Harvest mizuna leaves regularly: cut the outer leaves to keep the plant in a vegetative state for as long as possible.
• Sow successionally every 4 to 6 weeks for a continuous supply of young plants rather than relying on a single sowing that will eventually bolt.
• Choose midsummer-tolerant varieties such as 'Red Knight' mizuna for summer sowings: some varieties have better heat and bolt tolerance than standard green mizuna.

Frequently asked questions

Why are my mizuna leaves curling?

Mizuna leaves curl most often from flea beetle damage or downy mildew. Flea beetles (Phyllotreta species) chew small holes in the leaves and the tissue between the holes curls and dies in heavy attacks. Mizuna, like all Asian brassica greens, is attractive to flea beetles and vulnerable to attack from spring through early autumn without insect mesh protection. Downy mildew (Hyaloperonospora parasitica) causes pale yellow patches on the leaves with a grey downy growth beneath; affected leaves curl and yellow. Downy mildew is most common on mizuna grown under cover in autumn and winter in poorly ventilated conditions.

What is mizuna and how do you eat it?

Mizuna (Brassica rapa var. nipposinica) is a Japanese salad green with finely divided, feathery, dark green leaves on white stems. It has a mild, slightly peppery, mustardy flavour milder than rocket. Young leaves are used raw in salads and sandwich fillings; older leaves are used in stir-fries, soups, and Japanese hot-pot dishes (particularly the Japanese dish nabemono). Mizuna is one of the most cold-tolerant salad leaves for UK growing, producing fresh leaves through much of winter when grown under a cold frame, cloche, or unheated greenhouse. It is harvested as a cut-and-come-again crop: cut the outer leaves at 5 centimetres above the base and the plant regrows for multiple harvests.

When should I sow mizuna in the UK?

Mizuna can be sown in the UK from March to September for outdoor crops, and from September to March under cover (cold frame, cloche, or unheated greenhouse) for winter salads. The best outdoor sowing times are March to May for spring crops and August to September for autumn crops: these seasons avoid the peak flea beetle activity of summer and the risk of premature bolting in high temperatures. Midsummer sowings (June to July) are possible but require careful flea beetle management with insect mesh and consistent watering. Under cover, mizuna is one of the most valuable winter salad crops, producing leaves through November to March when little else is available.

Is mizuna frost hardy?

Yes. Mizuna is one of the hardiest salad crops for UK winter growing. It tolerates temperatures down to about -8 to -10 degrees Celsius without significant damage, and under the protection of a cold frame, cloche, or unheated greenhouse, it provides fresh leaves through most UK winters. Hard freezes may temporarily collapse the leaves, but mizuna plants typically recover as temperatures rise. The leaves of frost-affected mizuna develop a particularly sweet, mild flavour that many growers consider superior to summer-grown mizuna. Remove any blackened outer leaves after a hard frost and new, clean growth will follow.