Gadenin日本語Get the appMesh from sowing day is the whole strategy
Napa cabbage leaves are thinner and softer than those of headed cabbage, and every brassica pest prefers them. Because the crop is sown in late summer, the seedlings emerge at the exact point in the year when flea beetle and cabbage moth numbers peak. A seedling stripped in its first fortnight never builds the frame it needs, so protection has to be in place from the day you sow, not from the day you see damage.
Fine insect mesh, around 0.6 mm, stretched over hoops and sealed to the soil on all sides is the single most effective thing you can do. Leave enough height for the plants to grow to 18 inches (45 cm) without touching the mesh, because moths lay through fabric that rests on leaves. Lift it only to weed and water, and close it the same day.
Mesh only works if the ground underneath is clean. If the bed grew brassicas recently, root maggot pupae and clubroot spores are already in the soil and the mesh seals them in with your crop. Rotate first, then mesh.
What shows up, and when
Different pests arrive at different stages, and knowing the sequence lets you inspect for the right thing at the right time rather than looking at everything vaguely. Check the undersides of leaves twice a week, and check the center of the plant once it starts to cup. Most of these problems are trivial when caught in the first few days and serious a fortnight later.
| Problem | Early sign | Stage it hits | What to do |
|---|---|---|---|
| Flea beetles | Dozens of small round shot holes in seedling leaves | First three weeks | Mesh from sowing. Keep the soil damp, which slows them |
| Cabbage caterpillars | Ragged holes and dark green pellets in the leaf axils | Weeks two to eight | Mesh, plus hand picking and crushing egg clusters on leaf undersides |
| Aphids | Gray or green colonies packed into the growing point | Warm spells, any stage | Strong water jet early. Once inside a closed head they cannot be removed |
| Slugs and snails | Slime trails, then holes deep between the wrapper leaves | From cupping onward | Clear mulch back from the stem, trap and hand collect at night |
| Cabbage root maggot | Plant wilts in the afternoon despite damp soil | Two to five weeks after planting | Collars at the stem base at planting. Rotate ground |
| Tip burn | Brown papery leaf margins on inner leaves only | During head filling | Even watering and adequate soil calcium. Not treatable once seen |
| Soft rot | Slimy brown patch at the base or top of the head, sour smell | Wet autumn weather | Cut the plant out and remove it. Improve drainage and spacing |
Slugs inside the head
This is the problem that spoils otherwise perfect Napa cabbage. Once the leaves close over, the inside of the head is dark, sheltered and damp, which is precisely what slugs want. They move in during the filling stage, feed between the wrapper leaves, and you find the damage only when you cut the head in the kitchen. By then several outer layers are holed and fouled.
The defense is to make the base of the plant an unpleasant place before the head closes. Pull any mulch back three or four inches (8 to 10 cm) from each stem once cupping starts, so the soil surface around the plant dries between waterings. Remove yellowing lower leaves that lie flat on the soil, since they are the daytime shelter. Then patrol with a torch on damp evenings and collect by hand for the two or three weeks that matter, which is far more effective than anything else.
- Pull mulch back at cupping Clear a dry collar of bare soil three to four inches (8 to 10 cm) around each stem. Slugs cross dry ground reluctantly and it also reduces stem rot.
- Strip the flat lower leaves Any outer leaf lying on the soil and starting to yellow is shelter, not photosynthesis. Snap it off and take it out of the bed.
- Patrol after dark on damp evenings An hour after dusk, with a torch, working along the row and checking the top of each head where the leaves meet. Twenty minutes twice a week through the filling stage is enough.
- Water in the morning only Morning watering lets the surface and the leaf bases dry before nightfall. Evening watering hands the slugs a damp night on a plate.
Tip burn is a water problem, not a disease
Tip burn shows as brown, dry, papery margins on the inner leaves of the head, while the outer leaves look perfectly healthy. It is not an infection and nothing sprays away. It happens when the fast-growing inner leaves cannot get enough calcium, and calcium only moves with water, so anything that interrupts the water supply during head filling causes it: a dry week, an unexpectedly hot spell, or soil too acid for calcium to be available.
Prevention is straightforward and it all happens before you see the symptom. Get the pH into the 6.0 to 6.8 range well ahead of sowing, mulch to buffer the soil moisture, and water deeply and regularly through the filling stage rather than in irregular soakings. Heavy nitrogen late in the crop makes it worse by pushing the inner leaves to grow faster than the calcium can follow.
Bolting, and what to do with a bolted plant
A plant that sends up a thick central stalk with pale yellow flowers has bolted, and it will not head. In a late-summer crop this is almost always a spring-sown plant or one raised through a cold spell. Once the stalk is up, the outer leaves toughen and turn bitter within about a week, so there is no benefit in leaving it standing.
It is still usable if you act quickly. Cut the whole plant while the flower buds are still tight and green: the flowering shoots eat like a mild sprouting broccoli, and the younger leaves are fine cooked. Then note which sowing it came from. Bolting is a timing record, and a season of dates showing which sowings bolted is the most useful thing you can carry into next year.