Slug Damage on Pak Choi Cut with Nemaslug Nematodes Across a 5-Metre Plot

Pak choi, Brassica rapa subsp. chinensis, is quick enough to feel almost reckless in slug country. It germinates fast, then puts on soft leaf within about three weeks, exactly the kind of tissue the grey field slug, Deroceras reticulatum, seems to find first. In a 5-metre bed sown at 15-centimetre spacing, I had roughly 33 plants up before the first obvious chewing appeared. Four days later, holes had opened across the outer leaves on at least a third of the row.

I used Nemaslug as the main response. It is a formulation of the parasitic nematode Phasmarhabditis hermaphrodita, produced by BASF and sold in the UK through suppliers including Green Gardener and Harrod Horticultural. The point of using it was to reach the slugs where they were spending the day, below the surface and under damp cover, instead of only reacting to the bites I could see above ground.

The nematode enters slugs through the mantle cavity. It carries a bacterium that stops the slug feeding within three to five days, then kills it below the soil surface. That makes the result less visible than pellet products. There are usually no bodies lined up on the bed, so the first sign of success is slower damage on new growth.

Waiting for soil that would carry the treatment

Nemaslug needs soil at least 5 degrees Celsius to remain viable, and the manufacturer gives an effective working range up to about 20 degrees Celsius. I did not choose the application date by the calendar. I checked the bed with a basic soil thermometer and waited until the soil was clearly above the lower limit.

The pack I used was rated for 40 square metres. I mixed it into a watering can at the dilution printed on the sachet, then watered it over the 5-metre pak choi row and the adjacent brassica block.

The bed was already moist after overnight rain, which mattered more than convenience. These nematodes move through the thin water film between soil particles. In dry soil they can be left near the surface and dry out within hours.

After applying the suspension, I kept watering for several minutes. The aim was to carry the organisms down into the zone where slugs shelter during daylight. I also avoided treating a crusted, dry surface, because that would have wasted the application.

This part of the job was untidy in the ordinary garden sense. A watering can never distributes like a laboratory rig, and the bed had dips, firmer patches, and brassica leaves already lying close to the soil. I treated the crop area broadly instead of trying to draw a narrow line along the seedlings.

The treatment did not make the bed immune. It reduced the feeding pressure enough for fresh leaves to stand a chance, which was the practical target.

Coir changed the way the replacement plants grew

The replacement pak choi was raised in coir compost in 9-centimetre pots on a north-facing windowsill. I used coir instead of a peat-based mix, and the difference showed quickly. Once saturated, it wetted evenly, but it drained faster than peat, so the pots needed checking twice a day during the first week.

Coir is close to nutrient-neutral out of the bag. It carries almost no nitrogen, phosphorus, or potassium of its own, so it is a poor place to leave hungry seedlings without feed. Pak choi held in it for more than about ten days began to show pale lower leaves, the usual sign of nitrogen shortage.

I started a dilute liquid feed from the second true-leaf stage. The advantage of the cleaner medium was fewer fungus gnats and no sciarid larvae feeding on peat organic matter. The cost was having to provide every nutrient the crop used while it was still in the pot.

For pak choi, that compromise worked because the crop was not staying under glass for long. Transplanting at three weeks limited the time the coir had to act as the only nutrient source.

The root balls came out dense and white, with root tips visible against the pot wall. Once planted into the treated bed, they took hold without the check I often see when small plants come out of a heavier compost.

Chempak calcium and the tomatoes by the fence

A bottle of Chempak Calcium was already within reach because the tomatoes trained against the same fence had blossom end rot. The dark, sunken patch at the base of a tomato fruit is a calcium distribution failure inside the plant, usually driven by uneven watering more than a true lack of calcium in the soil.

The Chempak product supplies calcium in a form the plant can take up through foliar and root routes. I was already applying it to the tomatoes fortnightly.

Pak choi does not suffer blossom end rot, but calcium still supports firm cell walls in brassica leaf tissue. After the transplants had established, I gave the row one dilute application.

The tomatoes showed the more important lesson. Fruit that set during a dry spell developed the rot; fruit that set after regular watering resumed did not. Calcium feed can correct the shortage the plant is experiencing, although a steady water supply is what lets the plant move that calcium into developing fruit.

Sowing kept the bed moving

Succession sowing kept the bed productive after the first row was compromised. Pak choi bolts readily in long days and after any check in growth, so I started a fresh batch every 14 days from mid-season, with about a dozen seeds in each batch.

That rhythm meant the damaged row was not the only chance at a crop. It also meant the Nemaslug application had to protect seedlings at slightly different stages, from newly transplanted plants to leaves nearly ready to cut.

Companion rows helped, but not as slug armour

The pak choi row had a short block of spring onions on one side and coriander running toward flower nearby. I did not treat either plant as a dependable slug repellent. Soft brassica leaves still needed direct slug control.

The coriander earned its space for another reason. Once in flower, it drew hoverflies, and hoverfly larvae feed on aphids that can settle on the undersides of maturing pak choi leaves. That value was separate from the slug problem.

Garlic and chive borders are often credited with deterring pests. Allium foliage contains sulphur compounds with measurable insect-deterrent properties in laboratory conditions. In an open bed, those effects were too diluted to matter much for a mollusc moving at ground level by moisture and scent trails.

The change that did matter was removing shelter. I cleared leaf litter and upturned pot saucers from the bed edge, taking away damp daytime hiding places within a metre of the crop. The companion rows were useful for predator support and for using the bed space well, without creating any protective aura around the brassicas.

Wood ash and crushed eggshell barriers also had a short trial at the perimeter. I abandoned both because they lost their mechanical deterrent effect as soon as they got wet, and this bed was watered daily.

Harvest was better, though still chewed at the edges

Eighteen days after the Nemaslug application, the second row of pak choi was ready to cut. It was not spotless. A few leaves still had fresh nicks, and one plant at the outer edge never caught up with the rest. Even so, the pace of damage had changed. Newer leaves were much cleaner than the first sowing, and many of the holes visible at harvest were older wounds that had stretched toward the margins as the leaves expanded.

I cut the heads at the base with a knife and left the roots in place. Several stumps pushed a small secondary flush of leaves within a week, enough for a light follow-up picking.

The nematode population declines once slug hosts are depleted and as soil temperature and moisture shift. Protection is therefore temporary. On the manufacturer’s stated timeline, one treatment can last up to six weeks under suitable conditions, and that was enough to cover two successive pak choi rows in this bed before another application would be needed.

I could not tell from this bed whether repeated six-week treatments would reduce the resident slug population over a whole season or whether pressure would rebuild from surrounding untreated ground after each round. The clearest weak point was the damp strip just outside the treated area, where shelter remained close enough for slug pressure to creep back.