No matter where I have lived or gardened, slugs have been my constant companion. They used to drive me spare. I did all of the usually things like beer in a shallow saucer, boards in the garden that you would turn over in mid-morning to scrape off the slugs and on and on. Then I found Iron Phosphate (FePO4) and my life got much better. It controls the little boogers and doesn’t harm anything. It’s completely organic. But … it does depend on the slugs being on the ground for some part of the day/night so they can munch on the Iron Phosphate. And that’s the rub with Chinese Cabbage.
When the cabbage is small, the slugs can be controlled because there is lots of room between the cabbages and you can get both the slugs and the Iron Phosphate on the ground at the same time. But the cabbages grow rapidly and just as they are about to seriously head up, their leaves cover the ground. So … the slugs crawl up in the cabbage and there they now stay … munching their way throughout the head and living out the goodlife in the dark interior of the cabbage. This is one where floating row cover does no good because the slugs come from the ground up.
So … just before the plants cover the ground around them, make sure you apply the slug control and hope that you get them before they get your cabbages.