Answer: I can think of two possibilities for what is damaging that plant. One is that a bird is tearing away leaves to use as nesting material or because there are insects on it. If this is the case, you might try confusing the bird with scare tape (flashy metallic material) or by stringing some heavy fishing line above the plant.
The other possibility is that slugs are attacking it at night. Since they work at night you would only see a slight trail of shiny slug slime as evidence. This can be controlled by hand picking (at night with a flashlight) or by sprinkling some diatomaceous earth on the soil surface around the plant.
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