Last year I grew ornamental kale from seed, but when I returned from my
summer holiday all the plants had been completely eaten by caterpillars, so
into the compost bin they all went.

I'd like to try again, but I'll be going on holiday again. So, can anyone
suggest how to keep the caterpillars off the plants?

Ornamental kale plants are quite expensive to buy (why is that?), so I'd
really like to grow my own.

Many thanks.