Go to Elona fast. There, find the elven academy and when graduating, choose gardening. After a quite tough fight (where you have to beat demons with plants), you'll get the Botany Diploma (I think that's what it's called), which increases the stats of plants (Thorns, Ents) quite a lot.
You'd also do well to employ Fauns, they boost thorns as well. Dryads too, I think. I think in Orcs on the March, you can always find Royal THorns and Fauns in the Hut on Debir - I always did.
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