The only plants I found to be hardy enough to withstand Sevs is Java Fern and Anubias, even those they dont eat, but do like to play with causing damage.
Vallis they like to eat, Sevs are herbivours so plants tend to take a battering, try putting plenty of veg in the diet that you feed them and they may be less inclined to eat the plants around. Personally I gave up with plants with Sevs and got some decent silk plants.