PC's becoming plant-type

[Dausuul]

Anyway - I expect him to take the plant/flail, and when he does, I'm going to ask for a reflex save, as the plant attempts to grab hold of his arm. If it manages to do so (I haven't figured a DC, gonna roll it on the fly), then it's going to start requiring a fort save each week thereafter. On the first failed save, the plant will clearly become a part of his arm, as opposed to just attached, and on the second, it'll transform him entirely into a plant-type creature.

I'm kind of worried about balance, however. By doing this, I'm essentially giving him free low-light vision, immunity to poison/sleep/paralysis/polymorph/stunning/critical hits, though I won't be giving him the mindless-type qualities (plant creatures normally have immunity to all mind-affecting effects). I'm looking at this, and I don't think that it'd be a big change, but I wanted to put it up here, see what other people think this sort of change could have on a game, and whether it'd throw it out of balance.

Balance-wise, that's probably worth a +1 LA.

What I'd be more concerned with, though, is the fact that you're making a drastic change to his character without his permission. IMO, that's something DMs should try to avoid. What if he doesn't want to play a walking plant? At least make sure he has the means to get rid of it if he wants to.

This would make a neat curse, but it sounds like you're envisioning it as a permanent change to his character, which is quite different.