I'd still say it's a bit on the weak side, since you need to use some of your precious feats to make it compete with other classes, but it's probably playable as a caster now.
I would change Hardness to DR 5/Fire, Electricity or Cold. Hardness and DR are basically the same thing, but one is used for objects and the other is for creatures. The mechanical effect is the same, but it's an established standard which I would stick to in order to make it less confusing for players and DMs.
Why are they more susceptible to electricity and cold? Plants generally contain less water than mammals, so they're normally less conductive, and they manage cold just as well as other creatures in the same areas.