rocking the plant power

I don't have that specific combo but I have a L50 plant/fire dom. I went with fire mastery on that character for the extra area damage (seeds + carrion creepers + rain of fire + fireball = tons'o damage) and because Embrace of Fire enhances the damage in all fire powers.

For anything but a plant I might suggest going primal forces mastery to continue the energy theme and add a third good melee attack but while energy mastery does offer some ok area damage in energy torrent + explosive blast, ice mastery is generally a better choice if you want to add area damage to anything other than fire. Sleet adds a very nice resistance debuff and ice storm tops of your damage, plus it offers the ice version of dull pain (hoarfrost) which is a nice emergency heal and frozen armor is a good place to slot an LoTG +recharge IO.