I too felt mana starved as a druid--the first game I played as her, I thought I was being clever by pumping out tons of seedling pods through the Samara Tree (because they'd magically turn into free actions later on! And one would grow even faster with my little Tataree sprinkling magical mana dust on it!), but got overwhelmed with skeletons, and eventually conceded when I had one creature versus ten creatures on the boar.
The second game, I used the Samara Tree much more conservatively, only letting free pods drop, and I set them up defensively so if a crowd pressured me they could pop into something big and scary. If you only use a three mana tree to drop a pod, you can get a three mana discounted summon every three turns beginning on turn six... pretty crummy trade off if you try to use offensively, but it worked better defensively and let me utilize the rest of my mana pool more efficiently. One popped for a heavily discounted Togorah that protected my druid while my Raptor Vine fed off my druid to stave off death. It sucks that one of the only living creatures in a Necromancer deck is the Necromancer himself.
Edit: this was just with the prebuilt decks and we were toying around with their capabilities. I can imagine a Samara tree with a harmonize and a dedicated Tataree dropping a pod every turn, but for the cost, I'm not sure it's worth the limitations, what with only one pod per zone and their fragility if used offensively (though then again, the sheer quantity of them on the board can prove to be a deterrent).
