Edit: The situation where the DM says, "Actually Bob, only half the clearing has vegetation in it. The other half, where all the monsters are standing, doesn't have any vegetation in it, your entagle is useless... once again." is far too common a possibility otherwise. *shrug*
There's a reason the 4E DMG addresses the "rule of Yes" at length.
I LOVE the idea that druid magic can make plants grow where there were none before. Seriously, how cool is this:
At first it seems your spell failed. Then, a small green sprout appears beneath Trevolax's feet, followed by another, then another, then the floor erupts with grape vines that grow thick and fast. Trevolax manages to leap out of the way, but the grasping vines quickly manage to twine around his minions' feet - holding them fast!
For added coolness, leave the vegetation in place after the fight.
Sorry. There's no natural vegetation here. Your spell fails. Bob, you're up.