The detail work is nice, I especially like the ruts by the bridge.
How does foliage prevent it from being playable? As long as you don't pull a GW and make the foliage so dense that models can't be placed into it, you're golden. I suggest forgetting about tall grass, though, unless you want to do the "sprite" method seen in video games, where grass is represented by a loose criss-cross of very thin stands of tall grass. It may look convincing while still allowing models to be placed within it. If you build trees and shrubs into the board, just make sure there is spacing between the plants to put models in, and put different color flock in the spaces that count as cover.
Otherwise, make the trees removable so that tanks can roll through them.
Alternatively, you could leave the tree stands separate. You can make a detailed board that has open areas where terrain can be added or removed.
