After hunting around for something suitable that I could use to cover the base, ordering and being bitterly disappointed with the wired moss, I actually found an imitation grass tile at Bunning's. It was quite large, but I figured I will probably use it for other props down the track.
Because the tile was quite stiff, I had to remove the Ent and mushroom from the base. I then used the wooden base as a template to cut a circle of grass and glued the grass onto the base using PVA glue. Using a scalple I cut a hole where the Ent was to sit and then all that was needed to finish the edges was a little touch up paint, which I mixed to match the grass.
I glued the Ent and mushroom back in place with my glue gun. Then using a couple of different brown acrylic paints, a little harvest gold, a touch of the green that I'd used for the edge of the base, I added streaks of the different colours all over making sure to get in all the grooves. Then I painted a blue grey wash over the top, before adding a final wash of pearl white. This made the bark look shiney, as if it had been weathered smooth by wind and rain.