The grass is certainly landable - I've landed on both the grass and the hard depending on glider activity / wind direction. Actually, depending on which bit of grass you pick, its smoother than some of the hard - which has got tufts of grass growing through joints.
The thing to avoid is crossing the hard/grass boundaries at any apprecialbe speed - in some places it's a non-event and in others it could do your nose gear some damage (if it's on the ground) at more than a slow walk.
The other "gotcha" is that there are still some of the original drain grates in the grass close to the grass/hard boundary of the field at the N end of the airfield (the one with T-Hangars in it) - these are a good few inches below the surrounding ground.
That said the wheels on a glider are around the same size as the mains on a 172 and there are no parts of the airfield which are not landable in a glider. Just stay at least 30 feet away from any fences (gliders don't get any closer than that due to wing span - so there could be some undetected bumps).
Best advice is to keep the nose wheel off as long as possible - usual rough/soft field stuff.
Andy