Yeah, it is a problem in lots of zoos, just look at London's new gorilla exhibit.
According to the zoo's description of the exhibit, this part of the exhibit is supposed to be the grassy sunny areas that Lowland Gorillas are known for going to in the wild. I you look in the back there is a lot of vegitation, but like most zoos they don't let the gorillas actually touch the plants because they will destroy them. But the zoo does have many hidding places for the gorillas, theres even days that they don't even come to the front of the exhibit for like half of the day.