Before taking off I didn't lock properly the rear canopy of my glider. So when the plane started to tow me the canopy got open as my glider was bumping on the track before taking off. I realized about that when I was flying a few meters high because of a huge bang due to the breaking of plexiglass the canopy is made of. I informed the pilot in front of me and since the wreackage of the canopy was slowing down both my glider and the plane we took the decision of getting back to downwind zone in order to unhook the towing rope just before final. But I did a second mistake because I released the rope just a bit before the plane could drag me on to final right to the airstrip. I was about 80 meters high, just too low to overcome the trees and too late to look for an alternative field for landing. So I decided to try to land anyway on the airstrip finding a narrow opening amid the trees placed at the start of the landing strip, but I touched a tree with the right wing of my glider, which suddenly yawed on the right side toward the taxiway. Fortunately I succeeded in getting through the concrete blocks marking the taxiway without hitting them and at last I stopped my glider operating my brakes fully.