The pilot reported that he was conducting a local glider flight. After soaring for about 1 hour and 45 minutes, the pilot began to return to the departure airport. The pilot stated that the glider encountered a downdraft that would not allow him to return safely and he elected to land in a hay field about 3 miles northwest of the departure airport. The pilot further stated the approach was normal and at the "proper speed". While turning onto final approach the glider's right wing clipped a tree and spun around clockwise into trees. Postaccident examination of the glider revealed that the left and right wing leading edges incurred substantial damage due to impact with trees, and about three feet of the left wing tip was sheared off. Both spoilers were found in the full extended position. The pilot reported no pre-impact mechanical malfunctions or failures with the glider that would have precluded normal operation.