On December 22, 1996, about 1320 eastern standard time, a Let Blanik L-13, N48052, registered to the Tampa Bay Soaring Society, collided with trees while returning to land at South Lakeland Airpark, Mulberry, Florida, while on a Title 14 CFR Part 91 personal flight. Visual meteorological conditions prevailed at the time and no flight plan was filed. The aircraft received substantial damage and the commercial-rated pilot and his dog were not injured. The flight was originating at the time of the accident. The glider pilot stated he did not check that the spoiler handle was locked in the spoilers retracted position before takeoff. The spoiler handle was not locked and after takeoff the spoilers deployed. The tow pilot and several people on the ground tried to call him on the radio but he did not hear the transmissions. He knew they were flying slow and he thought the tow plane might be having a problem. When the tow pilot moved his rudder to notify the glider pilot of the spoilers being out, the glider pilot released from the tow plane. The glider pilot turned back toward the airport, but with the spoilers out he descended rapidly and collided with trees about 1/4 mile from the runway.