The airplane was substantially damaged following an impact with trees on takeoff from a wet and muddy grass airstrip. The pilot stated that it had rained the day and night before the accident. The pilot reported that the airplane did not reach rotation speed by the end of the runway and that he should have aborted takeoff after realizing the aircraft was too slow for liftoff. No anomolies with respect to the airframe, powerplant, or systems were determined to have existed prior to impact.