On May 26, 1996, about 0800 mountain daylight time, N626RP, a Cessna 180, operated by the owner/pilot, collided with a tree and was substantially damaged during landing rollout in Grangeville, Idaho. The private pilot and his passenger were not injured. Visual meteorological conditions prevailed and no flight plan had been filed. The personal flight was conducted under 14 CFR 91. The pilot stated that he attempted to land on a grass airstrip that was about 1,000 feet long. He stated that he touched down about mid-field, and he could not stop the airplane until it collided with a tree at the end of the airstrip. The left landing gear support structure was torn from the airplane and the fuselage was damaged. The pilot stated that his approach was "too high and hot" and he did not realize that the grass on the airstrip was wet, causing a decrease in the airplane's braking effectiveness. No preimpact mechanical malfunctions were reported.