Arbor Day (1990)

Planting a sapling for Arbor Day, Elmer Jacobs watches in horror as a grizzly bear mauls his parents to death. He falls catatonic for two decades, mocked by indifferent doctors and a sadistic hospital staff. When he finally snaps like a brittle branch, he harvests his revenge using gardening tools. Meanwhile, oblivious teens party in his abandoned house, only to be pruned from life, one by one.