THE FLIGHT WAS DEPARTING AN AIRSTRIP THAT WAS IN POOR CONDITION. THE PILOT STATED THE LEFT MAIN GEAR 'SLIPPED' INTO A DEPRESSION IN THE RUNWAY'S SURFACE DURING THE TAKEOFF ROLL. THE AIRPLANE CONTINUED TO SLIP OFF THE RUNWAY UNTIL THE LEFT WHEEL STRUCK THE WILLOWS GROWING ON THE RUNWAY'S EDGE. THE PILOT STATED IT WAS TOO LATE TO ABORT THE TAKEOFF AND THE AIRPLANE OVERRAN THE END OF THE RUNWAY.