The pilot was departing, in a tailwheel-equipped airplane, from a 600 foot long, by 50 wide, off-airport site that was surrounded by trees and overgrown brush. The pilot reported that before starting his takeoff roll he completed the predeparture checklist, and then he configured the wing flaps by placing the flap handle into the first notch. During the takeoff roll, and as the airplane achieved a takeoff speed, the pilot applied an additional notch of flaps. The pilot reported that as the airplane became airborne, the left wing contacted an area of overgrown brush, which briefly pivoted the airplane to the left. The pilot then applied full right rudder to correct for the pivoting and realign the airplane over the site, resulting in a reduction of airspeed. The airplane subsequently descended into an area of brush-covered terrain at the departure end of the site, sustaining substantial damage to the wings and fuselage. The pilot noted that there were no preaccident mechanical malfunctions or anomalies that would have precluded normal operation.