The helicopter drifted backwards during a confined area operations training and the tail rotor blades struck tree branches while hovering over tall grass. The 11,330 hour flight instructor and the 16,800 hour pilot receiving instruction reported that they were both familiar with the training area being used, the flight was properly briefed, and they had both worked together as a crew in the past. The crew reported that the pilot performed a high recon of the landing area, noting the surface winds, approach and departure paths, and the conditions of the landing zone. The approach was planned to terminate at a hover. The grass at the landing zone is normally less than a foot long; however, due to the record rainfall this year, the grass was 3-to-4 feet tall. Both pilots underestimated the effects of prolonged hovering over tall grass. The tall-waving grass disoriented the pilot and allowed the helicopter to drift into the trees. The flight instructor was clearing the left side of the helicopter during the approach and hover, but was unable to react in time to take the flight controls to stop the drift. The flight crew immediately landed the helicopter in the confined area when they felt a vibration from the tail rotor. The helicopter was recovered to the operator's maintenance facility for damage assessment. Examination of the helicopter by the flight crew, the operator, and the FAA inspector, confirmed that both tail rotor blades were destroyed, the mounting bolts for the tail rotor 90 degree box were damaged or broken, and the tail fin gearbox mounting spar was also damaged.