They cause problems in many parts of the country by taking over farmland, out-competing native plants and causing fire hazards, and they are very difficult to remove.
One of the largest forestry companies in the Nelson Marlborough region is OneFortyOne, which owns a total of 80,000 hectares of forests across the top of the South Island.
Every September for nearly 70 years school children have been coming out to Coronation Forest at Golden Downs near Nelson to plant seedlings that they have been growing.