The competition, which attracted 100 entries last year, is divided into five categories: pine, fir, spruce, container grown and wreaths.
Each finalist can submit two of any one species per category and all trees entered must be 1.8m to 2.2m in height. The judges will score the trees on their foliage, colour, shape and marketability to determine the overall winner.
The winner will deliver an outdoor Christmas tree to 10 Downing Street in December, and the wreath at Number 10 will be provided by the winner of the wreath category.
The winners will be announced at an event on 30 October at the Berkshire College of Agriculture.