Trees shall have a minimum caliper size of three inches measured six inches from grade at time of planting, and evergreen trees shall have a minimum height of seven to eight feet of planting. All plant materials, planting practices and specifications shall in accordance with standards established by the American Association of Nurserymen. Trees shall be nursery-grown, free of disease, substantially uniform in size and shape and have straight trunks. The minimum branch height at planting shall be six feet, except where planting is on a sight triangle, in which case no branches shall be below eight feet.