Arborvitae grow from the outside green tips and as they grow the inner parts (close to the trunk) of the branches lose their green needles. The best way to keep an arborvitae at a desired height is to do light trimming/pruning of the new green growth in the spring of each year. Take care not to cut too far back into the brown empty area close to the trunk. This empty space may not fill back in, so it’s best to keep your trimming to the green parts of the plant.