When you put a baby plant in the ground, you want to give it all the care you’d give any other baby. Luckily, young evergreens are somewhat easier than kids or puppies: their nutrition needs aren’t that special, and you don’t need to spend time worrying about extra feedings. Just stick to a regular evergreen fertilization schedule and you’ll be fine.
There are several types of arborvitae (Thuja spp.) and even more cultivars. Due to their naturally conical or pyramidal growth habits, they are a popular landscape tree for hedges or specimens. “Green Giant” (Thuja “Green Giant”) grows to a full height of 40 to 60 feet and spread of 12 to 18 feet and is hardy in U.S. Department of Agriculture plant hardiness zones 5 through 8. “Smaragd” (Thuja occidentalis “Smaragd’), a type of American arborvitae, grows to a full height of 14 feet and spread of 4 feet and is hardy in USDA zones 2 through 7.
Several types of fertilizer work fine for conifers, but all of them should be heavy on nitrogen. The best one to use is a lawn-type fertilizer with a ratio of 4-1-2 or 3-1-2 and does not contain any chemicals that control weeds. Other high-nitrogen commercial fertilizers also work, such as 12-6-4, 16-8-8 or 20-10-5. If you want to go the natural route, use manure, but be sure it is well aged, or you could end up burning the tree’s roots.
Mix fertilizer applications into the soil around the tree, avoiding roots. Use a rate of 1/3 pound of fertilizer for every foot of tree height, and apply either in early spring or late fall, once a year only. Dig manure into the ground around the tree as well. If you want to give young trees an extra boost at planting time, sprinkle 1 tablespoon of balanced fertilizer like 12-12-12 around the base of the tree on top of the soil after you’ve filled in the planting hole.
Because young trees are more susceptible to chemical damage than older trees, be careful not to over fertilize them. If you are worried about this, you can refrain from fertilizing until your tree shows signs of needing it, usually when foliage begins to lighten. Whatever you do, resist the urge to put fertilizer in the hole you dig to plant your tree; while this works for some types of plant, it's a surefire method of damaging or even killing your young arborvitae.