When to fertilize arborvitae?
I suggest that you can dodo in the late winter or early spring, depending on your location's climate. You could try using a balanced fertilizer like 10-10-10 or 12-4-8.
You can use Espoma Organic Plant-Tone 5-3-3 Natural & Organic All Purpose Plant Food when you start to see those new green shoots in the early spring.
The best time to fertilize is before new growth starts, around early spring. Another good time to fertilize is during the planting season.
It looks like this plant is just like the others that were fertilized during the growing season.
I think early spring is probably the best time.