I’d suggest feeding them now as they come out of dormancy. Arborvitae are an exception among evergreens in that they don’t like acid soil so most evergreen foods like Holly-Tone with added sulfur are NOT appropriate for arbs. I use Tree-Tone which has all they need but no sulfur. Take a small pipe (I use and old tire iron) and poke 5-7 holes around each plant into the soil out away from the trunks about as far as the outermost branch tips. Make the holes about a foot deep and an inch or two wide. Fill the holes almost to the top with the Tree-Tone and that’s it! One feeding now when the season is just starting is all that is needed. After a few years when they are well-established, feeding isn’t really necessary.