Cedar/arborvitae, unlike other evergreens don’t prefer acidic soil so a traditional evergreen food like Espoma Holly-Tone with sulfur isn’t appropriate for Arborvitae. Espoma Tree-Tone is what I’d suggest.  The food should be placed in the soil by poking hole in numerous locations along the hedge and the food placed in the holes.  If the food is simply scattered on the surface, very little of the nutrients make it to the roots.  Here’s a link to a blog post all about spring feeding:  https://blog.timesunion.com/gardening/breakfast-in-their-beds/7457/