Best fertilser for Buxus

I have some healthy box plants and would like to keep them that way - apart from a general fertiliser is there anything else I can use/buy to keep them a lovely dark green?

 

Hiya Stanley
A general fertiliser is fine. My choice is always organic, viz. Fish blood and bone, which is a little slower in effect but more sustained. "Feeds" the soil too.
I would not over feed.......plants grown slowly and surely is better than nitrogen rich quick boost quick fade.
However, mulching is something I recommend .....helps conserve moisture and soil comdition.
Enjoy your plants