Worms are of no benefit in a container gardening situation and can actually make soil conditions less desirable for the plants (impede good drainage).

If you are unable to lift trees and root prune/refresh the soil periodically, the arbs will eventually fill the container with roots as well as suck up available moisture and nutrients. You will probably need to be more attentive to both watering and fertilization to maximize growth with any other plants.

I concur that the worms wouldn't FIX the problem, they probably wouldn't hurt either. I'm not sure it would be root bound in that short of a time, but I'm not familiar with the growth habits of Arborvitae either. Perhaps a girdling problem?

Is the container draining properly?

Yes. I drilled several holes in the bottom and lower sides of all the containers. It tends to dry out fast due to the sun. We try to water every day in the summer.

I guess I will try to fertilize and amend the best I can. Probably in the fall or spring I can amend at least some of the soil that isn't next to the Arborvitae .

No need to amend container soil after planting. Proper fertilization should take care of all plant nutrient needs.

Is it in potting mix or garden soil?

It is a combination of perlite, peat moss, manure and top soil.

+1