Spring: April through May
Once the ground has thawed and is workable, spring is the most forgiving window there is. The soil is warming, spring rain does part of the watering for you, and the tree gets a full growing season to push roots out into the surrounding soil before it has to face a Michigan winter.
This is also the busiest window. If you want screening in place before summer, it's worth getting on the schedule early — spring bookings tend to fill first.
A spring-planted arborvitae still needs watering through its first summer. Spring rain gets it started; it does not get it established.

