Answer: Arborvitaes are hardy down to zone 3 and are widely planted in zones 5 and 6 so I don't think it's your climate that is stressing the plants out. I wonder if the planting site and soil preparation - and perhaps watering practices might be the problem.
For best performance, choose the correct planting site. It should be in full sunlight. If you are planting these arborvitae to make a dense hedge they should be planted on 3 foot centers. If you are not in a hurry for the hedge to fill in plant them on 5 foot centers.
Dig the hole about 50% larger than needed. Fill the hole with water. While the soil is absorbing the water mix the excavated soil with compost or peat moss When the water has soaked into the soil replace enough of the soil mix in order to bring the arborvitae up to the same soil level it is growing in the container.
Using a long knife or saw blade go around the inner circumference of the container and loosen the root ball from the container. Carefully remove the root ball and place it into the hole. Make certain the arborvitae is at the proper level desired and then fill in a few shovels full of soil. Position the arborvitae so it is perfectly upright.
At this time it might be a good idea to add a liquid root stimulator (Root stimulators may be purchased at most garden centers ).
Fill in a few more shovels full of soil and tamp it in around and over the root ball. Water again to set the soil firmly in place around and over the root ball. Then add any remaining soil and tamp down firmly.
After planting the arborvitae should receive a thorough watering once each week unless there is sufficient rainfall.
Best wishes with your landscape!
Q&A Library Searching Tips