Answer: Based on your description and on the precipitation records for your zip code, I would suspect that they may be suffering from the hot weather on top of dry soil conditions. Windy conditions would also contribute to them drying out. Along streets, you may find that winter road salt residue has contributed to the problem as well.
Your new arborvitaes are still trying to become established and would do best with an evenly moist soil, damp like a wrung out sponge. Whenever there is an extended dry spell such as this during the next two years you should consider deep watering them to soak the roots. You can check if you need to water by digging into the soil with your finger. If it is damp, you do not need to water but if it is dry, then a slow, thorough watering would be helpful.
Also, keeping a layer of organic mulch over the root area will help reduce watering needs. It should be about three inches deep year round and spread out flat over the root area; do not allow it to touch the bark or trunk.
IF you think it is not related to moisture stress, then you might also want to check with your local county extension to see if there is currently a particular pest or disease problem with the arborvitaes in your local area.
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