Hot weather can cause sunscald or sunscorch, especially if there is a shortage of water. In hot summer weather, water evaporates from the leaves faster than the root system can replace it, which causes the leaf discoloration. This may cause twigs to turn brown and eventually drop off. You can help avoid this by soaking the soil to a depth of about two feet once every two weeks or so. You might also try mulching around the soil beneath the arborvitae to minimize water evaporation. Hope this helps!
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