Answer: Mulch should not affect your arborvitae, unless it is very thick and is packed up against the trunk or main stems of the plants. Mulch actually helps slow evaporation which does keep the soil moist, but not to the extent that it might cause root rot. The major problem with poorly draining soils is that they hold so much water that the oxygen can be driven out which can suffocate the roots of your trees and shrubs. Roots need oxygen as well as moisture. Hope this clarifies things for you!
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