Answer: You may be overwatering your arborvitaes. They need fast draining soils and a deep watering once or twice a week if the weather is hot. The key is to water them deeply to encourage deep root growth and then to allow the soil to dry out between waterings. I'd remove the soakers and soaker hoses and water by hand. It helps if you make a watering well or watering basin beneath each arborvitae by mounding up a few inches of soil in a circle about 12" away from the main stem. Fill the basin with water, allow to drain, then fill a second time. Do this once a week (twice if the weather is hot and the soil is dry an inch below the surface). Watering in this way concentrates the moisture over the root mass and allows it to trickle down, wetting the entire root system. I think when you provide a fast draining soil and water as suggested above, you'll see a big improvement in your arborvitates. Good luck with your plants!
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