Answer: The last part of your question is a tough one! I think it ultimately depends upon how long the planting area was used as a litter box, and by how many cats. A single cat and a large planting bed shouldn't be problematic; a small bed and lots of cats over several years could certainly make the soil inhospitable to plants. So, I can't address that issue. However, I would recommend digging up one of the plants to inspect the root system and the soil. Soaker hoses are great for perennials and annuals, but unless you keep the water on for extended times, may not be so great for deeper rooted shrubs such as the arborvitaes you are growing. After digging up one of the shrubs, check to see how deeply the water is penetrating. It could be that the roots are not getting enough water to sustain life. It is generally recommended that you water newly planted trees and shrubs once each week, applying one inch of water per plant. The best way to do this is to build a water well or watering basin beneath each shrub, extending out about 12" from the trunk. When it is time to water, fill the basin, allow to drain, then fill it a second time. Watering in this way will concentrate the moisture directly over the rootmass and allow it to trickle down, wetting the entire root mass.
I think if you adjust your watering, your arborvitaes will perk right up. As for the dead ones, replace them now with shrubs of the same size and a few years from now you won't notice a difference in their sizes.
Best wishes with your arborvitaes!
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