Answer: Your arborvites will be happy with one deep soaking per week during the growing season. The general recommendation is one inch of water per week, applied slowly so it has a chance to trickle down and wet the entire root mass. If you water too often, you'll keep the soil too moist and the roots will suffocate. I'd cut back to one application of water per week. If you've built basins beneath each arborvitate, all you'll need to do is fill the basin with water, allow it to drain, then fill it a second time. Watering in this way concentrates the water over the root mass. If you're not sure how well your soil drains, or whether you're applying too much water, you can do a simple test. Water as you normally do and then wait 3-4 days. Dig a hole near the roots of your plant. If the soil is still moist 3" beneath the surface, you won't need to water for a few more days; if it's dry, it's time to water. After a few checks, you'll get to know your soil and how quickly it dries out, which will give you some indication of how often you'll need to water.
Best wishes with your landscape!
Q&A Library Searching Tips