Answer: Because your arborvitae will eventually grow six feet wide, you'll want to keep them at least 6 feet, and preferrably 8 feet away from the side of your house. Planting them closer will crowd them as they mature, and will require severe pruning to keep them from growing into the wall, both of which can compromise their health. If you want to plant closer, choose a columnar rather than a pyramidal arborvitae. Columnar plants grow 25-30 feet tall yet remain only 2-3 feet wide at maturity. Again, you'll want to keep them at least 3 feet away from the wall to provide room for growth plus some room for good air circulation.
Best wishes with your landscape!
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