Answer: There are so many different arborvitaes that I don't know what the growth habits of yours might be. But arborvitaes make excellent hedging plants and don't mind being topped. They will grow into one another for a nice dense screen. The only concern about spacing so closely is that the roots will compete for moisture and nutrients. As long as you pay attention to their watering needs, they should be fine. You might also consider planting them close now and then removing every other one in a few years so they won't be quite so overcrowded. You could simply replant them at either end of your privacy screen to extend the screening. Best wishes with your landscape!
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