Answer: Spring is the best time to prune arborvitaes but you'll want to carefully consider your plan of trimming off the tops. As they are now they narrow at the top which is what allows them to slough off the snow. If you trim them flat they will collect snow and lean over under the weight, or some of the branches might break off. If you carefully taper the tops when you trim you can avoid this problem. You can safely remove about 12 inches of growth without harm. If you want to remove more, do it over a 2-3 year period of time. Best wishes with your arborvitae.
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