Answer: Emerald Green (also known as Smaragd) Arborvitae is rated hardy to USDA Zone 4, but that rating is for a shrub planted in the ground where the roots are insulated from the cold so leaving it in a pot will be very risky. The best way to hold the shrubs would be to sink the pots into the ground, another method you might try is to somehow insulate the pots by surrounding them with a very thick layer of mulch. (Also insulate below the pots if the porch is raised so that air flows under it.) Unfortunately, burlap will not be sufficient insulation. One more thing to consider would be to plant the shrubs and replace them every year since ultimately they do grow rather large -- about 10 to 15 feet tall and about 3 feet wide. Finally, as it is on a covered porch you will still need to water it until the ground freezes.
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