Green Giant Arborvitae growth rate is at an impressively fast rate, up to 4 feet per year in the proper growing environment.
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It is suggested to remove no more than 12 inches or so at a time from Thuja Green Giants that are already short hedges.
I can have them trimmed yearly so they wont look my leylands did, if that would help.
In other words, the sides should slope inwards slightly.
ALL conifers grow indefinitely, as long as they live.
Small amounts are not a problem and larger infestations can be treated with systemic insecticides.
But I think that all this proves is that Green Giant is a bit deer resistant if you have plenty of other things near by that they would much rather eat because I hear lots of stories about deer eating Green Giant Arborviate.