Has anyone had an experience planting now in a mountain area? We are designated a 6(a) hardiness zone but are at the top of a mtn. and it can go as low as -10 Fahrenheit. Is it too late for new hostas, astilbe and ferns (which do great there under the shade once estabilshed)? Is it too late for double knockouts in the sun? What kind of upright tree ( like aemerald greer arborvitae, holly, or leyland) in a huge 30 inch diameter pot can withstand the cold in a container. TIA