Answer: Growing container evergreens (or overwintering container plants at all for that matter) is a challenge in a cold climate such as yours. The reason is that the roots are not as well insulated as they would be in the ground. This is made even more difficult on a rooftop where it tends to be extra windy and there is nonstop reflected heat and sun all winter. The sun adds to oscillating temperature differences when it heats the soil, and can scorch or dry the foliage for added stress on the plants. In my experience this type of situation means that no plant is going to be totally reliable for you. (Even at street level in Manhattan where it is sheltered and the microclimate is warmer, most of the evergreens in pots are replaced each spring.) Cold tolerant needled evergreens such as the Arborvitae and as well as junipers would be possible choices. I would not suggest dwarf Alberta spruce due to the summer heat. Broadleafed evergreens such as hollies and rhododendrons are more liable to winter burn because they have a larger exposed leaf surface, and do not tolerate windy sites, so they are not good choices. Too, they would likely bake to death in the summer. Rooftops tend to be very hot in summer, just like a tree-free parking lot becomes, due to relentlessly reflected heat. I should also mention that the larger the container the better the chance they will survive as the roots will be better insulated and less affected by the freeze/thaw cycles we have, especially in the spring. The soil should be a good quality one formulated for container use, these are relatively light weight soil-less mixes that hold both moisture and air and also drain well mso they do not become water logged. (You will need to have a way to water these regularly whenever the soil is not frozen.) Your containers must also be sturdy to hold the soil volume you need and withstand the year round exposure including freeze/thaw/. They have to have drainage holes in the bottom as well. I might mention also that weight may be a concern, depending on the size of the containers, but for example a large concrete container filled with wet mix and a large tree can be extremely heavy -- you may need to check with your building engineer as far as how much weight can be accommodated safely, especially if this is an older building. I realize this sounds a bit discouraging, but your project is not a simple one. It may require some experimenting to get it right. Your local retailers and/or garden designers with experience in rooftops in your local area may be able to make more specific suggestions. Best of luck with your garden!
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