Last summer a big box store had a clearance sale of Citrus trees, some had only the rootstock still growing. I planted my Citrus Trifoliate, Poncirus Trifolata outdoors since they are supposed to be hardy to zone 5. I'm in 5A. Grew nicely all summer. Almost 3 foot tall. This fall I kinda covered it up a bit to help it get through the winter. On a random walk this morning I stopped by to see how it is faring. To my suprize it still has all of its green leaves. We have had -10 already. 3 days where the high was +1. 40 mph winds when it was below zero. How can it still be green?