Well, this year I learned that cold hardiness is largely based on micro climate, not just considering how warm you can keep your tree in winter ( south side of house, etc, but also how cold can you keep your tree during those weeks of warm weather we get here in the southeast US in January and February. I have a high tunnel that is opened/ closed with doors on both ends that I've been protecting my Ichang and poncyrus hybrids with for the past three winters with water barrels next to each tree. I'm on top of a mountain, south facing slope, with high tunnel running along slope horizontally west to east. High cold winds in winter. The doors don't seal very tight. The west side gets the highest coldest wind. The trees inside all did great last year with cold January and February and an easy transition into spring with no roller coaster temps. I keep doors open to vent tunnel to keep trees dormant when temps above freezing. This past January and February however had a lot of highs in 70's and even with doors open, it got too hot in tunnel and trees started pushing new growth in March when we got a night of 14 degrees and the next night 16 degrees. My low this winter was 7 degrees in January and trees were fine then. My Dunstan citrumelos that were protected somewhat from winds by other forest and orchard trees in the open yardwent through entire winter without dropping leaves. My citradia on south side of house with a water barrel on the west side of tree died down to the level height of the barrel. This citradia is also three feet away from blacktop driveway and had broke dormancy when 14 degrees hit. One big Dunstan out in open without a barrel lost half of its height due to getting full winds in the face. The trees in tunnel were more severely damaged on the east end of tunnel and the east side of each tree; the cold side of the trees is now growing better than the warmer east side of the trees. The order of the tunnel trees from west to east are as follows: Dunstan ( full green leaf with no damage and no water barrel ) , Ichang lemon ( now growing on west side of plant with few live shoots on east side and lost 3/4 height, no water barrel), pink seedling grapefruit from grocery fruit ( dead, water Barrel), Thomasville ( lost only top foot of growth from 6 ft tree but is green branches that still have not started to push growth yet, barrel), Ichang lemon ( water barrel, lost upper 5ft of 6 ft tall tree, starting to push growth from base trunk), Nansho daidai (barrel, lost 2 ft of 5ft tree and half the overall width of tree, now pushing growth vigorously at base and some half way up tree/branches, citradia ( lost upper 5 feet of seven foot high vigorous and wide tree, barrel, some sprouts from base and one sprout on branch 1 ft up from ground), rusk citrange ( barrel, lost upper two feet of growth and half of width of a 7 ft tall tree, pushing growth all over tree now), mortan citrange ( east end of tunnel by door, barrel, lost upper three feet of 7 ft tall tree, starting to push growth from base and a few sprouts on branches half way up tree. ) point: Dunstan is hardiest of all, protected or not. Ichang on colder west end of tunnel near door is doing better than the Ichang that was in warmer center of tunnel even without water barrel protection, and all fared better than citradia ,which is supposed to be the cold hardiest of them all, because citradia is in the center east end up hill end of the tunnel, was growing vigorously into fall, heat rises, etc. I should have taken tunnel down in February when highs in 70s , but it's too hard to put up again since plastic is stapled to heavy wooden framework to withstand high winds up here. All citrus need to stay dormant unti no more freezing temps in spring, and if breaks dormancy, needs to be kept from freezing, or you will get big damage regardless of variety.