This is Poncirus. Hard to believe we can grow citrus outside this far north, but this one is it. It is a small tree, though a bit irregular and untidy in its habit. Ours kind of rambles through other bushes. Probably about 8 feet tall at this point, though it has only been in 8 or 9 years and may get bigger. Partial shade seems to suit it just fine, though ours gets some afternoon sun. The thorns are an inch or so long and so even in winter this plant offers some interest in the garden. It blooms with sweet smelling white flowers in the spring, and if a late frost doesn't get them, they will produce fruits which ripen to an orange color and which, though too sour to just eat, make great marmalade. They're small, but very thin skinned, so they're easy to work with even though you need quite a few to do anything with. A highly recommended plant.