There are few plants left in my garden from the previous owner, but one I kept was the Poncirus trifoliata or Hardy Orange. It would have been a real bitch to try to get rid of, and this fact led to my appreciation of it. Right now it is covered in colorful fruit which are fuzzy, yellow and about the size of a ping pong ball. They have a wonderful aroma that reminds me of how Kumquats taste. I have read that you can use the fruit as a substitute for Lemon, or it can be used for marmalade. They are incredibly seedy and I think there would have to be a world-wide Lemon and marmalade shortage for me to make the effort. In spring it is covered with attractive white flowers. The stems stay green in winter as do the vicious thorns which were the main reason I did not want to attempt its removal. Birds love to build nests in it as it is virtually cat proof.
Poncirus trifoliata gets anywhere from 8 to 20' tall, prefers full to part sun, and I have never watered mine in 13 years. It is native to Korea and northern China. It is indeed a citrus plant and is often used as root stock for more popular members of the family. It is listed as hardy to zone 5b into 9. We have sold this at work occasionally, but we usually prefer the cultivar 'Flying Dragon' which has unusually twisted branches and curved thorns that look like green talons. If you are interested in seeing this plant there is a picture in a previous post here. Poncirus and Citrus are members of the Rutacea family. While doing some fact checking the inner geek in me was amazed to learn this family includes some widely divergent members including Rue (Ruta gravolens), Skimmia japonica, and two Zanthoxylums - the Tooth Ache Tree ( Z. americanum) and the Sichuan pepper (Z. simulans).