Trifoliate Orange, Poncirus trifoliata (syn. Citrus trifoliata), is a member of the family Rutaceae, closely related to Citrus, and sometimes included in that genus, being sufficiently closely related to allow it to be used as a rootstock for Citrus. It differs from Citrus in having deciduous, compound leaves, and pubescent (downy) fruit. It is native to northern China and Korea, and is also known as the Chinese Bitter Orange.
The plant is fairly hardy (USDA zone 5) and will tolerate moderate frost and snow, making a large shrub or small tree 4–8 m tall. Because of the relative hardiness of Poncirus, citrus grafted onto it are usually hardier than when grown on their own roots.I'm in USDA zone 7, and I can attest that my plant (the "Flying Dragon", which is a dwarf variety) is very hardy -- it has done fine through blizzards and ice storms!
I planted this in my front yard 4 or 5 years ago, and this year it has fruit:
If you notice, the branches are "twisty", with long bendable thorns. It's a very interesting-looking plant. I've been waiting until the fruit drops to collect it, and this is what I've gotten so far. It looks like I'll get nine fruits this year.
The fruit is a little bigger than a golf ball (about 3 cm in diameter), fuzzy like a peach, and smells like a pear to me.
There are a lot of seeds inside.
I thought from the way it smelled that this might be a bit like a kumquat, but the rind and pulp taste just like a lemon.
I recommend this for anyone in a zone where you can't plant other citrus.
I decided to save the seeds -- I don't know what they will grow or if they will grow ... but if anyone would like some let me know by commenting here and I'll mail them to you.