Trifoliate Orange

Trifoliate orange is native to northern China and Korea and was introduced to Japan sometime before the 8th century, making it, by a wide margin, the most cold-hardy member of the broader citrus family. It has been used as rootstock for grafting more tender citrus varieties for well over a century, and remains the standard commercial rootstock for satsuma and many other citrus types today specifically because seeds are cheap and abundant from fruit that has no direct market value of its own, unlike some alternative rootstocks whose seed must come from fruit that does. The dwarf ornamental cultivar "Flying Dragon" was first recorded in American nursery listings as early as 1823, but did not receive serious commercial attention until botanist William Saunders reintroduced it in 1869 specifically for rootstock use. The fruit itself is essentially inedible fresh, intensely sour and bitter, though it is occasionally used for marmalade, and the plant is also grown purely as an ornamental thorny hedge, reportedly capable of surviving temperatures as low as minus 20 degrees Fahrenheit.