Unripe Japanese Bitter Orange
Sunny day Poncirus flower
Frost and standard Poncirus trifoliata leaves
ripe Japanese Bitter Orange
cross section of Poncirus fruit
Other plant Poncirus flower
Poncirus trifoliata seeds
unusual Poncirus trifoliata leaves
Weather beaten Poncirus flower
Known as the Japanese Bitter Orange, or Trifoliate Orange, this is the hardiest close relative of Citrus. A native of China and Korea, it is a deciduous shrub armed with vicious spines and will survive temperatures as low as –20C. It grows satisfactorily in the UK producing small, yellow and slightly furry ‘oranges’ which are unpalatable due to the presence of a bitter oil called Ponciridin.
In the USA it certainly grows as far north as Boston, MA  - visitors to the Arnold Arboretum will find several fruiting plants.
There are many named varieties of Poncirus trifoliata, the most common being Rubidoux, but it is difficult to find any references to precise differences between them. However, it seems clear that there is one group of large-flowered forms and another group with smaller flowers. Additionally, there is a dwarf form, with twisted stems and curved thorns, called 'Flying Dragon'.
Poncirus Trifoliata and citrus are graft compatible. Commercially many citrus varieties are grown on Poncirus rootstocks, which provide resistance to some diseases, increased cold-hardiness and high quality fruit.
I have two large, mature plants of Poncirus, as well as  seedlings of 'Frost' (said to be exceptionally vigorous),  'Kryder 5-5' (said to have large fruits), and Rubidoux. Most years I collect and grow seeds from un-named plants growing at various sites in Southern England.
Tours Botanic Garden Poncirus in spring
Poncirus trifoliata flower
Cannizaro Park Poncirus in summer
Ripe Autumn Poncirus fruit in tree
Oxford Botanic Garden Poncirus trifoliata
Poncirus trifoliata through the seasons. Trees of different ages, free-growing and pruned. For more details, locations and photos see my interactive map.
Old, pruned Poncirus trifoliata in Turin
One of my trees produces rather tatty,  flowers only about 1.5cms across. But given a hot, sunny day they expand to wispy-petalled flowers of about 4cms.
My second tree, now transplanted from the greenhouse to outside has flowers in April about 7.5cms across.
Sometimes an odd leaf is produced that is not 'trifoliate' - here are examples with four and five leaflets.
The 'Frost' variety (left) has a shorter petiole and more serrated leaf margins
Here is the very first fruit that I managed to grow, after a 12 year wait from buying the plant!
The golf-ball sized fruit is shown in July 2000 and when mature in October the same year.
It contained around 30 seeds, ready to be planted for citrus rootstocks.
Poncirus trifoliata 'Flying Dragon' 
These strange contorted plants are used as Bonsai specimens and also as a dwarfing rootstock for citrus varieties. They are equally hardy as the standard form. Even though they usually come true from seed, they are not considered a separate species. With a bit of imagination I think I can just see the winged dragon flying away at the top left of this photo!!
Poncirus Trifoliata first crossed with the sweet orange, Citrus sinensis, about 100 years ago, has produced the citrange. (The old name for Poncirus was Citrus trifoliata, combined with orange gives citrange). There are several named varieties of citrange – I am presently growing seedlings of  Morton, Troyer and Carrizo, as well as some budded plants of Benton. Although they seem to survive the winter outside in England, none has yet flowered.
For further information about citranges click the ' Hardy Citrus Varieties' link button below.
Poncirus trifoliata 'Flying Dragon' in autumn
Click the photo for some more of my artistic Flying Dragon pictures
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