Poncirus trifoliata Seeds | Trifoliate Orange
LATIN NAME: Poncirus trifoliata
COMMON NAME: Trifoliate Orange
Plant Profile
Growers plant the trifoliate orange for its roots rather than its fruit. Almost every sweet orange, mandarin and grapefruit in a cool district sits on top of one, and the fruit itself is a seedy ball in a thick fuzzy skin that few people bother to eat twice.
Linnaeus placed it in Citrus, Rafinesque made the separate genus Poncirus, and modern genomic work has pushed it back inside Citrus, beside the papedas. Horticulture has never accepted the change, so both names are correct in practice. Its hybrids with Citrus carry names of their own: the citranges and the citrumelos that fill commercial nurseries.
The fruit is small, densely fuzzy, and packed with oil glands that squirt when the peel is bent. There is very little juice, and what there is tastes sharply sour against a resinous background, so the fruit goes into marmalade and liqueurs rather than onto a plate. Each fruit holds several large seeds, and those are the easiest route to a rootstock plant.
Wood of the trifoliate orange survives winter cold that would kill sweet orange outright, and the trait passes to the scion through the rootstock rather than through the fruit. That is why cold-climate citrus is grown as a grafted tree with a trifoliate root and a mandarin or citrange top, and why the graft union must sit above the soil line in well-drained ground. Cold air pooling on a frosty flat will still kill a hardy tree.
As an understock it brings resistance to citrus tristeza virus, to Phytophthora root rot and to the citrus nematode, plus tolerance of cold and of heavy soil. It is not perfect: it is sensitive to exocortis viroid, it yellows on high-pH ground, and some scions grafted onto it start slowly. It also shows partial tolerance of Candidatus Liberibacter asiaticus, the bacterium associated with citrus greening.
Science & Biotechnology
The bitterness of the flesh and the bitterness of the peel are different chemistries. The flavanone glycosides poncirin and naringin, with neohesperidin close behind, carry the sharp, lingering flavour of the pulp, and poncirin takes its name from the genus. The rind also holds coumarins, poncimarin among them, alongside imperatorin and related compounds. Peel oil is mostly limonene with myrcene and gamma-terpinene, so it smells of lemon and resin.
A second layer of bitterness comes from limonoids, the oxygenated triterpenes that give all citrus juice its edge; limonin and nomilin are the two that matter most in a bite of the fruit. Limonin forms as the fruit is damaged or processed, which is why fresh-pressed juice turns bitter after standing.
Uses & Benefits
This is the only deciduous member of the orange subfamily, dropping its leaves in autumn while its relatives stay evergreen. The leaf is a single organ divided into three leaflets, carried on a winged stalk that is jointed where it meets the blade. Large flattened spines grow from the stem, and the bare green branches still work as a hedge in winter.
Where a hedge must be stock-proof, this is the plant. The dense branching and long spines make an effective barrier that needs little clipping, and the white spring flowers and yellow autumn fruit give it a second reason to be there. The contorted dwarf form sold as ‘Flying Dragon’ is slower and suits containers far better.
Growing from Seed
Citrus seed has no resting period and does not tolerate drying, so sow it fresh from a ripe fruit.
A mix of two parts loam, one part compost and one part sharp grit suits both seedbed and container, at neutral pH. Seedlings vary in vigour and spine length, so choose the straightest, least thorny stems for grafting and keep the thorniest for hedging. Never let the mix dry out completely, and never leave the pot standing in water.
Water when the top centimetre dries, and feed citrus fertiliser from spring until late summer. Once established the plant is hardy, but roots in a pot are not, so move containers against a warm wall for winter. Scale insects and spider mites are the usual indoor problems, and they build up fastest in still, dry air.
Our Take
I keep a trifoliate orange in a pot purely as rootstock material, and it earns its space every spring. Sow fresh seed as soon as the fruit colours, because stored seed loses its vigour quickly. For other citrus that take grafting or a warm sill, try Citrus junos, which is itself hardy, the cold-tolerant Ichang papeda, and the free-flowering Citrus cavaleriei for a hedge in mild districts.
Related Reading
More species in the same orbit as Trifoliate Orange: