A Field Guide to Trifoliate Orange and Its Hybrids: Citranges, Citrumelos and Citrangequats

Field Guide · Trifoliate Orange and the Hybrids · Southeast

A thorny, deciduous, bitter little tree from northern China is the reason citrus grows north of Florida at all. Here are the tree and the hybrids the tree fathered: citranges, citrumelos and citrangequats.

9 min read · By Fiona von Grey · Aiken, South Carolina

I.The bitter ancestor

Poncirus trifoliata, the trifoliate orange, is a citrus relative that behaves like a hawthorn. The tree comes from northern China and Korea, grows as a large shrub or small tree somewhere between thirteen and twenty-six feet, and drops all of the leaves every autumn, which is very nearly unheard of in a family built on evergreens. Each leaf carries three leaflets (hence trifoliata), and crushed ones smell of spice. The green stems bristle with thorns an inch or two long. In spring the white flowers open, larger than a true citrus blossom and, by most accounts, with much less scent. The fruit that follows is the size of a lime, covered in peach-like down, packed with seeds, and so bitter that most people call the fruit inedible fresh. Gardeners with a marmalade habit have dried and powdered the rind as a condiment, but nobody grows the plant for the table.

They grow the tree for the cold. The species is rated hardy to USDA zone 6, and published figures for a dormant, established plant run as low as minus twenty degrees Fahrenheit. That is a long way from a Valencia orange. Whether the tree belongs in Citrus at all is still argued: Walter Swingle moved the plant into a genus of their own, Poncirus, in 1943, and David Mabberley and Dianxiang Zhang folded the tree back into Citrus in 2008. Genome work has kept the question open. We use both names, depending on whom we are quoting.

One more trait matters for what follows. The trifoliate orange flowers at a different time from most true citrus, which in the wild keeps the two groups from crossing on their own. That is a barrier that breeders, with a paintbrush, are happy to step over.

II.A freeze, a breeding program, and a pun in Latin

Florida's citrus groves took a severe freeze in 1894, and the United States Department of Agriculture went looking for a tree that could survive the next one. Two USDA botanists, Herbert Webber and Walter Swingle, began crossing sweet orange with the trifoliate orange, hoping to keep the fruit of the first and the constitution of the second. The pairing is not an easy one. Plenty of seedlings kept the thorns and the bitterness along with the cold tolerance. A handful turned out well enough to name.

That work produced three families, and they are the plan of this guide. A citrange is sweet orange crossed with trifoliate orange. A citrumelo is grapefruit crossed with trifoliate orange (the "melo" borrowed from pomelo, grapefruit's relative). A citrangequat goes a step further, crossing a citrange with a kumquat, so that three genera meet in one tree. Each family trades some of the trifoliate's cold tolerance for fruit that looks more like citrus, and the table below shows how the bargain tends to work out.

III.Citranges: sweet orange crossed with the thorn

The founding citrange appeared in a grove at Eustis, Florida, in the 1890s, in the care of a Colonel G. H. Norton, and was described by Webber and Swingle in 1905 as 'Rusk'. Sweet orange was the mother and trifoliate orange the father. The fruit is small, about two inches across, deep orange with a reddish flush, thin-skinned and very juicy, with a sprightly acid flavor and a trace of bitterness. In those years the fruit was eaten fresh with sugar, stirred into citrangeade, and made into pies, marmalades and jellies. The tree was also commended as an ornamental, which is fair: glossy leaves, fragrant spring flowers, orange fruit into winter.

Citranges have had a quiet second career underground, as rootstocks, and some of the best-known names in that world ('Troyer' and 'Carrizo', among them) are citranges. But a few are worth growing for their own sake, and the one we recommend is 'Morton'.

IV.Citrumelos: the grapefruit hybrids

Swingle made the grapefruit cross in 1907, using 'Duncan' grapefruit and the trifoliate orange. The seedling that became famous is the 'Swingle' citrumelo, and the cultivar's fame is entirely underground. The USDA's citrus collection records the tree as resistant to tristeza virus, Phytophthora foot rot, citrus nematode and cold, and the tree went on to become the most widely used rootstock in Florida. The rootstock is moderately salt tolerant, and unhappy in heavy clay or limy soil. Some trees on that rootstock declined on certain soils, and some scions are incompatible, so even the success story has footnotes.

For the garden, the better citrumelos are the ones that carry fruit worth cooking with. 'Dunstan' is the connoisseur's choice: growers have long ranked the plant as the best of the trifoliate hybrids, among the hardiest of all citrus, with golden fruit nearly four inches across and a flavor like a grapefruit picked a little shy of ripe. The glossy leaves fall in autumn, leaving a strong winter silhouette. 'Dallas', which came to us from the citrus enthusiast Tom McClendon, makes large, round, thick-rinded fruit with strikingly big seeds. Neither is a fruit bowl fruit. Both are marmalade fruit, syrup fruit, cocktail fruit, with sugar and a little patience.

V.Citrangequats: three genera in one tree

In 1909, at the USDA station in Eustis, Swingle dusted the flowers of the Nagami kumquat with pollen from a citrange, and the seedlings were sent out across the South to prove themselves against the cold. By the custom of the day each took the name of the town where each first fruited. 'Thomasville' fruited in Thomasville, Georgia; 'Sinton' in Sinton, Texas; 'Telfair' in Telfair County, Georgia. 'Thomasville' and 'Sinton' were named and described in 1923. The citrangequats are often described as among the first three-genus hybrids ever made by hand from known parents, which is still a pleasure to say aloud. Of the three sisters, Thomasville thrived, Sinton hung on, and Telfair seems to have slipped out of cultivation.

Citrangequats are the most edible of the trifoliate hybrids. The fruit is egg-shaped, yellow to deep orange, sharply acid, and rewarding squeezed like a lime, sweetened into an ade or cooked into a marmalade with real bite. The fruit ripens in winter, from January into April, long after the other citrus fruit in the neighborhood has finished. Thomasville is the toughest of the group, rated about ten degrees Fahrenheit once established and suited to the ground in zones 8 to 10. The tree grows upright and vigorous, with thorny branches and three-parted leaves. Sinton leans toward the kumquat parent: simple leaves rather than trifoliate ones, a denser and more compact plant, and fruit so richly pigmented that chemists studying the rind found a carotenoid new to citrus and named it sintaxanthin after the tree.

VI.The root story, and two newer USDA oddities

It is worth saying plainly how much of the citrus world stands on trifoliate roots. A grafted citrus tree is two plants pretending to be one, and the plant underneath does the unglamorous work of roots, vigor and disease resistance. Citrus grafted onto trifoliate orange tends to be hardier than citrus on its own roots, which is why so many cold-hardy trees in this region are grafted onto some trifoliate selection. That is also why the hybrids in this guide matter beyond their own fruit: the same blood runs through much of what grows north of the citrus belt.

Two recent USDA releases show where the story is going. 'US-1516' is a rootstock from a cross the USDA made in 1975, African pummelo by 'Flying Dragon', the contorted, fiercely thorned dwarf form of the trifoliate orange. The seedlings were watched in Florida for forty years, and Kim Bowman's program released them in 2015, into the middle of the huanglongbing epidemic that has hollowed out Florida's groves. We sell them ungrafted, which is an unusual and honest thing to sell: a tree for the tinkerer who wants to practice budding or simply grow the trifoliate-blooded foundation and see how far north the plant will go.

'Weeping Dragon' is a different kind of gift. Bowman released the plant in 2016 from a cross of 'Flying Dragon' and 'Cipo', a sweet orange whose limbs arch toward the ground. The result is a citrus that weeps: branches spill over and cascade, so a plant in a tall pot reads less like a citrus and more like a small weeping tree. The fruit is small and sour, handsome and better for looking at than eating. Cold tolerance has not been formally rated, so treat the plant as reliable in the ground in zones 9 and 10, a gamble in a sheltered zone 8 spot, and a container plant farther north.

VII.Growing them: a short field manual

All of these like full sun, sharp drainage and shelter from the worst of the winter wind. Most do well in a large container that can be moved under cover for a hard freeze, a habit that also lets a gardener farther north push the line. Plant in the ground where winters are mild, and site the tree where the spring fragrance and the winter fruit can both be enjoyed, with the thorns back from the path.