The development of cold-hardy citrus didn’t begin in a backyard orchard—it began in the laboratories and trial fields of early 20th-century horticulturalists. These early scientists, often working in southern universities and under USDA auspices, grappled with a singular challenge: how to make citrus—tender, evergreen, and frost-fearing—survive and fruit in the colder reaches of the American South.
Their solution lay in Poncirus trifoliata, an unlikely hero in the citrus family tree.
Poncirus trifoliata: The Frost-Resistant Ancestor
A native of northern China and Korea, Poncirus trifoliata is the only deciduous member of the citrus family and can withstand temperatures down to -10°F. Although its fruit is small, bitter, and resinous, its tolerance to cold, pest resistance, and disease resilience made it invaluable as a rootstock and hybrid parent.
Introduced to the United States in the mid-1800s via botanical exchanges, Poncirus trifoliata soon attracted the attention of plant breeders attempting to hybridize it with true citrus species to confer hardiness. This effort blossomed particularly in the early 1900s.
Pioneers of Hybridization and Early Research
1. Dr. Walter T. Swingle (1871–1952) – USDA Citrus Pioneer
Swingle, a legendary USDA botanist, is widely credited with introducing and classifying Poncirus trifoliata in American citrus research. He saw its value early and began hybridizing it with various citrus types, coining the term Citrange for the offspring of sweet orange (C. sinensis) and Poncirus.
Key Research Contribution:
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Swingle introduced numerous cold-hardy hybrids, including ‘Rusk’, ‘Morton’, and ‘Troyer’ citranges. These were primarily used as rootstocks, but some bore usable fruit in mild climates.
Notable Publication:
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Swingle, W. T. (1911). "The Botany of Citrus and Its Wild Relatives of the Orange Subfamily (Aurantioideae)." Contributions from the U.S. National Herbarium.
This foundational document not only classified citrus and its relatives but also advocated for hybridization with hardy species.
2. Dr. H. Harold Hume (1875–1965) – Florida Citrus Authority
A British-American horticulturist, Hume was an early advocate for citrus culture in cooler regions of the South. He contributed extensive observations on Poncirus trifoliata and its hybrids, particularly in his work at the University of Florida.
Notable Publications:
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Hume, H. H. (1926). “The Cultivation of Citrus Fruits.”
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Hume, H. H. (1943). “Citrus Fruits and Their Culture.”
Both works highlighted the horticultural promise and challenges of cold-hardy hybrids, and helped build the case for planting in marginal climates.
3. Eugene May and the USDA Horticultural Field Stations
USDA horticulturists like Eugene May in Texas and Florida developed hundreds of hybrids in the 1930s–50s using Poncirus trifoliata as a breeding parent. Many of these were never commercially released but served as progenitors in later breeding lines.
Example: The Citrangequat, a hybrid of a citrange and kumquat (itself hardy), was developed during this period and later gained notoriety in home orchards for its tolerance and tangy fruit.
The Rise of Satsuma and Ichang Papeda as Breeding Partners
In the mid-20th century, attention shifted from the heavily bitter Poncirus hybrids to more flavorful, slightly less hardy options.
Satsuma Mandarin (C. reticulata var. unshiu)
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Originally from Japan and China, Satsuma was introduced to the Gulf States in the 1870s by Jesuit missionaries.
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While not as hardy as Poncirus, Satsuma trees could survive to 15°F and were favored for their sweet, seedless fruit.
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Breeders began using Satsuma as a parent in hopes of preserving flavor while pushing hardiness limits.
Ichang Papeda (Citrus ichangensis)
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A little-known Chinese species with exceptional cold tolerance and much better fruit quality than Poncirus.
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First described in Chinese flora in the early 20th century but gained breeding interest in the 1970s.
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Key to the creation of hybrids like ‘Ichang Lemon’ and other high-resilience varieties with improved taste.
Cold-Hardy Citrus and Post-War Hobbyist Networks
By the 1960s and 70s, while government breeding slowed, amateur growers and regional universities picked up the torch. This was the era in which Bob McCartney would eventually enter the scene, surrounded by growers like Joe LeVert and Herschel Boyd who had begun to trial these USDA hybrids in real-world conditions. Many of the “unreleased” cultivars, once stored in university collections or passed via grafting wood among enthusiasts, were rediscovered, trialed, and sometimes renamed.
This was a slow, grassroots rediscovery—but it was also responsible for bringing previously ignored varieties into the hands of niche nurseries and specialty growers.
Legacy and Lasting Impact
Today, cold-hardy citrus stands as a testament to over a century of breeding, experimentation, and collaboration. The early USDA efforts under Swingle and Hume provided the genetic foundation. Later, grassroots networks like that spearheaded by Bob McCartney allowed these hybrids to be tested, refined, and distributed in meaningful ways.
What was once considered fringe horticulture is now an exciting frontier for food resilience and regional flavor. And in every cold-hardy kumquat or satsuma that ripens in a Zone 8 backyard, there’s a little echo of Poncirus trifoliata, Walter Swingle’s vision, and Bob McCartney’s practical determination.
The Most Edible and Cold-Hardy Citrus Varieties
The modern market now offers a wide range of cold-hardy citrus, but a few cultivars have proven exceptional in both flavor and resilience:
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‘Changsha’ Mandarin (Citrus reticulata) – This historic Chinese cultivar is among the hardiest of true mandarins, surviving temperatures down to 10°F. It produces small, deeply flavored fruits with a rich mandarin tang—though the many seeds can be a drawback.
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‘Ichang Lemon’ (Citrus ichangensis x C. reticulata) – A robust hybrid with large, lemon-scented fruits and strong cold tolerance. The juice is tart but usable for cooking and drinks.
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‘Benton’ Citrangequat (Citrus × georgiana) – A triple hybrid (orange × trifoliate × kumquat), it produces small, sour-sweet fruit that ripens early. Hardy to 10°F or lower.
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‘US 119’ – A hybrid developed by USDA with excellent cold tolerance and a true citrus flavor, often used in breeding programs and home orchards.
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‘Ten Degree Tangerine’ – A rumored name for one of the best-tasting hybrids to survive in Zone 8. While elusive, the variety reflects the ambition of the cold-hardy citrus world.
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‘Thomasville’ Citrangequat – A widely grown cold-hardy variety with an unusual lime-kumquat-orange flavor. It’s hardy to around 10°F and bears prolifically.
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‘Nordmann’ Seedless Satsuma – While not as hardy as trifoliate hybrids, this cultivar survives brief dips into the teens and produces sweet, seedless fruit. Protected planting (e.g., near a south-facing wall) is essential in marginal zones.
Woodlanders has purveyed many interesting and successful cold-hardy varieties over the decades, in large part thanks to a wider network of citrus enthusiasts including Joe LeVert of Aquinas High School, Thomas McClendon, and Hershel Boyd of Madison Citrus Nursery. We have curated two beautiful orchards in Aiken, South Carolina with over 25 varieties of citrus growing and producing incredible fruit year after year.
The Challenges of Cold-Hardy Citrus
Growing citrus in colder climates isn’t without hurdles. Even the hardiest cultivars can be damaged by sudden temperature drops below 15°F—especially if new growth hasn’t had time to harden off. But cold alone isn’t the only challenge. A number of other considerations complicate the picture:
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Rootstock compatibility – Not all hardy scions are vigorous when grafted onto cold-tolerant rootstocks. Some combinations result in stunted or delayed growth, poor fruiting, or shortened lifespan of the tree.
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Fruit quality – Many early hybrids leaned heavily toward bitterness or resinous flavors, thanks to their Poncirus trifoliata parentage. Breeders have had to walk a tightrope between flavor, hardiness, and productivity.
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Freeze-thaw cycles – In areas like the Southeast, it’s not just how cold it gets—it’s how often temperatures swing back and forth. These fluctuations can damage bark, cause splitting, and weaken young trees.
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Market skepticism – For years, cold-hardy citrus varieties were viewed as curiosities—too sour, too seedy, or simply too strange for widespread adoption. Only in recent decades have select cultivars begun earning broader appreciation.
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Potential invasiveness – As more cold-hardy hybrids enter the landscape, some researchers and conservationists have raised flags about the invasive potential of certain vigorous rootstocks and hybrids, especially those with Poncirus genetics. While not currently a widespread problem, Poncirus trifoliata has naturalized in parts of the southeastern U.S., and its hybrid offspring may pose similar risks in sensitive ecosystems if left unmanaged.
Despite these challenges, many growers have proven that with careful siting, seasonal protection (such as frost cloth or thermal mass), and thoughtful cultivar selection, cold-hardy citrus can thrive well north of its traditional growing zones—bringing an unexpected flash of zest to even the frostiest gardens.
