Invasive Bur Cucumber Spreads Rapidly Across Western Georgia

A climbing vine from North America is quietly tightening its grip on the lowlands of western Georgia, and a decade of fieldwork has now documented just how far it has come. Sicyos angulatus, commonly known as bur cucumber, was first recorded in the country in 2012, when established populations turned up on agricultural land in the valley of the Chorokhi River. New research published in the journal Caucasiana brings together ten years of route surveys, herbarium collections, and interviews with local residents to map the species’ spread, describe the morphology of Georgian populations, and assess the traits that make this annual vine such a formidable invader. The picture that emerges is of a plant that arrived almost unnoticed, took hold along riverbanks, and has since expanded year after year into croplands, wetlands, and disturbed natural communities.

The genus Sicyos is the most diverse lineage within the gourd family, Cucurbitaceae. Linnaeus established the genus in 1753 with three species, and the count has grown steadily ever since: 31 species were recognized by the end of the nineteenth century, roughly 40 by 1990, and recent estimates place the total at 64. Most of these are climbing annual herbs, and their natural range centers on southeastern North America, northern South America, and parts of southern and southeastern Australia. One member of the genus, Sicyos angulatus, carries a particularly unwelcome reputation. In its native range it is classified as a weed, and it has been listed among the five most noxious weeds in regions where standard weed control programs struggle to contain it. That pedigree alone makes its arrival in the Caucasus a matter of genuine concern for agronomists and ecologists alike.

Bur cucumber’s journey to the Old World began in the nineteenth century, when it was introduced to Europe as an ornamental plant. The first record of the species growing in the wild came from the Balkan Peninsula in 1927, and over the following decades it naturalized in moist habitats across central, southern, and southeastern Europe, including Austria, the Czech Republic, Hungary, Italy, Romania, and parts of Russia. From the second half of the twentieth century onward, its range expanded dramatically: the species has now been reported from Sweden, Croatia, Norway, France, the United Kingdom, Spain, Germany, Slovenia, and beyond, as well as from Korea, Japan, China, Siberia, Australia, the Caribbean islands, Turkey, Ukraine, and India. Historical accounts also placed the genus in the European part of the former Soviet Union, in the Dnieper River basin and along the Volga-Don Canal, although an early indication of the genus for the Caucasus was never confirmed by later botanists.

Against that backdrop, the Georgian discovery in 2012 was less a surprise than a confirmation of a slow-motion invasion already underway. The first specimens were found on agricultural areas in the Chorokhi River valley, and the extent and frequency of the populations suggested the species had been present for some time. Interviews with local residents filled in the missing history: people living near the river recalled first seeing the plant ten to twelve years earlier, initially in small numbers along the riverbanks before it gradually pushed into farmland. Taken together, the evidence points to the early 2000s as the likely period when the first individual specimens appeared in the Chorokhi Valley, meaning the vine had been quietly establishing itself for roughly a decade before science formally caught up with it.

What followed was a steady drumbeat of new records. In 2013, a single specimen was seen on the left bank of the Chorokhi near the village of Tkhilnari, in disturbed habitats and agricultural areas, and in 2014 the plant reached the Chorokhi Delta. By 2015, botanists had found specimens around the Korolistskali, Bartskhana, and lower Mejinistskali rivers. The species then jumped northward: in 2017 it appeared in Ozurgeti near the Nabzhuara River, in 2018 along the Sajavakho-Chokhatauri-Ozurgeti highway near the Supsa River and in maize fields, and in 2020 new populations emerged at the Gubazeuli River near the village of Nabeglavi. In 2021, small populations were recorded in Kobuleti around the Achkva River, alongside a remarkable invasion in the middle reaches of the Mejinistskali and Bartskhana basins. By 2022, the vine had reached the banks of the Rioni River, cementing its presence across the lowlands of Kolkheti.

Understanding why this plant succeeds so well begins with its architecture. Sicyos angulatus is an annual herbaceous vine that typically stretches four to ten meters, dying back completely each winter and regrowing from seed. In Georgia, researchers recorded stems reaching an extraordinary sixteen meters. The angular, hairy stem carries branched tendrils in groups of three or four, and the leaves are simple, five-lobed, and up to twenty-five centimeters across, borne on stout hairy stalks. The tendril mechanics are remarkable in themselves: when a tendril tip touches a support, it wraps around it, then twists into spirals that reverse direction, an unusual behavior known as perversion. The result is a spring-like anchor that lets the vine haul itself over virtually any plant it encounters, from alder trees in the Chorokhi Delta to citrus shrubs in commercial plantations.

The plant’s reproductive machinery is equally efficient. Bur cucumber bears separate male and female flowers on the same plant, with male flowers arranged in panicles and female flowers packed into spherical clusters of eight to twenty. Flowering in Georgia runs from mid-July to September or mid-October, with pollination carried out by bees and flies as well as by the plant’s own pollen. What appears at dispersal time to be a single spiky fruit is actually a cluster of individual fleshy fruits radiating from a central point, each one to two centimeters long, covered in sharp white prickles, and containing a single large brown seed that is never fully released from its casing. Those prickles are not merely decorative: they hook into animal fur, turning livestock and wildlife into unwitting seed couriers.

The numbers behind this strategy are staggering. A single plant can produce between 4,500 and 78,000 seeds in a season, and in Georgia those seeds germinate by the end of April, with germination continuing throughout the growing season. Field surveys in the invaded zone found dense populations averaging twelve to eighteen individuals per square meter on moist and wet soils, with average heights of six to eight meters and individual specimens reaching twelve to sixteen meters. The vines develop numerous side shoots, and coverage at invaded sites climbs to ninety to ninety-five percent, smothering every plant beneath them. Because the species reseeds itself prolifically and its seeds can germinate across an extended window, each growing season adds new ground to the invasion.

The consequences for Georgia’s agriculture and biodiversity are already visible, even if they have not yet been quantified. In maize fields and citrus plantations, the vine significantly reduces both the quality and quantity of harvests, and its spiny fruits cause skin irritation in people who handle them. Three species from two native genera of Cucurbitaceae had previously been recorded in Georgia’s natural flora, making bur cucumber a genuinely novel addition to the country’s plant life, one with no native competitors adapted to check it. The humid, mild climate of western Georgia appears tailor-made for the species, which grows, flowers, fruits, and matures seeds without apparent limitation, and its spread is concentrated precisely in the river valleys, wetlands, and farmland that define the region’s economy.

The authors of the new study are careful about what remains unknown. The effect of Sicyos angulatus on local biodiversity and on the regional economy has not yet been measured quantitatively, and the species’ full potential range within Georgia and the wider Caucasus is still an open question. What the ten-year record does establish is the trajectory: a fast-growing, intensely climbing annual that has adapted readily to local soil and climate, expanded its distribution every year since its arrival, and now threatens both agricultural lands and already disturbed natural communities. Whether bur cucumber becomes a manageable nuisance or a defining weed of the Kolkheti lowlands will depend on how quickly monitoring and control efforts catch up with a vine that has, so far, always been a step ahead.

Subject of Research: Distribution and invasion biology of the alien plant Sicyos angulatus in Georgia

Article Title: New distribution records of the alien plant – Sicyos angulatus L. ( Cucurbitaceae ) from Georgia

Article References: Mikeladze, I., & Bolkvadze, G. (2023). New distribution records of the alien plant – Sicyos angulatus L. (Cucurbitaceae) from Georgia. Caucasiana, 2, 9-13. https://doi.org/10.3897/caucasiana.2.e96039

Image Credits: AI Generated

DOI: 10.3897/caucasiana.2.e96039

Keywords: Sicyos angulatus, bur cucumber, invasive species, alien plants, Georgia, Cucurbitaceae, Chorokhi River, Kolkheti lowlands, agricultural weeds, plant morphology, biological invasion, field survey