Italy’s Alien Plant Watch Logs New Invasions and Naturalizations Across the Country

Italy’s botanical surveillance network has released the eighteenth installment of its ongoing inventory of alien vascular plants, a report that documents new records, status upgrades, and one notable exclusion across the country’s regional floras. Published in the journal Italian Botanist, the update covers taxa in eighteen genera, ranging from wattles and honey locusts to succulents and ornamental bulbs, and reflects the collective work of field botanists who submitted verified specimens and observations to the national herbarium network. The contribution is part of a long-running series that keeps Italy’s alien flora checklist current, a task of growing urgency as climate change, trade, and ornamental horticulture continue to move species far beyond their native ranges.

The most consequential finding concerns Acacia melanoxylon, the blackwood acacia, a Australian tree long grown in Mediterranean gardens. Although the species had recently been reported as merely casual in Sardinia, field surveys in 2024 revealed numerous individuals in all life stages spreading through natural environments, particularly under humid and shady conditions along watercourses. Populations were documented at Rio Leni near Villacidro, on Monte Arci, at Monte Nieddu near San Teodoro, and at Berchida near Siniscola. On this basis, the authors upgraded the species from naturalized to invasive for the whole of Italy, and from casual to invasive for the Sardinian flora, a formal recognition that the tree is now reproducing and spreading without human assistance in sensitive riparian habitats.

Sardinia emerges as a hotspot in this update for reasons rooted in land-use history. At Domus de Maria in the province of Sud Sardegna, along an ancient Roman road, botanists recorded naturalized populations of Allocasuarina verticillata and Brachychiton populneus, both Australian trees introduced in the second half of the nineteenth century and used as ornamentals and for reforestation. The area was planted by the Regional Forestry Agency from the late 1950s through the 1970s with a suite of non-native species, including Acacia saligna and A. pycnantha. The Allocasuarina population covers roughly one hundred square meters and includes adults, juveniles, and seedlings, while the Brachychiton population has been naturalizing in coastal scrub since at least 2018. These cases illustrate how historical afforestation with exotic species can create persistent recruitment foci decades later.

Several other status changes in the report involve familiar ornamental plants making the transition from garden escape to established member of the flora. In Sicily, the hybrid succulent Kalanchoë × houghtonii, first reported as casual there in 2017, now shows an expanding population on Monte Pellegrino in Palermo, reproducing both sexually and through plantlets that form on the margins of its leaves. Additional populations grow on a harbor cliff at Terrasini and along a motorway spur in Palermo, and the species has even been observed on the roofs of the city’s historic center. Similarly, Freesia leichtlinii subsp. alba, a South African iris, has formed abundant seeding populations along roadsides at Monte Inici near Castellammare del Golfo, with further colonies found in southeastern Sicily, prompting its upgrade from casual to naturalized.

The islands also yielded a species entirely new to the Sicilian flora. Aloë microstigma subsp. microstigma, a spotted aloe native to the Cape Province of South Africa and popular as a low-maintenance garden plant, was found naturalized on rocky uncultivated ground at Cala Francese on the island of Lampedusa. The population most likely originated from pruning debris discarded from nearby houses, a common and underappreciated pathway for succulent escapes in Mediterranean environments. A second, smaller population was subsequently observed on a sea cliff at Cala del Passo on Favignana, suggesting the species is quietly spreading across the Egadi and Pelagie islands.

On the mainland, the update records a series of firsts and promotions. Gleditsia triacanthos, the honey locust of North America, is newly confirmed as naturalized in Molise, where about ten trees ten to twelve meters tall were found along a path on the outskirts of Campobasso, at least five of them mature and fruiting abundantly, with conspicuous young regeneration all around. In Umbria, Hesperocyparis arizonica, the Arizona cypress, has climbed from casual to naturalized status on the vertical rock walls below Orvieto near the famous Pozzo di San Patrizio, its stable population of fruiting shrubs evidently derived from cultivated specimens in nearby urban parks. In Liguria, the Spanish broom relative Retama monosperma subsp. monosperma is now considered naturalized, colonizing abandoned terraces, field margins, and even the cracks of retaining walls along the Aurelia road between Ventimiglia and Bordighera.

Two further regional firsts underline how urban and ruderal habitats serve as gateways for alien establishment. Liriope spicata, an East Asian groundcover, was recorded for the first time as naturalized in Sardinia at Senorbì, where plants spread along a railroad edge from propagules of specimens cultivated more than a decade earlier in the garden of a cantonment house that no longer exists. In Emilia-Romagna, the tiny Asian Mazus pumilus was found for the first time in the region growing in the joints of the cobblestones around the Basilica di Santa Maria della Steccata in Parma, a reminder that pavement fissures can function as miniature ecosystems for opportunistic colonizers.

The report also documents the consolidation of previously casual species into self-sustaining populations elsewhere. Melia azedarach, the Chinaberry tree, is now naturalized in Sardinia, with adult specimens and seedlings recorded at Oristano, Selargius, San Vito, and Cagliari, all presumably derived from ornamental plantings in adjacent gardens and thriving in disturbed, sandy roadside habitats. Phyla canescens, a creeping South American verbena, has expanded to occupy roughly two hundred square meters within managed green space in Lombardy, earning its promotion to naturalized there. Platycladus orientalis, the Oriental thuja, has become naturalized in Trentino-Alto Adige on the basis of a mixed-age, regenerating population near Merano, while Prunus amygdalus, the almond, long cultivated across Sardinia with little tendency to escape, has in recent years been found as adult and subadult specimens far from old plantations, regenerating by seed in both the southern Campidano and Marmilla and the northern Logudoro and Mejlogu districts.

Not every entry in the update adds a species; some correct the record. Convolvulus tricolor subsp. tricolor, the annual dwarf morning glory, has been excluded from the flora of Campania. Historical reports from the Phlegrean Fields near Naples are now attributed to the native subspecies C. tricolor subsp. cupanianus, while plants described from Capri in 1939 were recorded as intermediate forms whose status, whether escaped or indigenous, could never be confirmed, and no supporting herbarium specimen survives. The exclusion demonstrates the discipline’s reliance on verifiable specimens: without a physical record, a species cannot stand in a regional flora, however charming the historical account.

Beyond the floristic records themselves, the contribution compiles nomenclatural and distribution updates drawn from a wide body of recent literature, made available as supplementary material and integrated into the Portal to the Flora of Italy. The series operates as a distributed early-warning system: contributors submit short notes with voucher specimens to the FI Herbarium in Florence, where records are verified before entering the national checklist. As the eighteenth installment shows, the line between a garden plant and an invader is crossed quietly, one seedling at a time, and only systematic, specimen-based tracking can catch these transitions while management options remain open. For a Mediterranean biodiversity hotspot threaded with roads, railroads, cliffs, and cities, that vigilance is not academic housekeeping but a frontline conservation tool.

Subject of Research: Updates on the distribution and naturalization status of alien vascular plant species in Italy

Article Title: Notulae to the Italian alien vascular flora: 18

Article References: Galasso, G., Domina, G., Adorni, M., Alessandrini, A., Ardenghi, N. M. G., Bacchetta, G., Barone, G., Bartolucci, F., Boker, D., Calvia, G., Cuena-Lombraña, A., De Luca, A., Del Guacchio, E., Fois, M., Fortini, P., Lallai, A., Lonati, M., Lozano, V., Marengo, G., … Lastrucci, L. (2024). Notulae to the Italian alien vascular flora: 18. Italian Botanist, 18, 155-166. https://doi.org/10.3897/italianbotanist.18.141896

Image Credits: AI Generated

DOI: 10.3897/italianbotanist.18.141896

Keywords: alien flora, invasive species, Italy, Sardinia, Sicily, naturalization, vascular plants, biodiversity, botany, Acacia melanoxylon, ornamental plants, floristic records