The distinction between a casual and a naturalized alien species is more than bureaucratic taxonomy. Casual aliens are plants that appear outside cultivation but cannot sustain themselves without repeated reintroduction, usually because the local climate or habitat does not suit them. Naturalized species, by contrast, form self-sustaining populations that persist through seed production or vegetative reproduction over multiple generations. Invasive species go further still, spreading aggressively and displacing native vegetation. Tracking these transitions matters because each upgrade in status signals a plant that has crossed an ecological threshold, one that may be far harder to manage once it is firmly established. The new notula, the twenty-first in the series, records exactly these threshold crossings for a long list of species across Italian regions.

Among the most consequential updates is the promotion of Bidens pilosa from casual to invasive in Sardinia. This therophytic species, native to tropical and subtropical America, was first reported on the island in Golfo Aranci a decade ago. It has now spread to different parts of Sardinia, and in some places it behaves as a true invader. Near Porto San Paolo in northeastern Sardinia, researchers observed two metapopulations of hundreds of individuals, each covering approximately 120 square meters, and a similar area was occupied near a tourist village in the municipality of Pula. The plant’s success is no accident: its achenes are equipped with awns and hooked bristles that cling to animals and human clothing, turning every passing hiker, dog, and vehicle into a dispersal agent. The species was also documented infesting crops in pots, roadside lawns, wetlands, and seaside promenades at sites including Decimomannu, Quartu Sant’Elena, and Loiri Porto San Paolo.

Sardinia emerges as a hotspot of alien plant establishment in this installment. Boerhavia coccinea, an invasive species new to the island’s flora, was discovered at Foxi Manna in the municipality of Tertenia, where individuals multiplied rapidly in just a few years in a former poplar plantation turned parking area, and the population is now expanding into neighboring areas. Lantana camara subsp. aculeata, recorded as casual on the island more than two decades ago, is undergoing rapid expansion along the eastern and southern coasts, particularly between Golfo Aranci, Olbia, Porto San Paolo, San Teodoro, and Budoni, where it colonizes Mediterranean macchia, fallow land, and roadsides. Stenotaphrum secundatum, originally introduced for lawns in tourist settlements, has naturalized at sites from Portoscuso to the back-dune habitats of Villasimius and Costa Rei. Even the delicate-looking Sparaxis bulbifera, an irid from South Africa probably planted in flowerbeds, has jumped the garden fence and now grows wild along roadsides in Ussaramanna, Olbia, Orosei, and La Maddalena. Asparagus setaceus, once considered merely casual, has naturalized near coastal tourist sites across western and southern Sardinia.

On the Italian mainland, the picture is equally dynamic. In Calabria, two ornamental climbers have officially graduated to naturalized status. Campsis radicans, the trumpet creeper, was first reported as casual in the region only a few years ago, but botanists now document multiple well-established populations throughout Reggio Calabria and Crotone, with individuals reproducing both by seed and by basal shoots. Wisteria sinensis, the famous Chinese wisteria, shows the same pattern: populations with multiple individuals bearing basal shoots, some fruiting, and evidence of both vegetative propagation and seed reproduction. Catharanthus roseus, the Madagascar periwinkle familiar from garden beds, has likewise been found flowering and sustaining itself across numerous sites in the Metropolitan City of Reggio Calabria, from Pellaro to Bagnara Calabra and Melito Porto Salvo, earning its promotion to naturalized.

Some records carry regulatory weight. Cenchrus setaceus, the fountain grass, has been found for the first time in Liguria, with a few tussocks growing along the margins of the Torrente Verbone riverbed near Vallecrosia. This is no ordinary garden escape: the species is included in the list of invasive alien species of Union concern under Regulation (EU) No. 1143/2014, implemented in Italy by Legislative Decree No. 230/2017, which requires its eradication where feasible or its control throughout the entire territory of the European Union. The discovery of a new regional population of a Union-listed invader is therefore not just a botanical curiosity but a matter of legal obligation, highlighting how early detection through floristic surveying feeds directly into management decisions.

Water and wetlands prove especially vulnerable corridors for alien spread. Cyperus eragrostis, a sedge from the Americas, has been upgraded from casual to naturalized in Lazio after years of repeated reports around Lake Bracciano, where a population of numerous well-developed individuals now forms persistent stands along the shore at San Celso. Large populations also occur at Vignarello and Vigna di Valle, and multiple colonies line the Tiber River through Rome, from Ponte Milvio down to Isola Tiberina and Ponte Marconi. In Emilia-Romagna, Cortaderia selloana, the pampas grass, has naturalized along the Parma River near Lesignano de’ Bagni, where a population of 15 individuals, including adult plants up to 3 meters tall and numerous seedlings, was observed, together with a second site at Langhirano. In Liguria, Cyperus alternifolius subsp. flabelliformis has naturalized along a canal in Ventimiglia, probably introduced accidentally through garden waste dumped at a nearby embankment, a reminder that careless disposal of garden clippings is itself a dispersal mechanism.

Trees and woody species are also making their move, with implications that unfold over decades. In Molise, Cupressus sempervirens, the Italian cypress so iconic of Mediterranean landscapes but alien in this region, was recorded for the first time as naturalized in the Natura 2000 site at Collina Monforte near Campobasso, growing in woods alongside holm oak and the invasive Ailanthus altissima. Quercus rubra, the northern red oak from North America, is new to the naturalized flora of Molise, with a mature fruiting tree 12 meters tall and young individuals regenerating in an open Turkey oak and beech wood near Capracotta at 1,240 meters elevation. Perhaps most striking is Pinus wallichiana, the Himalayan white pine, which has naturalized in Piemonte at Sostegno near Biella: two adjacent wooded areas covering roughly 1,000 square meters contain about ten specimens with trunk diameters of 10 to 15 centimeters and heights up to 10 meters, several already reproductively mature, with numerous seedlings and saplings in the understory of both a conifer plantation and a downy oak forest. In Tuscany, Taxodium distichum, the bald cypress, is new to the regional flora, with populations in the Migliarino, San Rossore, Massaciuccoli Regional Park showing heterogeneous age structure and, at Vecchiano, forming stands where the species becomes locally dominant in seasonally flooded habitats.

Smaller and more inconspicuous aliens are establishing themselves in surprising places. Hydrocotyle sibthorpioides, a creeping herb, has naturalized in the interstices of basalt paving stones in the northern courtyard of the Royal Palace of Naples, thriving where sheltered from trampling in seasonally damp, sandy gaps, and was probably introduced as a contaminant from nurseries. Elaeagnus umbellata has naturalized in Tuscany along a hedge at Bucine, with fruiting adults and seedlings. Eragrostis curvula is new to Umbria, where multiple generations thrive along a roadside in an industrial zone at Narni, their seeds spread by passing traffic. Salpichroa origanifolia, a South American solanaceae, forms dense monophytic populations along the roads of Monticchio Bagni in Basilicata, probably derived from nearby ornamental plantings. Lonicera japonica, the Japanese honeysuckle, is new to Molise as an invasive species, climbing over fences in disturbed roadside habitats north of Campobasso. Even in the alpine north, Othocallis amoena has naturalized in humid undergrowth at Levico Terme in Trentino, with three groups of tens of flowering and fruiting individuals each, marking a first for both the region and Italy.

The series itself is a model of collaborative, verifiable biodiversity monitoring. Each record in the notula is backed by a herbarium specimen, deposited in collections such as the FI Herbarium in Florence, CAG in Cagliari, or NAP in Naples, with precise coordinates, dates, and habitat descriptions. Contributions are submitted to designated editors, with records of new records and status changes going to Lorenzo Lastrucci and nomenclatural novelties to Gabriele Galasso, and each text limited to 1,000 characters. Supplementary material provides nomenclatural and distribution updates drawn from dozens of recent publications and corrections to previous accounts, all available through the Portal to the Flora of Italy. In an era of accelerating biological invasions, this meticulous, region-by-region accounting does more than update a checklist: it builds the early-warning system on which conservationists, land managers, and policymakers depend. The message from this twenty-first installment is clear. Italy’s alien flora is not a static list of garden oddities but a living, expanding frontier, and the plants crossing from casual visitor to permanent resident today are the management challenges of tomorrow.

Subject of Research: Distribution and naturalization status of alien vascular plant species in Italy

Article Title: Notulae to the Italian alien vascular flora: 21

Article References: Galasso, G., Domina, G., Adorni, M., Bacchetta, G., Bartolucci, F., Bertotto, G., Bonari, G., Bonivento, E., Calvia, G., Cancellieri, L., Cannucci, S., Cuena-Lombraña, A., D’Aleo, F., de Francesco, M. C., Del Guacchio, E., Di Lernia, D., Di Pietro, R., Fiaschi, T., Fois, M., … Lastrucci, L. (2026). Notulae to the Italian alien vascular flora: 21. Italian Botanist, 21, 215-231. https://doi.org/10.3897/italianbotanist.21.199273

Image Credits: AI Generated

DOI: 10.3897/italianbotanist.21.199273

Keywords: alien flora, invasive species, naturalization, Italy, Sardinia, biodiversity, botany, Bidens pilosa, Cenchrus setaceus, floristic records, plant invasion, Natura 2000