Roosmarijn Steeman, a botanist at the University of Brussels, recently knelt on a city pavement and recorded a pomegranate bush sprouting between the cracks. A few weeks later she noted a date palm thriving on the same sidewalk. Both discoveries are part of FloraBru, a four‑year project that aims to map every wild plant growing in the capital.

Steeman says the phenomenon is not isolated. "We see many Mediterranean plants doing really well in Brussels," she told UnionPress. The explanation lies in the urban heat‑island effect: concrete and asphalt store solar heat during the day and release it slowly at night, keeping city temperatures higher than surrounding countryside. Milder winters and the prevalence of stone‑filled spaces also suit species that are accustomed to rocky Mediterranean habitats.

From pomegranates to avocado trees

Other examples are emerging across Belgium. The so‑called "baby's tears", a tiny plant native to Sardinia and Corsica, now pops up in Brussels, Ghent, Antwerp and even along the milder coastal strip. Small goosegrass, originally from northern Africa, south‑west Asia and southern Europe, has been recorded in urban sites throughout Belgium, the United Kingdom, Germany and the Czech Republic. In 2022, botanist James Wong reported an avocado tree growing in a London garden, underscoring how far warm‑climate species have travelled.

While the media often link these arrivals directly to climate change, Filip Verloove, a specialist in non‑native plants at the Meise Botanic Garden, cautions against a simplistic view. "The warming climate is the context that allows them to bear fruits, but most plants are actually very immobile," he explained. "I've seen weeds that don't even manage to spread to the other side of a field."

How seeds move north

Nature does play a role. A 2025 study published by the Royal Society used GPS trackers on thrushes and showed that these birds can carry fleshy‑fruited seeds tens to hundreds of kilometres, effectively acting as a natural seed‑dispersal service.

Human activity, however, is the dominant vector. Seeds hitch rides on clothing, shoes, car tyres and the countless international journeys Europeans make each year. Steeman traced a clear pattern to Belgian coastal campsites: holiday‑makers travel south with tents and caravans, then return north, inadvertently spreading seeds on their vehicle wheels and bootlaces.

Ornamental gardening also contributes. "There are quite a few ornamental plants that were brought here from abroad to show off in gardens, but which have since become wild and started spreading on their own," Verloove noted. The butterfly bush, prized for its colourful blooms, exemplifies the risk. "We don't always see them as a danger, because they're so beautiful," Steeman said, "but if we don't remove them, they can start creeping everywhere between stones and even damage buildings."

Trade routes provide another conduit. Non‑native species often arrive hidden in agricultural shipments such as grain. In the port of Ghent, Verloove observed several South African weeds near grain quays, despite Belgium never importing grain directly from South Africa. Further investigation revealed that the weeds first colonised Western Australia's grain belt after trade with South Africa, then hitched a ride on Australian grain exports bound for Europe.

This discovery suggests that trade data could become a predictive tool for plant invasions. By monitoring the flora of a country's major export partners, botanists could anticipate which species are likely to appear in European ports before they are first spotted on the ground.

For European policymakers, the spread of warm‑climate plants raises several concerns. While some newcomers may enrich urban biodiversity, others threaten native species, alter soil chemistry and even damage historic stonework. Urban planners and local authorities may need to integrate regular monitoring into green‑space management, allocate resources for early removal of invasive ornamentals, and cooperate with customs officials to tighten phytosanitary checks on high‑risk cargo.

Public awareness also matters. Citizens who enjoy weekend hikes or beach holidays can help by cleaning footwear and vehicle tyres before returning home, reducing accidental seed transport. Community groups and citizen‑science platforms could play a part in reporting new sightings, feeding data into projects like FloraBru and enabling faster responses.

As European cities continue to warm and expand, the line between cultivated gardens and wild habitats blurs. The FloraBru findings remind us that urban ecosystems are dynamic, shaped not only by climate but by the everyday movements of people, goods and birds. How Europe chooses to manage this botanical influx will affect biodiversity, heritage sites and the resilience of urban green spaces for years to come.