Her observations are part of the FloraBru project, a four-year effort to catalogue all wild plants Brussels has to offer.

“We see many Mediterranean plants doing really well in Brussels,” Steeman told us. The fact these plants can thrive in the city has to do with the heat-island effect: concrete absorbs heat in cities, and only partly releases it during the night. Temperatures are generally higher and winters milder than in the countryside. “Also, Mediterranean plants typically like stone and rocky environments, which means they thrive in cities,” Steeman added.

Take the (scarily named) “baby’s tears”, a plant native to Sardinia and Corsica which nowadays finds it warm enough in cities like Brussels, Ghent, Antwerp, and Belgium's milder coastal area. Another successful migrant is small goosegrass, native to northern Africa, south-west Asia, and southern Europe, but now spotted in urban centres in Belgium, the UK, Germany, and Czechia. In 2022, botanist James Wong even found avocados growing in London.

“It is a common misconception that the climate crisis is the reason these seeds are moving in the first place,” Filip Verloove, an expert in non-native plants at the Meise Botanic Garden, told us. “The warming climate is the context that allows them to bear fruits, but most plants are actually very immobile. I’ve seen weeds that don’t even manage to spread to the other side of a field.”

There is one well-known taxi service carrying seeds north: migrating birds. A 2025 study published by the Royal Society using GPS-trackers found that thrushes can disperse seeds of fleshy-fruited plants over tens to hundreds of kilometres.

But although most plants are more the stay-at-home type, some seeds can travel even further. The main carrier is us: seeds hitchhike on our clothes, shoes, and cars on our billions of international trips each year.

Steeman of FloraBru has traced the pattern to campsites on the Belgian coast. “Holiday-goers travel south with their tent or camper in the summer. When they later take a trip to the Belgian coast, the seeds are on their car tyres, all over their camping gear, and between the treads of their hiking boots.”

We also bring plants knowingly. “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 of Meise Botanic Garden told us.

These ornamental plants are often very resilient, and could threaten native species. One example is the butterfly bush: “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 is another driver: it's very common for non-native plants to hitch a ride with agricultural shipments of things like grain. That's why many tropical plants also bloom near ports, according to Steeman.

In the Belgian port of Ghent, Verloove noticed several South African weed species growing near the grain quays – despite Belgium having no history of importing grain from South Africa. After digging deeper, he found that these weeds had first taken hold in Western Australia's grain belt via trade with South Africa, and then hitched a ride to Belgium in Australian grain shipments.

His finding suggests plant experts can use trade data as a forecasting tool. Rather than waiting to stumble across a new non-native species by chance, researchers can follow what’s already growing in the export regions of a country's biggest trading partners and predict what might show up before it even arrives.