The results surprised the researchers. Under controlled conditions, the urban plants were smaller than their rural counterparts, indicating more conservative growth. But when the stress treatments kicked in, that trade-off paid off.

When exposed to warmer temperatures, drought, or a combination of the two, the urban populations were hit less hard than the rural ones. Tolerance of those conditions increased in step with the level of urbanization at the original seed collection sites.

"We documented large differences in plant performance between rural and urban populations based on our experimental treatments that mimicked conservative predictions for warming and lack of rainfall for the area," said Drew Hiatt, the study's co-author and a doctoral student.

He added a caveat: the real world is messier than a growth chamber.

"In reality, environmental stressors rarely behave so regularly, and it's likely that we would see even greater differences under a more prolonged drought or a more intense summer heat wave," Hiatt said.

Why might cities act as training grounds?

Many urban areas are hotter, drier, and generally more stressful environments than the rural landscapes surrounding them. Paved surfaces trap heat, and water is often scarcer. Plants that live in these conditions face constant pressure.

The researchers' argument works like this: invasive species in cities adapt to those chronic stresses. Those same stresses — heat and drought — are exactly what climate change is expected to spread more widely in coming decades. An invader pre-adapted to city hardship could therefore be better positioned to survive as those conditions extend beyond urban boundaries.

"Expanding urbanization and the rapid increase in global travel and trade mean that the importance of cities as hubs for the introduction and spread of invasive species will only increase," Flory said.

What could this change for management?

The findings could shift how scientists and land managers think about invasive species in cities. Urban populations may not simply be sources of new invasions — the usual framing. They could also be generating populations specifically adapted to environmental stresses that will become increasingly common in the future.

That has practical consequences. If city-adapted invaders are tougher than we assumed, monitoring and control efforts may need to focus more attention on urban populations before they spill into natural areas, agricultural land, or protected habitats.

Flory argues that understanding how invasive species evolve in urban areas is now critical for predicting and mitigating their spread and impacts under future climate conditions.

How solid are the conclusions?

The study comes with clear limitations, and the researchers acknowledge them openly.

• The experiment covered a single species, daisy fleabane, in one region around Zagreb, Croatia.
• Growth chamber treatments simulated regular, conservative climate scenarios — not the irregular extremes of real heat waves and prolonged droughts.
• The researchers say more work is needed to determine how common this pattern is among invasive species generally.

It remains an open question whether these urban adaptations consistently make city-evolved invaders more successful in the wild, or whether daisy fleabane is an outlier.

Still, the core result is striking: within a single species, city life produced measurable, inherited differences in drought and heat tolerance that emerged under controlled experimental conditions. As urbanization and climate change continue on their current trajectories, the researchers suggest, the intersection of the two could give invasive plants an edge that native species lack — and that land managers will increasingly have to reckon with.

The study, by S. Luke Flory and colleagues, appears in Plants, People, Planet (2026), DOI: 10.1002/ppp3.70267.