Urban wildflowers may be harming bees

In cities around the world, empty lots and old industrial sites are being transformed into patches of color and life through the planting of wildflowers. These green spaces are meant to help pollinators such as bees and butterflies, providing much-needed food sources from urban wildflowers.

But new research suggests that the soil beneath these flowers might be doing more harm than good. A team of scientists from the University of Cambridge has found that wildflowers growing on previously developed land – including sites where buildings and factories once stood – can take up harmful metals such as lead, arsenic, cadmium, and chromium.

These metals don’t just stay in the plants. They end up in the nectar that pollinators feed on, introducing toxic substances directly into their diets.

Polluted roots of urban wildflowers

Many cities, particularly older ones, have soil that’s been exposed to decades of industrial activity. Sources including cement dust, mining runoff, and vehicle emissions leave behind long-lasting contaminants which can affect urban wildflowers.

In the post-industrial city of Cleveland, Ohio – where the study took place – over 33,000 vacant lots remain from years of population decline and industrial use. Researchers tested nectar from common self-seeding wildflowers growing in these urban spaces.

The wildflowers included white clover, wild carrot, bindweed, and chicory – plants known to attract bees and provide nectar throughout the growing season. Every plant tested contained at least one toxic metal.

Lead was the most common and found in the highest concentrations. The amount and type of metal varied by plant species.

Chicory had the highest total metal levels, followed by white clover, wild carrot, and bindweed. These are not rare or exotic flowers – they grow widely across cities in both the U.S. and the UK, often planted intentionally as part of pollinator-friendly initiatives.

Urban wildflowers affect bee health

Even small amounts of metal in nectar can affect bee health. Previous studies have shown that these contaminants damage pollinators’ ability to learn and remember, which directly affects how well they can forage for food.

In extreme cases, the toxins can lead to population decline and even death. Reducing the risk of metal exposure is critical for the success of urban pollinator conservation schemes, according to the researchers.

They also noted that certain plants are more likely to absorb these contaminants than others – a fact that should inform future planting strategies. One solution is to test soil quality before sowing wildflowers. Polluted areas might need to be cleaned up first.

In places where harmful metals are already present, managing wildflowers through regular mowing may help. Limiting their bloom time can reduce the amount of contaminated nectar available to bees.

Bees can benefit from wildflower soil

Dr. Sarah Scott is an expert in the University of Cambridge’s Department of Zoology and the study’s lead author.

“It’s really important to have wildflowers as a food source for the bees, and our results should not discourage people from planting wildflowers in towns and cities,” said Dr. Scott.

“We hope this study will raise awareness that soil health is also important for bee health. Before planting wildflowers in urban areas to attract bees and other pollinators, it’s important to consider the history of the land and what might be in the soil – and if necessary – find out whether there’s a local soil testing and cleanup service available first.”

A practical way to support pollinators

Pollinators are vital to the world’s food supply. They help plants – including fruits and vegetables such as apples and tomatoes – develop and reproduce.

Natural pollination is estimated to contribute billions of dollars to global crop production each year. But wild pollinator populations have plummeted by over 50% in the past 50 years, largely due to habitat loss, land use changes, pesticides, and climate change.

“Climate change feels so overwhelming, but simply planting flowers in certain areas can help towards conserving pollinators, which is a realistic way for people to make a positive impact on the environment,” said Dr. Scott.

This new research serves as a reminder that while planting wildflowers is a step in the right direction, the quality of the land they’re planted on matters just as much. A colorful patch of blooms may look like a haven for bees – but if the soil beneath holds a toxic past, it could turn into a hidden hazard.

The full study was published in the journal Ecology and Evolution.

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