What 360 pots of soil reveal about urban biodiversity

Stepping out onto the balcony or into the garden to see whether anything had begun to grow: for hundreds of participants in the citizen science project Aanwaaiers, this became a daily ritual last spring. Each participant placed a small pot of soil outdoors and then waited: would any plants spontaneously emerge from seeds carried in by the wind?

The starting point for biodiversity

That is exactly what Monique de Jager focuses on during her research on biodiversity. “Seed dispersal is the starting point for biodiversity,” Monique explains. One important way plant species reach new places is by seeds that ‘hitchhike’ on the wind. As a result, wind dispersal plays a crucial role in shaping biodiversity and connecting natural habitats. In cities, where vegetation is often limited and fragmented, understanding how this seed dispersal works is particularly interesting.

Pots of soil across the Netherlands

Tiny seedlings

Some participants saw the first plants appear within just a few weeks. One participant from Maastricht wrote enthusiastically: “They are still tiny seedlings, but it’s a start!”

However, not every pot grew visible plants. This sometimes led to disappointment. “Some participants came to me in disappointment because nothing had appeared,” Monique says. “And I would tell them: there was nothing in my pot either.” 

They are still tiny seedlings, but it’s a start!

Even these empty pots are valuable research data, Monique explains. They show where few or no seeds arrive, providing important information about differences in seed dispersal.

The Aanwaaiers Day

On 17 May 2026, the so-called Aanwaaiers Day took place. On this day, participants brought their pots — whether empty or containing plants — back to the Utrecht University Botanic Gardens. In total, around 360 pots were returned. Here, the visible seedlings were counted and the first results of Aanwaaiers were gathered. On average, each pot contained one seedling, with one exceptional pot with as many as 33 seedlings.

The graph below shows the average number of plants found per pot in each Dutch municipality. The more yellow a municipality appears on the map, the higher the average number of seedlings found per Aanwaaiers pot.

After the Aanwaaiers Day, the pots were grown on in a greenhouse until they were large enough to be identified. The pots contained, among others, Dandelions, Chickweed, Willowherbs, Elm, Umbrella liverwort, Hairy galinsoga and various species of Grass.

The link between environment and wind dispersal of plant seeds

Once Monique's team had identified which plants had grown in the pots, they started analysing the other data collected from the Aanwaaiers pots. Hanna Schreuder, a biology student in Monique’s research team, studied how the surroundings of each pot influenced the number of seedlings and the diversity of species found in an Aanwaaiers pot.

First, Hanna looked at the population density of each area: does the number of residents in a neighbourhood influence how many seeds reach a pot, or the diversity of plants that grow from them? The analyses showed no clear relationship.

She then examined the amount of vegetation in the surroundings, based on satellite images. Here, a clear relationship did emerge: the greener the area surrounding a pot, the greater the chance that plants would grow in it. In more vegetated areas, not only did more plants emerge, but a greater variety of species was also found.

The greener the area surrounding a pot, the greater the chance that plants would grow in it.

A greener city

These results were made possible by the many participants who took part in Aanwaaiers. “Without the help of so many people collecting data at home, we would never have been able to gather information from such a wide range of locations,” says Monique. Thanks to the 360 pots that were returned, a link was found between the amount of vegetation in an area and both the number and diversity of wind-dispersed seeds, an important step in understanding urban biodiversity.

This link also provides a starting point for further research. “We will continue to investigate how seed dispersal varies between neighbourhoods and which factors can explain these differences,” says Monique. “Ultimately, this knowledge could help make cities greener and enable natural plant populations to spread more effectively.”