Tree pits, or alcorques, are small patches of biodiversity at the base of trees where water can collect. Cities across Canada and the United States are increasingly embracing these little green refuges, letting wild flora and fauna thrive rather than chasing a sterile urban look. Municipalities are joining the movement, with many North American cities supporting less pesticide use and more natural recovery of plant and animal life in every corner of the urban landscape.
The decision not to sanitize every inch of these spaces has sparked debate. To some, dirt and weeds seem unattractive; to biologists and conservation groups, they are signs of an increasingly humane urban environment. During the pandemic, with fewer pedestrians in streets, flower beds, tree roots, and even sidewalks experienced a vegetative surge, driven by wind-dispersed seeds and birds. This hinted at a rapid rebound of insects and other small creatures that often go unseen.
Glyphosate and other carcinogenic products have largely disappeared. This shift benefits both the environment and public health. Cities such as Nantes, Paris, Ghent, and Amsterdam have led the way by letting growth occur naturally under trees, avoiding chemical or ecological sprays and refraining from uprooting what grows in tree pits. It’s an approach gaining momentum.
Girona and Valencia
In Girona, last year the municipal team chose to remove weeds selectively—only when they hinder mobility, obscure road visibility, or threaten historical heritage. “Plants grow; they are not weeds,” asserted the sustainability department. In Valencia, a 2018 urban ecology initiative embraced a policy of naturalization, with the program growing to support tree pits and promote resilient, low-water landscaping.
The aim is to reclaim thousands of empty tree wells with species chosen for sustainability and drought resistance, integrating them into broader municipal landscape plans. This shift is accompanied by a broader environmental sensitivity, including expanding new planting areas in cities like Barcelona and Las Palmas.
Landscape designers often accept that large specimen trees such as banana, mulberry, or palm trees are better placed where space allows for healthy root growth. Beyond aesthetics, trees and the vegetation under their shade contribute to climate resilience by trapping dust, absorbing carbon dioxide, and improving humidity and oxygen levels in the air.
These green canopies also temper heat during hot summers and sustain insect and bird life. Some experts warn that very large trees or those with shallow root systems can strain sidewalks and curbs, potentially causing damage if space is too confined.
know and protect
The Spanish Society of Ornithology (SEO/BirdLife) highlights that urban environments host a wealth of natural resources—birds, bats, invertebrates, and plant life—that deserve attention, protection, and support. Urban biodiversity strengthens ecosystems, contributing to climate resilience, pest management, recreation, and overall well-being for city residents.
The global biodiversity crisis has reduced familiar city species such as sparrows and swallows, underscoring the important role towns can play in their preservation. Protecting urban biodiversity also helps mitigate flood risk by using nature-based solutions in planning. This means structural pits that allow root growth and temporary runoff storage with porous materials.
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REPORT
Anna Valentín, urban biodiversity expert at SEO/BirdLife
“So-called ‘weeds’ are decisive for other species”
Not long ago, tree wells were expected to be free of weeds, treated with harsh chemicals, and kept bug-free. Today, more life is encouraged in urban areas.
The range of actions to renature cities includes rational pruning, plant diversification, and encouraging natural succession in tree pits and parks. Pollinators and other invertebrates are prioritized, with nesting structures introduced to support urban wildlife.
The pandemic revealed nature’s resilience, bringing a surge of plant and animal life in gardens and tree wells. As vegetation management shifted away from intensive control, many species that had been labeled “weeds” proved essential to urban ecosystems. Plants in tree wells contribute to structural diversity, attracting invertebrates that become food for young birds such as sparrows.
What could renaturation offer to city residents? Urban biodiversity provides services that improve quality of life—cooler microclimates, cleaner air, and carbon sequestration. It reminds people that a city’s health is tied to its living green fabric. A plant is needed for a bird, and a diverse vegetation layer supports a wide array of urban wildlife.
If there are no insects in cities, there are no birds, right? The idea is simple: no plant diversity, no insect life, no birds. Wild herbs, once pejoratively called weeds, are instead valuable urban flora. In the past, industrial agriculture cast them as pests; now they are recognized as important contributors to biodiversity in parks, gardens, ponds, and beyond.
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The environment department encourages ongoing awareness and observation of urban biodiversity, aiming to foster healthier, more resilient cities.