How to make your garden part of the ‘urban forest’

Gerald Mills has counted the trees in Dublin’s parks and streets. Now he wants to know what’s growing in our gardens

Here’s a little quiz for you: If you had to hazard a guess – just a very rough estimate – how many trees would you say are growing in our capital city, by which I mean the Dublin City Council area? And in terms of that same tree cover, how do you think Dublin compares to other European cities such as Lisbon, Helsinki or Copenhagen?

Finally, for a wheelbarrow load of extra bonus points, could you name three of Dublin’s most common non-native street tree species as well as three ways that the city’s arboreal population plays a significant role in mitigating climate change, urbanisation and pollution?

A man who knows the answers to all the above is Gerald Mills, an urban climate scientist and associate professor at University College Dublin's school of geography, with a particular passion for trees.

Twelve years ago, Mills began the process of "mapping" Dublin's tree population, starting with street trees growing within a sample area within the Royal Canal and the Grand Canal. Working alongside his then-colleague Tine Ningal, he visited and recorded more than 2,700 trees within this zone over the course of three years, to measure their species, maturity, height and canopy size, in order to assess their ability to capture and sequester carbon, filter air and noise pollution, combat urban flooding and provide shelter.


Intrigued by the data he gathered, Mills (working again as part of a small group) has since mapped all of Dublin’s trees, or what he calls the city’s “urban forest”, a herculean task commissioned by the Office of Public Works and supported by Dublin’s four local authorities.

Most recently Mills, a speaker at the Garden & Landscape Designers' Association (GLDA) 2022 seminar later this month, has worked with residents of Dublin 8, the arts organisation Common Ground, the artist Seoidín O'Sullivan and event facilitators Connect the Dots, to examine ways to develop what he calls "a community greening strategy" in the Kilmainham/Inchicore/Islandbridge area of the city.

Calling itself Mapping Green Dublin, led by UCD's school of geography and funded by the Environmental Protection Agency, one of the aims of this collaborative project is to help the capital's citizens become its most powerful agents of change.

“What we’ve learned is that the communities living in this sample area of the city were really pushing for positive change in terms of greening up their neighbourhood, but they were at a very unfair disadvantage in not having the relevant information to hand to argue their case. Which is where we were able to help in terms of forensically mapping 7,635 of its trees using very detailed aerial photography and light detection and ranging techniques.

“Once we had that information, we could very precisely calculate the positive role these trees play in the environment.”

When it comes to the city’s urban forest, his research shows that location matters, and that every neighbourhood is different in terms of its tree population, its environmental stresses, its communities’ needs and its access to green spaces. “It’s something,” he points out, “that became very apparent during the lockdowns when so many of us found ourselves denied that access because of the restrictions on travel.”

“Some parts of the city – for example Crumlin – have relatively few mature trees, while others – for example parts of the north inner city – are home to an aging tree population reaching the end of its natural life span.

“Likewise in some neighbourhoods you’ll find inspiring examples of new tree planting such as along Parnell Street, whereas in large parts of the docklands, you’d be hard put to find any trees at all. Similarly, some areas are very exposed to traffic pollution and noise pollution, others less so. And while Dublin does have some wonderful green spaces including Bull Island and Phoenix Park, these green spaces are unevenly scattered throughout the city.

“What this research makes very clear is that we now need to devise community greening strategies that are tailored to the specific, collective needs of each neighbourhood and that are led by the communities themselves, rather than what we have at the moment, which is this very fragmented approach, with lots of private individuals making individual decisions.

“Why, for example, can’t there be tax breaks for house-owners who decide to leave their gardens unpaved in nature-friendly ways, where the focus is on the planting or soft landscaping? After all, these kinds of private green spaces are contributing significantly to the natural wealth and health of the city, to its microclimate, to its biodiversity.

“Similarly, why do we continue to make yet more space in the city for bus, car and bicycle lanes at the expense of trees, which are such marvellous multitaskers in terms of their ability to improve the microclimate of the city and to help counter so many of the challenges of urbanisation? I’m all in favour of an efficient transport system for the city but not when it continues to eat away at these valuable public spaces to the point where there’s next to no public spaces left.”

Mapping the trees growing in the capital is one big part of the puzzle. Now Mills wants to enlist the help of Dublin’s gardeners to allow him to dig much deeper into that data by finding out more about the many thousands of trees growing in the city’s private gardens .

“We’ve learned a huge amount about the kinds of trees growing in Dublin’s parks and along its streets. Its giant London plane trees, for example which are such a significant part of the historic planting in older areas and which are particularly valuable for their resilience and their ability to tolerate urban pollution. Its mature limes, its Norway maples, its cork oaks.

“But we know surprising little about its garden trees, even though these comprise about 50 per cent of Dublin’s total tree cover. So at the moment we only have half the picture, which is where the city’s garden owners can make such a difference to our research.”

Mills agrees that almost certainly Dublin’s population of garden trees is far more diverse than its public plantings in terms of the sheer range of species. This is as much a reflection of the everchanging fashions in gardening and of most gardeners’ natural urge to seek out choice and the unusual as it is of the range of different growing environments the city offers, from the sheltered, leafy front gardens of Rathgar where Japanese maples and fig trees thrive, to the golden-flowered mimosa trees found in coastal gardens.

He also agrees that accurately recording this complex, species-diverse part of the city’s urban forest will be challenging, even for knowledgeable gardeners.

To help them, Mills recommends the Curio app, a digital tool which will assist garden owners with identification and care of their trees as well as allowing them to enter specific information about individual trees such as species type, health and dimensions into a database. “Any information that the city’s gardeners volunteer will be hugely helpful.”

If he himself had to sum up the present state of Dublin’s urban forest, how would he describe it? “I’d say that up until very recently it was very elderly but that’s changed for the better with the arrival of a significant young population of trees, which is great.”

But he also sounds a note of caution. “Important as they are, we need to be realistic about the role that Dublin city’s trees can play in terms of mitigating the problems of urbanisation. We’ll never, ever be able to grow enough trees in the city, for example, to completely solve the problem of Dublin’s emissions. But we can certainly plant enough to make a very positive difference to its environment and to the ways in which we interact with that environment.

See and

This Week in the Garden

Divide large clumps of established herbaceous perennials and either quickly replant or pot them on, making sure to water them well immediately after transplanting to encourage the quick establishment of a healthy, viable root system.

Many species of ornamental grasses can be treated in the very same way to give you lots of young, healthy plants to use in the garden or to give away to friends and family.

Protect the juicy buds of fruit bushes and fruit trees from bird damage by covering them with a layer of garden netting, making sure to keep it taut and secure and checking it regularly for any gaps or holes where birds might inadvertently get caught beneath it. Pigeons can also do damage to leafy brassica crops at this time of year so netting is also advisable.

Dates For Your Diary

Until the end of February, Snowdrop Month in Carlow, a month-long celebration of the county's snowdrop gardens at Altamont, Huntington Castle, Burtown and Shankill Castle. See for opening times.

Wednesday, February 16th (7.30pm-9pm) Creating Beautiful Borders, a Zoom talk by Jack Willgoss of UK-based specialist nursery Wildegoose Perennials on behalf of the RHSI, see

February 26th, 2022 GLDA Seminar, PLAN Trees, PLANT Trees, PLANET Trees; Creative Design with Trees in our Landscape, Streetscape and Gardens, online as a livestreamed event with a range of guest speakers, tickets €45-€70 at