In the garden: How trees can make our city a green haven

THE 26th annual Garden and Landscape Designers Association (GLDA) seminar, which was live streamed at the end of last month, highlighted the importance of trees in our environments, with a particular emphasis from some of the speakers on the urban environment.

Trees can soften buildings and hard landscaping and also help to mitigate the effects of air pollution by taking in carbon dioxide and releasing oxygen during photosynthesis.

The theme for the day was ‘Plan Trees, Plant Trees, Planet Trees’ and there were four key speakers: Henrick Sjoman of Sweden, Gerald Mills of UCD, Thijs Dolders, of Holland, and Charlotte Harris from the UK.

Trees were central to discussions, with Henrick looking at the value of large and healthy trees in an urban environment and how they can filter the air, produce oxygen and create valuable habitats. He emphasized the importance of selection when deciding what to plant where.

Interestingly, in a planned setting like a car park or a city street, if the trees chosen fail, then 25-40% of them will not be replaced, so we get one shot at choosing the right tree that will do well in what are sometimes difficult planting positions with lots of hard landscaping around them. It makes the species selection all the more important.

Henrick believes there is limited diversity among urban trees and we would benefit from learning more about less traditional trees that could be suitable in urban environments.

Three to four species make up half of the population of street trees, with Tilia (Lime) featuring high on the list of popularity. Parks in urban environments tend to have more diverse tree populations as growing conditions are more favourable.

Gerald Mills spoke about how trees can be part of the solution to greener cities, and compensate for the sometimes harsh urban effects. Green walls, paving, channels and roofs can all be part of a designed, engineered plan to green cities and make them better places to live, work and visit.

Trees provide shade, shelter, cooling, improved air quality, can capture and store atmospheric carbon dioxide, intercept rainfall and help control erosion. 

He highlighted the need for staggered planting to ensure continuous cover as they mature at different stages. 

He also spoke about a project he is involved with called Mapping Green Dublin, which aims to map all of the trees in the Greater Dublin area.

Thijs Dolders discussed nativeness as an important factor for designers selecting trees, and how a lot of native trees may not necessarily feel at home in an urban environment.

Selecting trees from other areas in Europe with similar environmental growing conditions will help expand our options going forward. Growing monocultures of one species in an avenue of trees, for example, may look very good, but it can result in them all disappearing at the same time as they reach over-maturity, or maybe a pest or disease will seize an opportunity in the course of their lifetimes. Thijs mentioned the Great Dixter biodiversity audit, which highlights the importance of gardens as diverse habitats.

Charlotte Harris, the last speaker on the day, is a landscape architect in the UK. She spoke about creating an urban forest on a rooftop project with over 35% canopy cover and the process of selecting the species for this project. 

She also spoke of the concept of ‘trauma informed design’ with reference to a project she worked on in Southampton, and how the birch tree is symbolically seen as a tree of hope and a large part of the design centred around this.

Finally, she spoke about the RHS Bridgewater project, which was recently completed in the north- west of England. Harris Bugg Studio won the competition for the design of the walled garden and drew inspiration from the route of the Bridgewater canal and the historic field hedgerow system in the area. About a third of the two acre space is occupied with a forest garden, one third an apothcary garden, and one third raised beds growing vegetables and fruit.

The 4m walls at the perimeter have been used to plant and train fruit trees into different shapes and patterns on the wall. A really interesting project, it looks like it would be worth a visit.

An inspiring seminar from the GLDA, and timely as we head for the end of March and as National Tree Week draws to a close.

Consider planting a tree this spring, the window to do so is drawing to a close and, as the old Chinese proverb goes: The best time to plant a tree was 20 years ago, the second best time is now. Happy Tree planting!