Could Verge Gardens be the Leaders in Cooling our Streets?

Street trees are an obvious way to reduce urban heat.

I've written about my street tree and how the native verge garden around it increased biodiversity and helped the young tree thrive. And I measured the ground temperatures to show the cooling effects of the shade from this tree and its larger neighbour.

But some recent conversations have made me reconsider my focus.

Getting down to ground level

Some ecologists in South Australia have been doing fascinating experiments on whether verge gardens can provide faster and cheaper ways of cooling than trees. They measure the effects of small plants shading the soil at and below ground level.

The team at Succession Ecology, led by Glenn Christie, are passionate about using natural processes to create, restore, build and manage ecosystems.

While their business is about large-scale regeneration of degraded sites, they experiment on their verges. 

Glenn Christie calls his verge garden, "The Sponge". In this post, he describes their method:

(regardless of whether it’s 30m2 or 3,000,000m2), there’s no irrigation, no fertiliser, soil amendments or mulch; just the rain gods.

and the ability of The Sponge to absorb rainwater and retain moisture compared to an untouched verge:

Both street verges had the same compacted dolomite overlaying builders rubble only 17 months ago.

The difference is what The Sponge enables. The Sponge is comprised of rapid growing, perennial groundcovers; they feed & protect legions of soil critters that are feeding both above & below ground.

Together, these groundcover plants & critters make soil; the roots of these plants break up the soil compaction; which allows for deeper root systems & subsequently, healthier plants; therefore, more food for critters; “lo & behold” we have a positive feedback loop; a 40 cm (plus) deep soil in only 17 months…

On my street verge.

In this post, he measures the temperatures of the various options:

the best/coolest outcome was from a “double shading”; the Sponge providing shade (& transpiration) on the ground in combination with the shade & transpiration provided by a Council planted tree.

Here’s the catch though…

The Sponge was able to provide its full shade of the bare ground in 6 month versus the street tree taking 20/30 years.

Do we have the luxury of that time?

Later in this meandering conversation , Peri Coleman (Delta Environmental Consulting) added an extra dimension by describing the benefits to the trees provided by these low-growing gardens:

cooling needs more than tree canopy, as Glenn’s site has demonstrated. The reason is that street trees in particular tend to be lollipop shaped - lots of canopy up top and a single stem below. This allows hot shrivelling winds to blow across the area below the canopy. This not only dries out the soil, it can actually stress the trees - eucalypts will shed bark to control their trunk temperatures, for example. Having multiple lower canopies that reach to the ground level, even if they are spatially interrupted on the horizontal plane, breaks up the wind fetch. The benefits are that the lower canopies’ transpiration adds to the cooling effect which benefits the trees as well as the soil denizens underneath. The gentling of the wind has big benefits for insects like butterflies, famous for sunning themselves in sheltered sunny “dells” - in European terms, think of an open annual flower bed surrounded by rosemary hedges.
Here in Australia I can’t praise enough the “Woody Meadows” approach (Vic) to getting a similar effect using native plants and a particularly interesting form of management that is very low cost.

What does this mean for cooling our cities?

My conclusion is that we've underestimated the role of verge gardens in cooling our cities.

Instead of supporting the more important street trees, maybe the low-growing verge gardens are the trailblazers - cooling the area while preparing both people and the soil for the street trees.

Of course, councils must continue to work to increase their tree canopy. Trees are long-term commitments that will pay off in decades to come. But we don't have that luxury of time to wait for trees to grow.

Planting verge gardens in front of our homes is the easiest place to start: the land is available right now with relatively simple governance provided by council policies. Every one of us can start now - planting alone, as part of community group projects, and in collaborative projects.

This conversion of verges and nature strips by residents will give us the rapid, scalable, and affordable action we need for cooling as well as increasing biodiversity, preparing the soil for the street trees, and a host of other social and health benefits.

Longer term, showing the benefits, building relationships and learning to collaborate with other people and organisations, also prepares the ground (pun intended) for finding innovative solutions for improving more complicated public spaces like our typical two-tier tree and grass local parks.

Join the Conversation

Many thanks to Glenn Christie (Succession Ecology) and Peri Coleman(Delta Environmental Consulting) for their words quoted above and the conversations that fostered them. It is through conversations with people from diverse backgrounds, sectors, and disciplines that new ideas emerge. We can do so much more together in loose networks than any one sector, discipline, or organisation can do alone.
 
The free course, Verge Garden Basics – Understanding the Space will help you understand the social aspects of verge gardens, the reasoning behind the rules, and how to avoid the risk of disputes. 

I look forward to hearing about your ideas and initiatives to make our cities and suburbs greener, cooler, and more sustainable.