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Green Up Campaign: Interview with Dr. Duncan Slater

Dr. Duncan Slater, senior lecturer in arboriculture at Myerscough College, tells us why he’s fully behind the Green Up Campaign and offers some key solutions to tree planting which would stop our supermarket car parks being full of dying trees.

Why do you feel like the Green Up campaign is needed?

Unfortunately, these high public-use areas, which are owned by commercial enterprises, are very often neglected after the initial installation of the landscaping scheme. They often lie at the heart of a town centre, or are key to how a retail area look for visitors and yet they are highly under-invested in and often poorly installed – especially the provision of good quality soil volumes in which trees and shrubs can flourish.

Research identifies that greener spaces around retail units results in higher footfall and more spending (K L Wolf, 2004). So, it’s a good investment for retail centres and supermarkets to ensure good, sustainable green cover outside their stores. As well as the aesthetics, large trees of good health in car park areas help to reduce air and surface temperatures, which, with acres of tarmac on very hot days, can be highly beneficial to the health of all visitors and staff. Such opportunities should not be lost but so often are.

Have you got any exemplary examples of supermarket car parks ‘done right’?

Sorbus outside Booths Garstang 2013-2020

Unfortunately, I would struggle somewhat with this. My nearest supermarket with reasonable tree and shrub cover is probably the larger ASDA store in Preston – where a set of lime trees (Tilia cordata) have been planted at the periphery of the car park site and have established reasonably well. However, they are still quite dwarfed by the lack of soil volume in the strips in which they are planted. All other supermarkets have very poor tree provision, except on the outer-most of their landscaping strips: none have good trees WITHIN the car parks. And that amounts to over twenty supermarkets that are local to me. Shocking, but true…

Why should supermarkets make their carparks greener?

There are several points to consider here. Two points are made above – that it is likely to improve customer experience and that it will also lower ambient temperatures for the car park users on hot days. This latter point we need to take seriously in the light of continuing climate change and very high temperatures now occurring in the UK.

Car park tree planting gone wrong 09 01

As well as the benefits of tree shade in the car park, well-established maturing trees can also lessen the risk of surface flooding – again, a problem that is becoming more frequent due to climate change. Private land owners, holding land of this size, should be looking to mitigate their water run-off, to limit the risks of flash-flooding – and growing trees well on these sites is part of the solution.

I would add two other important points: that the planning permissions of these developments were granted, in part, due to a satisfactory landscaping scheme and yet so few are delivered well. Really, due to this, it ought to be a legal requirement that the supermarkets supply the landscaping that they promised, rather than do the initial installation (often poorly) and then not achieve what the landscaping proposals said would be achieved. Living up to promises is important.

There are many other benefits – but I would emphasise these four in particular.

What innovative solutions are out there?

The fundamental issue is that, in and around the car parks that are constructed, the quantity of soil and the quality of soil provided for the landscaping scheme is typically inadequate to support tree growth in the long-term. Many car park trees are planted in less than one cubic metre of soil – and that is compacted and full of builders’ waste.

Car park trees – Walton-le-Dale 2019 05

To tackle this, there are essentially three key solutions:

  1. Set aside a generous area for landscaping right at the start of a development – fence it off and do not allow any vehicles to access the area or materials to be stored there. Preserve a large area of native soil, and then there is scope to establish large-growing trees. This is rarely done on supermarket sites – the whole site is “trashed” during the demolition/clearance/construction phases of these projects.
  2. Suspended pavement schemes (including plastic crate installations) that supply uncompacted and large soil volumes under the car park’s surface have proven to be very effective at improving the growth rates and health of trees planted in car parks. The short-term costs are quite high – but the long-term benefits of good tree growth will surely pay back for the investment. These schemes can also act as reservoirs for surface water run-off if integrated into a site’s SUD scheme (sustainable urban drainage).
  3. There has been considerable research into viable ‘tree soils’ that can be compacted to put a car park surface on top of them, but still allow for tree roots to develop in them. There are a number of these “tree soil compositions” now available and they have the added advantage of easier long-term management, compared with suspended pavement schemes (as they can be dug up, if necessary, using standard equipment, without compromising the structural integrity of the installation). There are some really good examples of such installations in Copenhagen, Denmark – and elsewhere, although there is probably still scope for improving the mixes used – and finding mixes that work best for particular tree species.

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