TO GUIDE RATHER THAN CONSTRICT
We are often asked, what root barrier should we use? Or should we use a root barrier at all? Or like earlier today… “The tree pit has been spec’ed with non-permeable root barrier around all four walls of the tree pit. Is this right?”. In this instance the answer was no, absolutely not, as there were only services that needed protecting on one side, with open ground to the other.
To understand whether a root barrier should be used or not, it is important to firstly consider a key feature of tree rooting systems. One common misconception with a tree is that what is above ground is reflected below, a bit like an egg-timer. In reality, a tree’s full shape is much more like that of a wine glass, with the bowl/canopy above ground, and an extended stabilising base, much like the diagram below.
In fact, for most UK tree species, 90% of the root growth is within the top 500mm of soil. It makes sense really, for as British Standards are keen to point out, any deeper than 300mm most topsoil will start to become anaerobic. So, the roots like plenty of uninhibited lateral growth to spread out.
Take for example an average 16m3 tree pit for a “Medium” class of tree, such as an Acer Campestre ‘Elsrijk’. The canopy after 25 years will be anything up to six metres in diameter, so from the earlier diagram, we can be pretty sure that the roots will want to extend further than 3 metres from the centre each way. How can they do this if their 4m x 4m tree pit has impenetrable root barrier around all four sides?
What we end up with in so many cases, is the rooting system becoming constricted, halting healthy growth; and the trees never reaching the full potential of the planners’ or tree Officer’s vision.
Of course, close proximity to buildings or footpaths, and service lines, especially water pipes, means that sometimes we need to gently deter, or guide the roots. This can be done by lining the tree pit or trench along the side where the paving or services lines are. You can also simply wrap the service line with root barrier. I mention water pipes in particular, and this includes sewerage pipes. If there should be even the tiniest crack or loose joint in the piping, this will enable warmth and moisture to seep into the surrounding soil. Tree roots are attracted to this source of moisture, and being sewer pipes, organic nutrients. The fine hair-like roots will work their way into the crack or loose joint and carry on growing inside the pipe. As the roots grow, the crack in the pipe widens, more moisture seeps out and we have an ever-increasing problem; until finally the pipe is so full of roots and trapped debris that it becomes completely blocked as we can see here in the picture.
Occasionally, the root growth will be so much that it actually splits the pipe completely, leading to a total collapse. On a sewerage pipe this is bad enough, but should it happen to a mains water line then you would have a serious flood to deal with, huge disruption, and a lot of expense to fix.
Compared with the cost of an overall tree planting project, and certainly any potential repair work, Root barrier is a negligible cost investment. Here at Green-tech we offer a range of products for most applications:
gt260 – which is a single-sided permeable barrier measuring 650 microns (0.65mm) thick. This allows water to pass through, but is impermeable to roots, making it an ideal all-round barrier.
gt325 – is a slightly heavier, but still very flexible 720 micron (0.72mm), fully impermeable barrier. The gt325 is ideal for protecting the newly planted tree roots from pollutants or acids that may be in the surrounding soil.
gt360 – is very similar to 260, except that it is coated on both sides, making it sturdier at 710 microns (0.7mm), and ideal for keeping two types of trees or shrubs apart should they need to be moved at a future date.
gt420 UV – is the heaviest of the standard root barriers at 780 microns (078mm) and also has an additional UV coating, so that it can be left partially above the surface. It is particularly effective against vigorous roots such as bamboo and reeds
These are just a few of the products, most having a 25-year guarantee, that form part of nearly any urban tree planting programme; where although we want to give the trees as much rooting volume as possible, to do this we may need to guide roots away from one area into another.