Manipulating Tree Roots With Stones.

Tree roots are nearly always now regarded as a malfeasance to development, to a point where an over riding assumption that tree roots will cause damage to property is dealt with, even from a ‘pro’ trees point of view, by a need for separation and the use of substantial buffer zones is now mandatory.

This is frustrating to landscape design and a brake on realising the potential profits gained from trees in the built environment. But it is a very much a modern problem, due to modern architecture and the materials used. How much has the now generic demarcation between industries, (including the education, research and science related to such industries) in the UK encouraged a presumption always in favour of the richer and larger construction industry with regards trees in the built environment?

The protectionism of expertise is all well and good until it isolates progress through joint research and discussion. A tree provides an amazingly versatile material, a significant element of design and added value as well as an ally towards real sustainable development in construction – and yet the arb / forestry world and the construction world remain largely at arm’s length from each other in the UK.

Tree roots were manipulated by our ancestors across the globe as an integral part of construction and to ensure longevity of buildings way beyond our limited expectations of the life of a building, certainly those post WWII. In Europe, in the most celebrated of rural landscapes, evidence surrounds abounds to this fact.

Whilst the rhizosphere of a tree is one of the least studied zones of the immediate natural environment, the patience of our forebears in testing and understanding methods of manipulating tree root growth to add cohesion to man made elements in the landscape should never have been so disregarded as it has been.

Modern understanding of the rhizosphere is severely constrained mainly due to the huge costs of the work needed to expose the area to be studied (making an archaeological dig look like a monster truck rally). And waiting until tree failure to be able to study such areas is limiting with regards actually understanding the subsurface interaction pre failure.  There has also for far too long been an overweight favour of separating the action of roots from the soil it sits in, and even in urban or technosols the two are so connected as to render any separation of study almost invalid.

A huge bank of knowledge which dispels many myths and therefore unnecessary concern with regards the action of tree roots on construction rests with the practitioners. And a very apparent action of tree roots of which there is much evidence in historic construction (Dry stone retaining walls, Banked and Cornish Hedges etc.,) is that gravitopism is heavily influenced by the quantity and position of stones in the soil.

Dissect an older Cornish hedge with trees growing on it and the primary root system does not penetrate straight down into the heart of the ‘backfill’ material in the centre of the construction, but fans out on either side to follow the inside edge of the stone surface.  This does not, as many assume quite wrongly, lead to failure of the hedge it does the opposite. The lateral or secondary roots envelop the stones, pulling them together. The roots themselves are regularly ‘air pruned’ creating a stable root system which increasingly strengthens the whole structure of the hedge.

The true ‘Cornish’ hedgers used a small layer of soil between each and every stone during construction to encourage this process. And in the South of the County many of these structures became extensions of the productive land by using cultivated wild plum varieties (including black and red Kea plums and the very rare crystal plum as well as of course the obligatory blackthorn ). There are many who believe that some hedge constructions were designed to fit the root systems of particular trees and given the fact that these structures were developed over incomprehensible timescales in comparison with modern construction and clearly the result of long established knowledge of using local materials then such a claim is far from ridiculous.

Free standing and retaining dry stone walls are also protected by the nearby presence of trees, either above the retaining wall (with the same process as per the Cornish hedge), or at the bottom of the wall, where gravitropism is replaced by a yet to be classified root growth; the roots growing upwards into the structure of the wall.

There are many other examples of the use of stones to manipulate roots for both human benefit and to ensure the longevity of a tree. In the alluvial areas of France the placing of large stones inside the bare roots of a tree at planting is standard practice, particularly as staking is ineffective here (as it is in many places). And there are examples from across the world where the use of ‘Vertidrains’ (nothing more than a hole backfilled with small clean stone) attract tree roots providing not just an anchor but a useful conduit of water, oxygen and nutrients flushed by surface water run off into the hole. This practice helps in mitigation of drought and times of excessive rainfall and contrary to popular belief the vertidrain actually provides a much more ready supply of water for the tree despite being free flowing. The secondary roots will infiltrate and multiply in such holes to the extent where after a year it is possible to pick up the stones / gravel used as one conglomerated piece.

The exasperation that tree roots (from an existing tree or newly established tree) will quickly discover the area where services are laid and or where the zone immediately adjacent to foundations or other construction is due to the trees innate ability to source convenient areas of water, as per the Vertidrain principle because the stones provide a ready and constant source, the resulting problems caused are further compounded by the inevitable consolidation that follows all groundwork, accelerating the damage caused. This leads to too many trees in the UK being felled for what they do instinctively – What if they had been provided with more attractive and bespoke ‘facilities’ in the first place?

In the UK the standard methodology of the moment is either too much or too little:

Planting pits are often treated as though a bed for a municipal display of annuals, stones removed, new compost etc., The resulting root system is loathe to leave such conditions in order to anchor and supply itself for a long life, it will establish itself quickly and when it needs more space to spread its roots the surrounding soil is no longer suitable for the now acclimatised to perfect conditions tree.

Many other new plantings take the ‘you’d be surprised how rough the conditions a tree will adapt to’ belief too far. So long as the obligatory ‘tubex’ is installed the needs and desires of root systems are simply ignored. Death by soil smearing* is a contributory factor for a huge percentage of young trees and stones, gravel or sand can ensure against this unnecessary death easily.

The over obsession by landscape and garden designers (as well as in the minds of the general populace) that the form of a tree takes precedence, leads to the often wrong assumption that a symmetrical and ‘perfect’ form is a sign of good health and potential longevity. It isn’t and unless we get things right below ground as best we can we will continue to see both trees felled for no reason and tree death simply due to ignoring the rhizosphere completely.


*The subject of my next blog.


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