Q.Mechanism Of ‘walking’ Trees And Hedges
I have an long east-west garden, with several varieties of hedges on that axis. I notice that most annual growth is on the south side of these hedges; what’s more, in the fight-to-the-light, entire plants ‘walk’ to the south, seemingly roots and all — always to the south. My background is civil engineering more than horticulture, and explain it to myself thus: When first planted, these hedges were small saplings with roots no more than wispy tendrils. After 20 years, I imagine root systems grown into substantial solid wood. All this new wood must occupy space. I imagine it displacing the soil: the soil itself must move. I’m guessing the volume of new material for each root system may be of the order of one to two cubic feet. Where you have 40 birch forming a hedge, expanding roots would act as a very wide bulldozer blade, pushing soil out of the way on a wide front. I’m now thinking this effect must combine with the astonishing drive of each plant to the light. This is what I am observing: ‘walking trees’.. Perhaps it is to do with the centre-of-gravity of each plant driving always southwards – root system moving to stay directly below that centroid – this combining with the ‘bulldozer’ effect…(?) Soil in my garden has a lot of clay in it. It also has a high water table in winter with occasional flooding from the local river. In summer the water table drops, probably below root-zone. As an engineer I know all about swelling and shrinking of clay in such conditions. Clay particles are dipoles; water molecules are also dipoles: I know about the enormous forces exerted by swelling clay. I see these effects combining thus: pressure on clay from root expansion builds up in summer. In winter, the clay is softened and lubricated by water. So, summer pressure is ‘released’, but the alternative swelling-forces take effect. On my east-west axis, the resultant vector of all these effects is always south. How am I doing.. Here is a final measurement. I have a huge and ancient apple trees just half a meter from the line-of-birch, and north of that line. This old boy is massive. His roots will be massive and set, filling most of the ‘root-layer’. He is not going to budge — not for newcomer birch trees. This means that the all my likely drivers must be to the south, away from old apple. Measurement shows that birch close to the apple ‘walk’ up to twice the distance of those further away. There is a distinct bulge in the birch line, around the apple. Here, over 20 seasons, this bit of hedge has been pushed south, (and pulled south by the light..) and the total ‘travel’ amounts to nearly half a metre! I have lately become aware of this movement all around my garden. I have a section of old box-hedge, also seems to have walked. This came to my attention when my neighbour bought a bigger car, and found she could not get it down her driveway, not withstanding all her efforts with a trimmer…. I would be glad to have feedback on my theories, and accepted expert opinion.
Certified GKH Gardening Expert
Most plants will grow toward the light. Trees and shrubs reproduce by underground suckers or by dropping seed. Birch trees do both. I suspect your birch trees are sending up new shoots from the root and growing into trees, which make it appear the hedge is "walking."
Box hedges need annual pruning to retain their shape or they will naturally continue to grow, unless you bought a variety that retains its size.
Terry, I think your observations are very plausible. I’d just describe the mechanism slightly differently from the tree literally being “pushed” south by its roots.
It is probably a combination of asymmetric growth towards the better light, root development, wind loading, and seasonal movement of your clay soil. With an east–west hedge, the stronger growth on the south side could gradually shift the centre of the plant and encourage a southward lean.
Your clay and high winter water table could make this even more noticeable. Repeated wetting and drying can cause clay to swell and shrink, while softened winter soil offers less resistance to movement.
The old apple tree is particularly interesting. Its established root system could create a relatively immovable zone, forcing the younger birches to develop around it. That could explain the bulge you have measured.
So I’d say “walking” is probably real, but caused by several small forces accumulating over many years rather than one bulldozer-like action from expanding roots.
Your measurement of nearly 0.5 m over 20 years is only about 25 mm per year, so surprisingly small annual movements could produce exactly the effect you're seeing.
