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Wind and Trees

wind and tree roots

So last time I was inspired by the horribly dramatic scenes of flooding across the country to write about how tree planting could help mitigate the risk. As I scan the weather forecasts for the coming weeks, I can see a worrying amount of little arrows and tight isobars. You don't need to know too much about meteorology to know that it's going to be windy!


As yes, trees are centre of attention again! Ok, so trees falling down because of strong winds can cause a lot of the problem but part of this is because they have been planted in the wrong place, or because they are getting old and rotten. And they say there's no such thing as bad PR!


In the tree world, we talk about 'wind-firmness' (I know, we love made-up words). There are two factors which help determine how wind-firm a tree is: the sail and the anchor! The sail is everything above ground, so essentially a tree is likely to be firmer if it drops its leaves during the stormy, winter months. In reality, this has only a light effect when compared to the effect of the anchor, which is the trees' root system.

There are 3 types of tree roots which affect how wind firm it is:

  • Flat Root. The roots grow fairly close to the surface which can be useful in stabilising ground and preventing soil erosion. However, it can make the tree susceptible to wind-rock where the wind gradually rocks the tree from side to side, compacting the soil and making it vulnerable to being blown over. Species includes Spruces, Birch and Poplars
  • Tap Root. There is one, big root which grows directly downwards and gives really good anchorage on deep soils. There are shorter roots growing out the sides which also help anchor the tree against strong winds. Examples include oak, many big broadleaves and pines
  • Heart Root. A mixture between the two, providing medium wind firmness. Examples include larches, firs and medium sized broadleaves such as ash and hornbeam.


Well, that's the theory of course. Tree root structure can be affected by the ground: iron pans and rocks can disrupt growth and steep slopes can prevent proper anchoring. Poor root structure can also be created by poor planting technique, but fortunately this is not normally a problem with our easy-plant root systems.

So you should avoid trees with flat root systems? Not necessarily! True, if you plant flat rooted species in the path of prevailing winds on exposed sites with long, narrow areas over which wind can pick up speed, then they might well fall over. But you can also protect those types of tree by planting more windfirm species in front of them. It is common practice amongst professional foresters to plant larch in front of sitka to help protect it against the wind (and for many other reasons). They can therefore plant sitka spruce trees in areas which might have otherwise been too windy, but take note that if the larch does blow down then the sitka tends to follow like dominoes.

Actual design of a woodland or windbreak strip is a subject for another posting or guide, but your starting point is to consider the type of trees you want. And remember that clumps of trees with strong tap roots have a much better chance than lone trees with flat roots.

20 January 2014 at 17:14 / Comment

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