The miracle world of tree props

What is a strut?

A strut or a prop is a static support system which is connected to the ground and used to stop part, or all, of the tree, from breaking off or falling over. I’m deliberately not talking about anchors in the tree which are not connected to the ground. Struts are made of many materials. This can be wood often including durable types of wood. They can also be made from steel which has been common since the 20^th century. They can also be supported with a steel cable which is anchored to the ground with a post. Characteristic of a strut is that it is static where a living tree is not. In all cases I’m talking about struts that are deliberately placed to strut part, or all, of a tree.

Iron strut Tilia cordata, Prague castle Czech republic, right picture shows trees reaction.

This kind of strut often stands on a ground plate, like a tile or manmade foundation. The challenge is how the strut is connected with the tree. There have been lots of experiments in the past, from steels straps to turning supporting plates, to pins through the supporting part. The world of the strut is the world of Willie Wortel, whose motto was ‘nothing ventured nothing gained’.

Left: Wooden strut supporting dead branch field Elm, Ulmus minor. Right: Iron pipes support Chinese red pine, Pinus tabuliformis in upright position, both Shenyang, China.

Functionality of a strut

Trees have the habit of letting branches fall off, or falling over with ‘roots and all’. Both change the shape of part, or all, of the tree which people usually don’t like to see. A risk related problem and nothing to do with the many supported examples of pleached trees discussed in my blogs. Take a Catalpa, a tree from the east of the United States. Usually found growing on river banks and after roughly 100 years in our climate zone the tree in its entirety starts to subside. Sometimes so gradually that its root ball mostly stays in the ground and turns with the tree. This tree is planted in our region and is then so stubborn that it wants to change position! We don’t want that so such a tree sometimes gets a strut. There are a number of examples to show that the tree can survive without being supported.

Resting Indian bean tree, Catalpa bignonioides, Soestdijk estate, Netherlands. Typical behaviour of this tree specie which proves to keep growing without props.

Then there are those trees which reach a veteran age and as part of their development lose structural branches. In fact with oaks for example this is all part of the process of becoming a veteran. In the past we haven’t always known how to deal with these branches and they have therefore been supported. Additionally there are cultivated trees which aren’t capable of holding their own branches once they reach a certain age. The best examples of this are probably weeping beech, Fagus sylvatica ‘Pendula’. A beautiful stationary tree with a near technically impossible branch structure. The bare, bowed backs of the structural branches are always sunburnt.  In reality, if these branches become too broad they can no longer be supported and break off. In these cases a strut is also used.

Rare cultivar of beech, Fagus sylvatica 'Tortuosa' in Bayeux, France. Supported by huge amount of cabling systems, by this the tree developed a canopy diameter of more than 40 metres.

The challenges of a strut

Lifespan and connection with the trunk, those are the challenges for a strut. A tree can go on endlessly, but all the materials we can use to strut them have a limited lifespan. Take wood, you can be happy if it lasts 20 years. Ok, take really durable tropical wood and make it 50 years but in the lifespan of trees that’s just a period of flu.

Linde, Tilia in Erdmannrode Germany with oak strut, the branch is formed and damaged by the V-shaped fork.  

You can make steel more durable and then it lasts quite a long time. But you do need to preserve steel if you want it to last more than 100 years, as it is subject to corrosion. So lifespan is a challenge, but the connection with the tree/trunk is probably the greatest challenge.

Leaning Chinese red pine, Pinus tabuliformis with steel strangling band around trunk. Also note small pipe size in relation to stem diameter. 

Lets talk first about the position of the strut. With branches and trunk you are restricted to certain points. Here the strut point in the ground and suitable position on the branch/trunk are crucial. Preferably one section is kept short to avoid movement in the strut, but the strut should also not form a tipping point giving the opposite effect. The position of the foundations is also restricted due to the roots of the tree. Essential roots (preferably no roots), should be removed to place a strut to save a tree. That would be counter productive. The location of the strut is not a science and should be customised where necessary and depends on gravity.

Kirchlinde Reelkirchen Germany, prop connected with pin through branch. 

Then the connection, a penetration has the advantage that the strut grows inextricably, but what also happens is that the strut part away from the connection does not have the strength to cope with the extra growth that later occurs. What happens is the tree keeps growing, exerting forces forwards and sideways on the strut which with time then constricts the tree. The strut is then restricted as it’s connected to the static ground. In short a strut connected to the tree can become misshapen, lose its function or even weaken the part of the tree it’s supposed to support.

Iron props, left on a four side splinted tree, right in the middle of a foothpath with significant strangling of a trunk. Both Hanoi Vietnam.

Straps around the trunk, steel strips or wooden posts often create pressure points that cause injury to the supported area. What you then get is that the tree will overgrow this. In the case of a band, it can lead to pinching and ultimately death of the supported part. Things can be ok, but often they aren’t. A static strut can also cause a branch to elongate because of the perceived support.

Creative iron prop Indian bean tree, Catalpa bignnnioides, letter h shape and swing in the municipality Heerlen, Netherlands. Right: hinged support with rubber inlay. 

Steel hinged bowls seem to work best, but they must have a rubber between the steel and the bark. But these do not guarantee lifelong functionality. Our shortcoming as humans is that we do not know the forces that the tree will experience as a result of the strut. As the years go by, we will increasingly recognise that functionality is changing.

Pine tree with limited leaning position and solid steel band around the trunk. Regulary checking the growth and widening the band is necessary buth often forgotten. 

Struts therefore need to be checked with a certain frequency, in my experience at least once every 5 years. Once you have installed a strut, the tree often cannot survive without it, because it has become dependent on the unnatural strut.

1000 year old linde, Rheinborn Germany. Central pole and steel props support ancient tree. In this case the cabling is not the problem but the static props damage the branches badly. 

I have seen a very prominent strut in German, a country that excels in supporting and anchoring trees. The 1000 year old lime tree in Reinborn, Germany, is an ancient veteran with a gigantic hollow trunk and a huge steel pole in the centre of the trunk. From that pole, as if it were a swing, all the heavy branches are anchored with a spaghetti of cables. A terrible solution in which we do not actually give the tree the opportunity to be a veteran tree and to have its crown reduced. This says a lot about our human behaviour towards trees. This strut, on the other hand, is extremely functional and has many fewer objections that I have listed above, so I immediately forget the aesthetic criticism.

Most creative support system by Monseigneur catalpa in Tegelen, Netherlands. Functionality can be discussed but the sculpture is fabulous!

Struts can be useful if no other tree-saving intervention, such as pruning, helps anymore and it is still worthwhile to maintain a tree. If you manage to develop a strut with a creative mind, it can also be very attractive.