LARGE TREE, SMALL GARDEN. CAN THIS EVER BE SAFE?

The question of how close to your house to plant a tree is a complicated one. After a dry summer there is usually a rush of claims to insurance companies after subsidence occurs. The problems generally happen in areas where there is clay soil as tree roots can remove the water from the clay producing shrinkage. This shrinkage may result in movement of foundations and the walls which they support. Exact information is hard to give but it is possible to give a few generalisations.

I take my information from a textbook on the subject - "Tree Roots and Buildings" by Cutler and Richardson published by Longman Scientific and Technical. They work from reported problems and give the distance from buildings within which 50% of the incidents occur. If the incidents occur close to a building for a particular species this is considered to be a less dangerous tree to plant than one which gives problems at a greater distance from the house. So Willows, Poplars, Oaks and Elms can give problems at far greater distances than birches, hollies, apples and rowans.

It may be wise to consult your insurance company before you plant a large tree. Be aware however that they are likely to give a very conservative response and that our cities would be far poorer places with many less fine trees if insurance companies were the arbiters of all tree planting.

There is another way to look at this problem. In many countries street trees are planted into prepared planting pits along the sides of roads. These are effectively large sunken plantpots which allow space for tree roots to develop without interfering with buildings, drains or other services. If you are keen to plant a large tree into a space near to walls, buildings, or services it may be possible to create a pit. For large trees you would need a pit around 6ft x 6ft and 3ft deep. This would have thick walls of blockwork or, alternatively, welded steel. You would be making an enormous container with drainage holes to prevent it turning into a sump or underground pool. Drainage holes should be protected by mesh to prevent tree roots from escaping through them and defeating the purpose of the pit. It would be essential to ensure that the container can drain properly and so it is unlikely to work if the pit is dug into clay unless you provide an efficient drainage system from the pit through the clay and away.

In order to water the tree effectively and, almost as importantly, to allow air to the roots a perforated plastic tube around 50mm in diameter should be buried within the pit and to the depth of the pit but with its inlet just above soil level. These tubes are available from tree and landscaping suppliers but can be improvised from drainage pipe if necessary.

If made well this pit should allow a tree to thrive in a limited space without danger to its surroundings.