Q: Is there a rule of thumb about how far a tree should be planted from the house to protect the foundation from tree roots? I've read 35 feet for a large tree in one book and 20 feet in another, but I guess it really depends on how big the tree will actually get.
Chris Morris
A: The size of the tree is only one factor. Also important is the type of soil, the kind of foundation under your house and the configuration of roots on your chosen tree.
Based on the size of the tree only, the recommended distance is said to range from about 40 feet for a forest giant to 12 feet for a well-disciplined tree such as a compact hawthorn.
But some trees are waterloving and will deliberately seek out house drains. Willow is one. Silver maple, poplars and cottonwoods are others. These create problems with septic fields as well as houses.
The layout of the tree roots also matters. Some trees have wide-spreading roots that are relatively shallow. These can vary from seven or eight feet deep to a few inches below the soil surface. But a few, walnuts for instance, have deep taproots. Oak trees produce large spreading roots that can extend to at least 80 feet.
The type of soil makes a huge difference. In dry spells, clay shrinks when foundation trees suck up the water it holds for their own growth. It swells again in rainy spells. This can make foundations unstable. Gravelly sand protects foundations somewhat better because it drains fast.
If you have a cellar-type basement where thick, concrete walls extend deep into the earth, you can take a few more risks with the trees you plant. But it's still not advisable because trees are enormously expensive to remove if the situation goes wrong-especially if large equipment can't get into your yard.
Ranch-style houses on slabs and other houses with shallow foundations can have the worst problems of all with tree roots.
I might mention that trying to contain trees in large pots then burying the pot in the ground in an attempt to keep the tree small is often doomed to failure. Tree roots can get through drainage holes and the nutrition they pick up strengthens the tree until it exerts tremendous pressure on the container. Many pots break.
Another point: sometimes a well-behaved tree becomes an agent for destruction. Animal pests have been known to use trees as a bridge to chew their way into people's attics (squirrels) or rip pieces off roofs to get inside (raccoons). This isn't a widespread problem, but I've met people who had these situations.
It's best to take a close look at these things and your risk tolerance before choosing a tree.
Anne Marrison is happy to answer garden questions. Send them amarrison@ shaw.ca.
