This is how tree roots were once thought to grow. Ill.: Peter, researchgate.net
That certainly sounds like an easy question, doesn’t it? Unfortunately, there is no easy answer!
Back when I was a beginning gardener 50 years ago, it was currently said that tree roots spread out as far as the drip line (the outermost circumference of the tree’s canopy, from which water drips onto the ground). We were told that to fertilize trees, for example, you should apply the fertilizer out as far as the drip line.
Then more serious studies of tree root length were published. The new norm became 1 ½ times the diameter of the crown for fertilization purposes and that is still often the measure used. And it can be fairly close to the reality of the situation in heavy, clay-rich soils, but in light, sandy soils, the roots can easily extend out 3 or even 4 times farther than the diameter of the crown.
The truth is that tree roots are opportunistic. They’ll grow longer and farther when they have the opportunity to do so … and a need to do so. And individual roots can wander much farther, up to about 8 times wider than the drip line.
Note, too, that tree roots tend to spread widely, but not deeply. They are usually fairly shallow, remaining near the surface of the soil where oxygen, minerals and moisture are most readily available. As a result, most tree roots are found within 2 feet (60 cm) of the soil’s surface, only rarely to 6 feet (2 m). That becomes pretty clear when you see a tree that has blown over in a storm.
Here are some of the factors involved in root spread:
- Because of competition for resources, trees planted closely together will generally have a less extensive root system than open-grown trees.
- Root systems often grow wider when they are unable to grow downward, such when there is bedrock at a shallow depth or a high water table.
- Roots tend to be longer when conditions are favorable.
- Dry, compacted soils tend to produce fewer but thicker roots, some of which will grow to extreme lengths.
- Tree roots will grow towards a source of moisture and will avoid permanently dry soils. They’ll tend to avoid house foundations, for example, where conditions are usually dry, but are drawn like a magnet to leaky pipes and drainage ditches.
- Pruning trees will not result in roots shrinking according to the reduced spread of the canopy.
- Barriers (walls, rocks, buildings, root barriers, etc.) will limit root expansion.
👍 Rule of Thumb: For practical purposes, calculate a tree’s root system to be 1 ½ times the diameter of its crown at maturity.
I find the “1 ½ times the diameter of the crown” rule to be useful in gardening.
Usually I can keep tree roots out of my garden bed at that distance by only occasionally pruning or by installing a simple barrier in the ground, whereas within that distance, fighting tree roots may be a constant and probably losing battle.
If you’re planting a tree, it’s likewise a useful guide: find out the diameter the crown is supposed to attain at maturity, multiple by 1 ½, and consider that zone as belonging to the tree. Yes, you can grow a lawn there (at least, if there is good light penetration) and also permanent plantings (perennials, shrubs, etc.), but you wouldn’t want to place a vegetable garden there or any garden where you are regularly digging, hoeing or planting.
And it is wise to respect the 1 ½ times rule of thumb when planting a tree near a house or other building, both for aesthetic and practical reasons (that will avoid branches rubbing against the structure, for example). Here’s an article about tree root damage to buildings: When a Tree Grows Too Close to a House.
If you’re planning substantial digging near a valuable tree, though, I’d suggest calling in an arborist for an evaluation of the situation.
Tree roots: yes, they certainly do get around, but with a little careful planning, they needn’t cause gardeners any serious problems.