A Tree’s roots serve two important functions: They anchor the tree in the soil so that its photosynthesizing parts (the leaves) are elevated and exposed to the sun; and the absorbing roots also take up the water and minerals that the tree uses to make carbohydrates for food and proteins for everything else. (Roots also must absorb oxygen, necessary for their own respiration and growth.)
Some trees have an elongated taproot where the root grows straight down to draw water and minerals from deeper soils. A tap root is better at holding the tree in place in windy locations or in loose sandy soils, too. Other trees have a spreading network of fibrous roots that absorb water and minerals from near the surface. A diffuse, fibrous root system occupies a large volume of soil close to the surface, and helps to control erosion. Either way, microscopic root hairs fan out from the principal roots, greatly increasing the surface area available for absorption. It is these minute root hairs (in combination with symbiotic fungi) that do the actual absorbing of mineral salts and water.
Diffuse shallow roots capture water and minerals near the surface, whereas taproots are better at finding water when the topsoil is dry. Most trees develop a two layered root system with diffuse roots near the surface to absorb unpredictable rainfall, and a second, deeper layer for when dry conditions prevail at the surface. Roots do not grow towards anything. They simply continue to grow as long as they encounter adequate water, minerals, and oxygen.
Tree roots can be damaged by disturbance such as cultivating, scraping, and installing other plants above the tree’s root zone. Furthermore, roots cannot grow where the soil is highly compacted. In fact, soil compaction from vehicles, livestock, foot traffic, etc., kills more roots than just about anything else. When a portion of a tree’s root system dies, a similar portion of the foliage and branches will die. Gardeners, builders, and home owners need to know how far out a tree’s roots are from the trunk.
It turns out that canopy width and tree height are not good predictors of root spread. The best predictor is trunk diameter. One recent study found that the ratio for young, fast growing trees was 38:1. That is, a tree with a trunk diameter of 6 inches has roots spread out in a radius of 19 feet! Older, larger trees have lower ratios. Many arborists suggest a root protection zone of 12:1. That is, we should protect the root zone out for at least 12 inches for every one inch of trunk diameter.
It’s a good idea to encourage tree roots to grow deep, where they will have access to water during dry spells when we might not be available to supply supplemental irrigation. Absorbing water and nutrients is the most important function of tree roots. Infrequent deep watering (as opposed to frequent shallow watering) will discourage shallow rooting and favor the development of deeper roots.
Take care of your tree’s roots and you take care of your tree! Read more on tree roots and how to move a tree.