Trees: Getting to the Root of the Problem

With winter coming, many people are worried about the stability of their trees.

Will they hold up to a strong wind storm? lf they fall, which direction are they likely to go? How can I tell if my trees are dangerous?

A professional arborist can visit your property and look for signs that a true is likely to topple.

Evidence of earlier root damage, roots pulling out of the ground, cracking in the soil around the root system all indicate a tree might be about to fall. Most problems can be traced to root-system injury. Trees with damaged roots might not show signs of damage for five, 10 or even 20 or more years after the injury.

Even trees that are attacked by insects, such as bark beetles, are often first weakened by root damage. Healthy trees with uninjured roots are better able to withstand wind stress, insects and fungus.

So how can you protect your trees from root damage?

Know your roots

First you have to know where the root system is in order to protect it. Because roots are difficult to see, few people understand how large a tree’s root system can be. Many people picture a deep, carrot-like tap root that supports the tree and provides water and nutrients. In our area, with typical heavy clay soils, even trees that do form tap roots often find the soil just too hard to push through.

Most of the roots are in the top few inches of the soil. This top layer is the most biologically active. Soil microorganisms actively break down old leaves and organic matter. The tree finds the most nutrients here. It is also a zone where there is more oxygen.As you dig deeper, the soil becomes more compacted, less air is available, and not many nutrients work their way down. For these reasons, most of the active feeding roots will be in the first 12 inches or so of the soil.

Root-spread is difficult to determine. Much depends on the environment in which the tree grows – the soil, the surroundings and other plants nearby. There is a balance between the leaves overhead and the roots underground. Roots provide water and nutrients for the leaves to convert with sunlight to food in the form of carbohydrates. A mature tree can use hundreds of gallons of water on a hot summer day.

A rule of thumb is: A tree’s roots will spread about 1 1/2 times the span of the branches. For a tree that spreads its branches 20 feet in one direction, you can expect the roots to spread 30 feet or more. If the roots are restricted in one direction by a building, foundation, road or driveway or other plants, the tree might adjust by spreading more roots farther in an unobstructed direction. As the tree grows taller, and the branches grow longer, the root system also must spread out.

Trees don’t like change

Established trees often respond poorly to changes in their environment. It takes decades for a root system to develop. Consider potential impacts before building in the root zone of a tree you want to save: Construction impact can take many years to be recognized. A tree that normally would have lived for hundreds of years might only survive 20 or 30 years following root damage.

Soil compaction by vehicles, foot traffic, or animals can crush the fine roots responsible for most of the tree’s nutrient and water uptake. Soil compaction also squeezes out the air and water the roots need to thrive. New roots have a much harder time growing through compacted soil, making it more difficult for the tree to recover from injury.

Cutting through the roots for utility lines or water lines can cause significant damage. Damage close to the tree is more serious than damage farther away. Root growth is similar to that of the branches – a thick root close to the trunk supports a large number of branching, fine roots farther from the tree. Also, cutting roots larger than about 1 inch in diameter can destroy a tree’s ability to gather water and soil nutrients.

Changes in grade can cause serious tree injury, also. Moving soil around can affect the way water flows underground. Equipment will cut through root systems. Piling soil on top of roots reduces the amount of air. It could take decades for the roots to grow toward the surface – if they survive.

Covering the root system with structures – even decks suspended above the soil surface on piers – changes the conditions surrounding the roots. Decks and other structures result in cooler, moister conditions longer in the summer, also allowing soil organisms to grow for longer periods and attack root systems. Also, nutrients provided by leaf drop can no longer reach the soil surface. Soil microorganisms that process organic matter do not get the sunlight they need to work efficiently. All these factors together can cause the established roots under the deck to slowly use up the nutrients stored in the soil and eventually die.

Avoidance is best plan

Avoiding damage to root systems is the best method for keeping trees healthy and standing in strong winds.

  • During construction, temporary fencing around the root zone – 1½ times the branch spread – will help.
  • Never pile soil on roots or around tree trunks.
  • If you need to run water or utility lines, tunneling under the root system might save the tree.
  • Remember: Most of the roots are in the top 12 inches.
  • Being careful during construction costs more, but saving a large tree might be worth the effort.
  • Preventing root-system damage helps keep trees standing in strong winds.

From the Calaveras Farm Advisor, 2002