Everyone understands that a tree cannot survive without roots, but few people understand how our actions can damage those roots.
Too often it is assumed that a tree root system shoots down into the soil in all directions making a kind of half-circle of wood spindles to hold the tree up. This could not be further from the truth. In reality, a tree root system is shaped much more like a plate than a ball, with the vast majority of roots in the top 10 to 12 inches of soil. This root structure is most clear when you see a tree thrown by wind and the large root plate can bee is seen like a disk protruding from the ground. Generally speaking the more compacted the soil and the drier the climate, the closer to the soil surface the roots will grow. This shallow rooting is why compacted soil is so detrimental to trees’ overall health. It’s easy to understand how parking a car, piling debris, or even walking on the soil surface could damage the tender roots just below the surface. The roots that feed the tree (fine feeder roots) only grow at the tip of the root and must push their way into new areas to find food and water. When soils are compacted this becomes impossible and limit the uptake of food and water. As trees get older they lose vigor, making it even harder to move through the soil.
With the recent mudslides, we now have many trees with literally tons of mud packed on top of the root systems. This will create an effect very similar to construction damage and should be dealt with in the same way. Below is a program to deal with trees affected by the mud. These practices will also help with more traditional compaction damage from any number of sources.
Safety First: The trees need to be assessed by an arborist (preferably one that is Tree Risk Assessment Qualification- TRAQ certified) to make sure it’s safe to save. Nobody wants to save a tree destined to fall into the living room.
Stop the Compaction: We need to understand what has caused the compacted soil and remove it. Sometimes this means fencing or signage to make sure vehicles and pedestrians don’t return to the sensitive areas. Sometimes it means removing the soil and debris deposited by storms or people. It’s a good idea to have a consulting arborist evaluate the damage if debris needs to be removed from the root zone so they can work with contractors to ensure no additional damage is done to the roots while digging.
Repair the Compacted Soil: Compacted soils can be repaired with a pneumatic spade that uses highly compressed air to break up soil without damaging roots. We have a process we developed at our lab called Root Invigoration that produces good results. During Root Invigoration, we can also incorporate biochar into the soil. This carbon-enriched additive increases beneficial soil microbial activity and improves water retention. Finally, we can make any adjustments to pH and make sure nutrient levels are correct for the tree.
Stay Ahead of the Pests: Most pests and diseases of trees and shrubs are attracted to plants under stress. Soil compaction is definitely a stressor! Borers and bark beetles are the most common issues after a tree becomes stressed, but root rot pathogens will also take advantage of the weakened defenses to try and get a foothold. Your arborist will know what insects and diseases are common to the plant and what steps are appropriate as every tree and every area are a bit different.
Monitor: When you have root damage there may be parts of the canopy that cannot recover. Only time will tell where those areas are, so keeping a close eye on the tree is a good idea. If you do have dead limbs they should be removed for the safety of the people who may frequent the location. As with all urban trees, nutrition can be a serious issue and should be monitored.
Compacted roots can be serious issue that is easy to overlook, especially in the midst of a large project or the disasters associated with heavy soil movement. This is one of many reasons to have a good arborist that you trust to help keep you safe and make the most of your landscape investment.