Tree Root Failures

Mike Davison, 3-22-24

Serrano Park Community enjoys about 1500-1600 trees creating wonderful canopies that cool the air and create shade. Many of our homeowners and guests are out with their families and dogs every day. That is why it disturbs most of us when we lose trees and especially large trees. We work with arborists and tree maintenance companies to try to maximize our canopy health while minimizing risk to property and people. We have been improving our general tree health and tree structures to successfully reduce large limb failures, but we cannot do a lot to reduce root failures. Root failures can be caused by a lot of things and our tree people are very good at catching these in time to prevent risk problems. However, we have had nearly all root failures due to rain saturated soil and high winds.

Trees are very good at adapting to their environment. They send out roots to find water and nutrients and to provide a good anchor for the environmental conditions they are experiencing. They grow compression wood and tensile wood as needed. They will lean toward better light conditions and strengthen the root structure accordingly. They do all this very well and our maintenance crews work hard at keeping them healthy and help correct any structural growth issues.

Normal growth environment

  • No wind – Nearly all of the time, we have very little wind. Out trees do not feel the conditions that encourage the buildup of tensile and compression wood or additional rooting needed to battle heavy wind.
  • Photropic lean – Many of our trees develop a lean to reach better sun exposure. They develop the rooting and improved wood support for this under the normal conditions that they experience.
  • Soil – The soil here is poor quality, usually dry and hard, containing mostly clay and sand. Water runs off and nutrients are only found in the upper layer. The tree roots grow to use the shallow water and nutrients and only need shallow anchoring roots to hold to the hard soil.
  • Little deep watering – By irrigating with sprinklers, much of the water on our slopes just runs off. Even the irrigation on flat areas is usually for shallow-rooted grass. We live in a desert where the deep watering of constant rain does not usually exist. The lack of deep water in the soil discourages deep root growth.

Problems occur when the tree’s environment changes abruptly

Changes in wind or soil wetness will create unusual conditions that the tree may not be able to endure. Both at the same time is especially troublesome.

  • Continuous rain – Constant moisture will soak and loosen the soil. As anyone digging holes in their flowerbeds can attest, letting some water soak in to the soil greatly softens it for easier digging, weeding and other garden practices. It does not take much moisture to soften a clay/sand composite, mostly it just takes time for the moisture to soak in. For trees that have grown to rely on hard soil for their footing, this creates a problem, in that the grip of the roots is greatly reduced. Trees with a lean may struggle fighting gravity, even without wind.
  • Strong wind gusts – The wind in a tall canopy creates a huge torque on the rot ball and trunk. Even with roots holding well, there may not be enough strength in the wood to handle the unusually large forces created. The force on the roots is especially large since there is a large lever and a fulcrum at the root ball. On a slope, the fulcrum moves to the root ball toe, increasing the lever length and tension forces in the roots. Compression wood in the root ball toe supported with soft soil increases the root tensions further. Changing winds create a wiggling tension in the roots similar to what we do when we pull a weed. Soften soil makes it easier to pull the roots loose or they may just break.
  • Drought – When we have long periods of drought and do not properly water our trees, we will get root die-back, the chance of root diseases, and other issues. This increases our problems when we finally get rain or wind.
  • Combined conditions – We will commonly get Santa Ana winds after spells of rain. If we had enough rain to soften the soil and high enough winds, the root balls can pull free more easily.

These trees on the Amapola slopes failed in February, 2017. The large root balls and long exposed roots indicate that this was due to a water saturated slope combined with high winds.

This is a large eucalyptus tree that failed in March of 2024 next to the clubhouse parking lot. These roots could not handle the constant rain and extreme winds. After several weeks of rains, we had some of the strongest winds in over 2 decades. Our poor trees were not able to cope with this.

This tree failed from the rain and wind plus being hit by the above falling tree. Note that the longer roots were snapped rather than pulled free of the soil. This may indicate that this soil was not loosened as much and may have held if not struck by the above tree.

This tree grouping also fell in March of 2024. This shows the very high sand content of this soil. Nearly all of the tree failures we had during these winds, were due to root failures with the root ball pulling free.

So, what can we do?

Unfortunately, not much.

Proper irrigation plays a role in creating deep roots and better growing conditions, thus less of an environment change when we get weeks of rain. Sprinkler irrigation does not work well for deep watering the trees and shrubs and actually encourages runoff and erosion. Drip line irrigation can deep water the drought resistant trees and shrubs optimally, encouraging deep root growth and reduced runoff. Unfortunately, the Serrano Park irrigation system is old and almost entirely sprinkler-based. The other problem is that extra maintenance is needed to use the same irrigation zone in different irrigation purposes; Occasionally deep water the trees while normally watering the shrubs. The Landscape Committee is working on a long-term plan utilizing rebates and incentives to correct this but it will take time and continued HOA Board support.

We ask that the community help reduce erosion and root damage by respecting the slopes and the plants.