Tree Care, Part 3: Caring for Mature Trees

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Mature trees are the gems of any landscape. They symbolize permanence and stability, two characteristics sorely missing in the world today. Planting a tree is an optimistic act evincing confidence in the future; protecting one demonstrates respect for the past; and appreciating one displays a joie de vivre.

Engelmann oak (Sept. 2002)

I have already discussed selecting and planting new trees, and training young trees. For those lucky enough to have beautiful mature trees, some of the practices that will promote continued good health are described in this post.

Down in the ground
Let us begin in the dirt, a place where I am very comfortable. Slithering through the soil are the roots of trees. Unseen but critically important, they serve several functions. Shallow surface roots take in water and minerals needed for plant growth. These and deep anchoring roots support the large structure above ground. Roots also store food reserves needed during slow-growth or dormant periods. Although the extent of the entire root system varies depending on the type and health of the tree, soil type, and water received (irrigation practices and natural water sources), roots usually do not extend beyond three feet in depth. Surface roots, providing most of the water and nutrients, extend far beyond the tree’s canopy, reaching a diameter two, three or more times that of the canopy. Damage to the root system, though not visible to the eye, often leads to the decline and death of these amazing organisms.

Most tree roots, excepting mangroves and the like, require good air circulation and water. A layer of mulch, especially from the tree’s own leaf litter and debris, on the ground under a tree’s canopy, is an excellent way to keep a tree healthy. The mulch moderates soil temperature, reduces water loss, and recycles nutrients into the soil as it slowly decomposes. Worms and other organisms living in and below the mulch create air pockets and facilitate good soil structure. Mulch also protects the soil from compaction due to foot traffic and heavy rains. Be careful to keep the base of the tree (the tree collar) clean and clear of mulch and debris.

When irrigating a mulched area, make sure the soil beneath the mulch gets wet. This may require watering for a longer period of time. One solution that is especially water efficient is to apply water beneath the layer of mulch with a soaker or drip hose. If you decide to do this, make sure you lay the hose so that there is an even and widespread distribution of water as far out from the canopy as you can go. You will be able to decrease the frequency of irrigation when the soil is mulched, likely resulting in overall water savings, regardless of how you irrigate. Check the soil moisture below the mulch before irrigating so that the area is not overly wet, a condition that can invite disease.

If you must have lawn, clear it away from the tree collar to avoid damage by mowers and weed-whackers, and to keep this sensitive area clear and dry. The problem with growing lawn beneath trees is that 1) it is a heavy feeder that draws nitrogen and other nutrients from soil; 2) lawn requires frequent shallow irrigation throughout the year, a practice particularly detrimental to our local native oaks; and 3) soil beneath turfgrass is usually compacted with little organic activity.

This crape myrtle growing in dense St. Augustine lawn has scars around the collar from encounters with weed-whackers.
Young oak tree with lawn growing around collar. The collar protector should have been removed and lawn carefully pulled out from around the tree. Notice the pillbugs and slugs that are enjoying the moist, protected area around the tree collar.

Mulch, soil or other construction debris piled on top of tree roots can quickly kill even large, mature trees. When left near the base or covering a significant area of a tree’s surface roots, decomposing mulch can get very hot and cook the roots. Soil or dense mulch can reduce air exchange near the surface, effectively suffocating the roots. Hot, decomposing mulch can also create conditions in which pathogenic root fungi thrive. Spread mulch or soil so that it is no more than a few inches thick – 2 to 5 inches is usually recommended.

Dense cover of spider plant is starving the roots of this avocado of water, nutrients and air.
Base or crown of this tree is clean and clear, but the spider plant is way too dense, its thick tuberous roots forming an impenetrable mat.
Milo works so hard! (2009)
I can hear the tree breathe a sigh of relief. I only hope I did it in time since avocados do get avocado root rot (Phytophthora cinnamomi). The rest of the lawn has since been removed, but this avocado had some serious damage during the windstorm. Only time will tell whether it recovers.

Change of grade or trenching due to construction can also be very harmful to mature trees. Before embarking on a project that requires digging or grading under or near the canopy of a specimen tree, recognize that this puts the tree at great risk. Ask yourself whether the project is worth losing the tree, since this is a real possibility. Maybe the trench can be routed around the tree, or the grade change can be mitigated in some other way. If you decide that work must take place near or under the canopy, hire a qualified arborist to advise you on the best way to safeguard the tree. Given the amount of time invested in a mature tree, it may be better to reconsider the project.

It is best to select trees that do not require ongoing soil modification, such as fertilizer. Mulch can do much to provide nutrients over time, especially leaf litter that falls from the tree. If, though, you have a mature tree that needs nutrients that are not present in your soil, even with organic mulch, supplemental fertilizer may improve its health and vigor. A qualified arborist should be consulted to determine what nutrients are needed and how best to apply them. Too much fertilizer can result in excessive growth, leading to long, spindly or overly dense cover increasing the tree’s susceptibility to wind damage. For native trees, additional fertilizer is usually unnecessary and can even be harmful. This is yet another reason that lawn beneath a tree can be problematic. If the lawn is being fertilized the tree may be getting too much nitrogen or other chemicals. Furthermore, some lawn food includes broad leaf weed killer that can damage a tree’s surface roots.

Add water 
Roots of trees, or any plants for that matter, generally grow where there is moisture and air. If a tree receives frequent, short periods of irrigation, its roots will usually grow in the top few inches (1-2 inches) right below the surface. Although most tree roots occur only a few feet below the surface (“…typically between 90 and 99 % of a tree’s total root length occurs in the upper 1 m of soil”, Peter Crow, Nov. 2005), a properly watered tree will have a deeper, healthier root system than one subjected to typical turfgrass irrigation practices. During the historic Santa Ana windstorm, many trees located in lawns toppled over. It is likely that they fell because their shallow roots were not well anchored in the overly wet surface soil.

Uprooted oak in Garfield Park. Magnificent specimen trees in Garfield Park have been dying for years, probably due to over watering (the grass is almost always moist, winter and summer), and lawn coverage right up to the base of these trees.

Trees with shallow surface roots are dependent on frequent watering since soil close to the surface dries out quickly, especially in our hot, dry weather. Not only will these trees lack deep, supportive roots, but they are susceptible to root rot since they are growing in warm, moist soil where pathogenic micro-organisms thrive. Native trees are not adapted to these conditions and often fail due to root fungi such as Armillaria and Phytophthora. Proper watering is essential for healthy trees. Mature native oaks, in general, should receive no summer irrigation.

Coast live oaks get watered on a hot day in September. Judging by how nice the grass is,
it appears that keeping it green is more important than keeping the trees alive.

Western sycamores, in contrast to oaks, are adapted to wetter areas and require irrigation until their roots reach a source of water below the surface, if one exists. Short, frequent periods of irrigation result in shallow roots that may never reach subsurface water. Sycamores planted in dry locations without subsurface water will likely require irrigation throughout their lives. All trees, native and non-native, should receive deep, thorough irrigation consistent with their growing needs.

Up in the air 
The worst thing that ever happened to trees was the invention and subsequent availability of inexpensive chain saws. This has led to some of the most atrocious tree pruning imaginable. One of the biggest surprises when we moved from the east coast to the west was the nonsensical pruning practices that we saw when we arrived. In general, trees are not butchered elsewhere like they are here. I have thought about this a lot, and cannot figure out why this is so. Do any of you have thoughts on this? Here are a few examples.


Most of the trees in the gallery above, except the one in the middle,  are examples of topping or heading back. When a limb is shortened, rather than cut back to its origin, the plant responds by sending out multiple branches from the cut. These branches are not well attached. Although many people have this work done in the hopes of preventing wind damage, in time this excess growth creates wind resistance (wind sail effect) leaving the tree more susceptible, not less, to wind damage.

Most liquidambars in my neighborhood have had the lead stem (the main central trunk) cut resulting in multiple leads high in the tree. I am not sure whether this is done because the owner feels the tree is too tall, or whether it is done to encourage a rounded crown. In either case, selecting the proper tree (one that has a desirable mature height and form) would be a much better solution.

Finally, much pruning is done to thin out branches, as shown in the bottom, center image. Rather than allowing the main branch to continue, it is cut, while secondary branches are left.  Again, the branches that are left are not strongly secured and are more likely to break in the future. Furthermore, additional thin stems often grow from the cut. Proper thinning cuts are barely apparent after they are made. In fact, one of the problems competent arborists face is that carefully pruned trees do not look like they have been pruned at all. The work is slow and pain-staking, yet the client feels like she has not gotten her money’s worth because the tree doesn’t look very different. Some arborists leave the cut material so that their clients can see how much was removed.

Pruning mature trees is a job best left to certified, professional arborists. Not only is it a very dangerous job, but the health of the tree and future safety are at stake. Check credentials to ensure that you hire a trained arborist. This is not the time to go for the low bid. Get a qualified person!

It is very difficult to know who is qualified. Certification helps, but business often requires giving the clients what they want, and so even certified arborists may not follow good tree care practices. The best thing you can do is to educate yourself on what these practices are. The International Society of Arboriculture’s (ISA)  excellent website, Trees Are Good, is a great place to start. I will try to give you a few tips here, but I strongly recommend that you check out the above website or have a look at the book, The Pruning Book by Lee Reich.

The four d’s
An arborist friend of mine uses the four d’s to determine which branches to cut when doing maintenance pruning. Limbs that are dead, dangerous, diseased, or deranged all require attention.

An excess of dead branches not only poses a hazard to the public, it may also endanger the tree since branches may break during inclement weather, tearing into healthy, live tissue.

Dangerous branches are those that, though not dead or diseased, pose a hazard to the public by their location. For example a branch that sticks out at eye level, though healthy, may prove highly dangerous to an unsuspecting or pre-occupied pedestrian. One that blocks a bicycle lane may require cyclists to dangerously enter traffic. Brittle and heavy limbs leaning over a house are also candidates for pruning.

The careful removal of diseased limbs, with proper cleaning of tools between each cut, can mitigate disease. Although trees can encapsulate damaged or diseased areas, this sometimes leads to structural weakness that becomes apparent during extreme weather, like the Santa Ana windstorm. Removing diseased branches before they get too large may not only help the tree, but also reduce structural damage.

Finally, “deranged” limbs are those that grow straight up, crossed, crowded, or in some other weird way that again endangers the tree or public safety. This fourth d is the one that is most important for young trees. Selecting scaffolding branches, removing crossed branches, and ensuring that there is good spacing between branches for future growth should be done as part of normal early tree care. Correcting the 4 d’s, not only results in a safer and healthier tree, but a better looking one as well.

The best way to prevent disease is to provide excellent care for your tree. Proper watering, maintenance of clean healthy soil throughout the root area with no mechanical disturbance, and good pruning practices, meaning only prune when required, will lessen the chances of disease. Still, climate change, air pollution, and the spread of some non-native (and native) pests can lead to disease. And even in the best of all worlds, disease happens. Often, for specimen trees, by the time you notice the problem, it is too late. A qualified arborist should be able to advise you on what to do next. No one likes to hear that the tree will probably die, but sometimes this is what will happen. It is a good idea to get professional advice from more than one source if a mature specimen tree appears to be in decline.

And the moral of the story is…
The winds on the night of November 30 – December 1, 2012 were very unusual. Certainly some of the tree damage can only be attributed to this freak storm. Some trees are naturally brittle, others are shallow-rooted regardless of the irrigation regime, and some of the older trees had structural weaknesses mostly due to their age. In spite of this, this wind event brought to our attention the value our mature trees, and the need to give them the best care that we can. This means using reliable sources to educate ourselves on the best accepted tree care practices. Whether consulting a landscaper or the internet, it is extremely important to use good sources. If you lost a beautiful tree in the storm, plant a new one now and take good care of it as a gift to those who come later.

Engelmann oak street tree in Pasadena, California.

– Colorado State Extension, Healthy Roots and Healthy Trees.
– Univ. of Arizonia Coop. Extension, Watering Trees and Shrubs.
– International Society of Arboriculture, Trees are Good – Tree Owner’s Information
– Forestry Commission, The Influences of Soils and Species on Tree Root Depth, by Peter Crow.
– US Forest Service, Tree Owner’s Manual
– University of California, Master Gardener Program,

California Master Gardener Handbook, Dennis R. Pittenger, Ed. 2002. Berkeley, UC California.

Planting and Care of Landscape Trees, Part 1, Pam Bone (90 min. video)

Planting and Care of Landscape Trees, Part 2,  Pam Bone (90 min. video)


Trees or Grass

Cleaning Up After the Storm

The Wind Event

Save Our Trees

What I Learned About Oaks

I’m Not an Arborist…

Tree Care, Part 1: Selecting and Planting Trees for Long Term Success

Tree Care, Part 2: Training Young Trees

Tree Care, Part 3: Caring for Mature Trees

4 thoughts on “Tree Care, Part 3: Caring for Mature Trees

  1. Barbara, thank you for your comprehensive series of posts about planting and caring for trees. If only I had received this information 30 years ago! Still, the replacement trees I have planted in the past few years can, as you said, be a gift for whomever comes after me. <br /><br />Have a lot of thoughts about why trees do not get the respect they deserve in Southern California… one day, we&#

  2. Great and much needed series of posts. A book?

  3. Thank you, both. Emily, a book indeed. Any suggestions?

  4. Janis – I&#39;d love to hear your thoughts on pruning crazies in So Cal.

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