Protecting Trees During Drought

HomeProtecting Trees During Drought


Now is the time to prepare for drought in the landscape, with the future of northern California’s water supply in question some foresight and prevention can go a long way. Trees need ground water to conduct photosynthesis this is an obvious fact that we all learned in school but the reason I bring it up is that people forget in order for a tree to decline some part of the process of photosynthesis has to be interrupted.  Once interrupted, it is secondary or opportunistic insects and diseases that will grow in population, causing increased decline. This is a benefit in forests. Because weakened trees recycle to the earth and provide organic matter and nutrients to the strong trees surrounding.



All trees will be affected, but trees that will be most affected are trees that are in poor health or declining, trees that have been over pruned and trees that have been grown in shallow irrigation, such as in turf grass and in mixed planting beds. Roots need oxygen as well as water and since water has not been scarce for decades, many trees are in irrigation that is frequent and shallow, forcing roots to compete at the surface to exchange gasses and absorb water.  These same roots that are close to the surface will be the first to dry out as rainfall and irrigation are less available, causing root loss and the trees ability to absorb what little water is available.

The key to keeping your trees healthy is prevention and monitoring, and many of these suggestions are not only free but will also save you money and help the environment in the process.


What you can do?

  • Mulch – Maintain a 3 – 4” layer of mulch under all of your trees and shrubs. Did you know? The most expensive portion of most landscapes is your lawn! Maintaining lawns causes more pollution than any other part of your landscape! Wood mulch collects carbon!  Remove as much grass under trees as you can and replace with mulch. (See the benefits of mulch below)


  • Irrigation – Deep water once a week during the hottest times or when trees show signs of stress.  Water evaporates from the surface down, so if you deep water, it will remain in the soil for much longer than if you surface water.  Mulch will increase this water retention.  The best way to water deeply, is slowly with a soaker hose.  This way you can control the irrigation in the root zone and not waste it on areas not needing water.

    Soaker Hose Placement

  • Do not prune trees in declineor in poor health and in healthy trees do not over prune.  Removing too much leaf matter is always an unacceptable practice, but in times of drought, it could kill a tree.  It reduces the amount of food the tree so desperately needs and reduces the shade in its root zone, which will increase water evaporation.

    Over-pruned Tree

  • Monitor pest and disease – Pest and disease issues increase during times of drought.  Have one of our ISA certified arborist evaluate your trees and shrubs at the first signs of infestation.  They can make recommendations on treatments before the pest or disease makes any impact on the trees health. (See Pest and Disease information below)


  • Do not use any Nitrogen fertilizers, slow growth in the crown will compensate for lack of water.  In fact we are only fertilizing trees with mycorrhizae and bio-stimulants.  These are naturally occurring organisms that help strengthen the trees root system. That is good for the tree.  We can slow down the growth even more and increase the trees fibrous roots by applying a tree growth regulator.  For more information on that please have an arborist come out and look at the tree.
  • This should be obvious, but you never know.  DO NOT CUT ROOTS, or change grade over root zone.  If there is a construction job or renovation requiring that roots be cut, an ISA certified arborist should always be consulted.  We have a lot of experience in these types of situations.
  • If a tree looses all of its leaves, it doesn’t mean it is dead.  Don’t give up on it yet.  Many tree species do this intentionally to save its resources for a better day.  Like next spring.  Increase water frequency and call an arborist.  You can do a little test yourself by grabbing a twig or small branch.  If it is flexible, it is still alive, maybe only barely but it still has a chance.  If the twig snaps when you bend it.  Its dead, but you should still contact an arborist in this matter.

With these tips your trees should make it through a bad summer, but these tips are sound for tree management every year, especially the mulching and water management.  Also don’t plant any trees or shrubs; it is not a good time for establishment.


Further discussion:

Reducing Compition and Selecting the Right Tree for the Right Place – Trees grow most efficiently associated with similar wood types and beneficial organisms, they benefit from the strength that comes with combining root systems and the protection peripheral tree crowns provide in inclement weather, stands of trees help shade and cool root systems and the differences in nutrient uptake from tree to tree is regulated by sharing leaf litter as it returns to the soil. Nature has provided the model for trees to remain balanced with their environment, in our landscape we grow ornamental trees alone with dissimilar plants that compete for resources.More common in commercial settings or large landscapes declining trees use resources that would benefit the sustainable specimen trees, trees in a drought situation that are weak or in severe decline should be removed where it would benefit the surrounding trees, short term considerations during the window of draught like subterranean roots of dead trees can rob nitrogen from the soil during decomposition and of course declining trees will be more susceptible to secondary pest which can be introduced to healthy trees or at a minimum increase population of pests.

Remove trees that are beyond saving that compete for resources and do not provide their fair share, golf course trees, park trees, campus trees where oaks and maple are surrounded with mostly dead mistletoe ridden ash trees can benefit from helping speed up natural selection and condense resources to viable the long term trees. This is valuable in mixed stands where trees beyond saving share stands with viable healthy trees

Lawn – nothing can replace plush green lawn in high use areas or where the kids play but how much of it is visual because it’s the standard. Landscapes with vibrant flowering plants complimenting yellow and purple shrubs growing out of uniform wood mulch are just as aesthetically pleasing and require considerably less maintenance. The amount of chemicals required for preventing or eliminating weeds, combined with high nitrogen that has the potential for runoff and damage to aquatic ecology is not great stewardship of the land.  Less turf grass equals less maintenance and less water usage.



As arborists, our job is to create and maintain favorable environments for trees and shrubs.  While doing so we need to look toward the environment that has achieved perfection in finding the soils that are balanced in nutrient content and filled with beneficial organisms and rich in organic material so that the cycle will continue to produce and support our goals.

Only in nature, amongst the undisturbed cycles, can you find a model worth recreating where trees benefit from the loss of each leaf, twig or branch and works in partnership with fungi, insects and bacteria to ensure nothing goes to waste.  Recycled parts of trees, working with their associates, create layers of protective nutrients that return to the roots while resisting the onset of greedy and invasive weeds that would serve as competition.  As scientists we cannot ignore what has been proven to work, we can only hope to learn from our findings and do our best to duplicate it with as little compromise as possible.

Our landscapes, however, are filled with compromises that work against nature’s process.  We must ensure that our trees do not shed branches on the people that grow them, therefore we prune.  With this in mind, taking away each piece we remove, raking the valuable leaves and small twigs that the winds bring to us for use and then we plant grasses and or plants that absorb more than their share of what is left.

Our only offering back to the soils are processed barks and sterile water.  As the cycle loses its balance, nature does what it is supposed to do by taking the weak and returning it to the earth. The process is slow but constant.

Trees no longer have to live in spite of us. They can thrive because of the lessons we have learned based on the models we know to work.  By taking all parts of the tree and grinding it into a recreation of what nature intended, we can begin the process of getting back our balance.

Simply by adding natural complete mulch, nutrients will return to the soil while it is cooled in the summertime and kept warm in the winter.  Weeds will not go away entirely but will be much fewer in numbers and easier to maintain.  Beneficial organisms will call home where pathogens may have moved in.  This simulation of nature combined with good habits and respect for the soils and the roots that grow in them will minimize the need for pesticides and expensive treatments saving you time and money which you can spend enjoying your trees as nature intended.



  • Adds organic matter and humus to soil
  • Reduces drastic swings in soil temperature
  • Increases retention of soil waters
  • Is a food source for earth worms and soil organisms
  • Reduces weeds
  • Reduces erosion

Mulch by definition is a material that covers soil.  Most of the studies that have shown drawbacks of mulch are done with it incorporated INTO the soil. This is where people confuse it with compost.  Mulch is not compost, although you can compost organic mulch and mulch with compost.  I just wrote that to try to confuse you so stay with me.

Mulch can be made of both organic and inorganic products and should always be used as a top dressing.  There is much debate in our industry over the depths of which mulch should be applied.  Most resources say to mulch at depths between 3-4” whereas my mulch hero, Dr. James Downer, has shown increases in tree growth and vitality with a fresh 12” layer of eucalyptus mulch maintained on avocado crops.

Adds organic matter and humus to soil

A rule of science is that nothing is lost and nothing is gained……. kind of.  What this means is just as you receive the benefits of the nutrients in the things that you eat, so does the soil benefit from what it decomposes. Thus, nothing is lost.  However, nutrients can be lost through volatilization such as NO4 release in standing wood chips…..kind of.

To simplify, organic mulch will incorporate organic material (OM) into the soil and that OM will carry with it what it was built from.  If you were to chip a small fruit tree in its entirety and the leaves had Nitrogen (N), the fruit had Phosphorus (P), and the Xylem had Potassium (K) you would get those nutrients except for what converted to gas during decomposition.  With that in mind, if you look at bark under a microscope you will find that it is mostly dead cells and waxes.  When used as mulch the levels of N, P and K that might have been available in the rest of the tree might not be present or available in the bark.  Even though it is part of the tree, nothing is gained from its association once it is isolated.

Reduces drastic swings in soil temperature

Mulch reduces the soil’s heat index year round by protecting the soil from direct sun exposure in the spring and summer months and trapping radiant soil heat in the winter. The importance of moderate soil temperatures is largely dependent on the crop and the geography of it.  Some trees and their roots do well in higher temperatures, while studies have shown that soil temperature between 93-97° can stunt growth.  Most of the top few inches of soil are affected by sun exposure and mostly in dry soils.  It takes five times the amount of energy to warm water than it does an equal amount of dry mineral particles.

Increases retention of soil waters

Mulch improves soil tilth by incorporating OM while allowing greater holding capacity and improving percolation.  In doing so, soil water helps reduce increases in soil temperature.

Is a food source for earth worms and soil organisms

Earth worms are instrumental in the incorporation of decomposed organic matter into soils. They create tunnels that air and water can move through and help ease soil compaction and disperse OM.  There are many instances where soils would benefit from tilling or mass introduction of organics in places where hard working absorbing roots occupy the soil.  Since mechanical means of incorporating OM are often invasive, it is vital to get the biology of soil working for you.  Mulch creates an ideal environment; moist and organic with moderate temperatures.

Reduces weeds

Many claims have been made of mulch eliminating weeds; this just is not true.  Where there’s a weed, there’s a way.  However, the first rule in weed control should be to select the most effective practice that is not harmful to people or the environment.  Seeds need sunlight to germinate; a consistent layer of mulch can prevent sunlight from reaching the soil.

Reduces erosion

Mulch is used for erosion control for four reasons. The first is to create a “shield” that reduces the amount of soil splashing from forceful downfalls of rain.  Secondly, it prevents soil from slipping when surface water move quickly across it.  The more fine particles, the better the mulch will be at bonding with soil particles.  This bonding creates clumping and requires a larger force to disturb it.  Mulch creates greater infiltration rates allowing a larger holding capacity of water which reduces run off.  Lastly, it creates a better rooting media for absorbing and anchoring roots of plants and trees since roots are an important factor in soil stabilization.


“Don’t be a wimpy mulcher”- Dr. James Downer




Chewing and sucking insects Systemic insecticides can be applied and usually only require one application.  These insecticides localize in the leaf and/or xylem and protect the tree throught the growing season.  Alternative treatments should be applied were bees may pollinate such as fruit trees and other flowering trees.  There are organic dormant oils and summer oils as well as insecticidal soaps that can be applied.  These treatments will require more applications throughout the active season.

Broad spectrum sprays should not be applied because they kill beneficial insects as well. Also more than one application is required, so they are not financially or environmentally beneficial. If they must be done plan on repeating every thirty days throughout the active season.

Family: Aphididae

Aphid there are more than 1200 species of aphid which come in all sorts of shapes and color they are the most common of all pests, they cause damage by extracting sugars from leaves, in doing so the leaves are less capable of manufacturing sugars and in time become smaller in size thus reducing the amounts of sugars that can be created. Aphids excrete a sugary substance called “honey dew” that attracts ants, wasps and some scale. Honey dew that collects along branches and trunks becomes colonized by sooty mold, a blackish gray mold that discolors bark. This can be used to identify infestations during the dormant season. Some species of aphid are more damaging than others and many have built natural tolerances to foliar sprays. Even more impressive are the wholly aphid that have a waxy coating that resembles thin strands of cotton which actual repel pesticides and fluids. Aphids along with all leaf chewing and sucking insects should be controlled systemically so only the target insect is eliminated and beneficial insects are allowed to remain and aid in natures process.

Super family: Coccoidea

Scale Scale insects are made up of many families, genus, and species from hard scale to soft scale and even cottony scale they all have a very effective defense system in the form of a waxy coat that protects them from insecticides and the elements. Scale insects are far less mobile then aphids and as they are mostly larger in size they are also more serious a threat to your tree, using a rasping mouth part they draw sugars from the twigs just below the leaves, some species attach themselves to the leaves. Scale insects should be sprayed with dormant oil in November, December and February. Or timed during crawler stage (spring) with insecticide. Some heavy infestations may need to be sprayed additionally with superior oil in the spring and summer months; in all cases Pruning to introduce more light into canopy along with fertilization is helpful to reduce populations.

Pit scale- Probably the smallest of the deadly insects it is often overlooked or misdiagnosed. When this tiny scale insect grows to populations of more than 8-10 per sq. inch it can cause defoliation. This is especially dangerous during the summer months when leaves are needed most and it predisposes the tree to other secondary insects or diseases. Treatment is difficult and takes dedication, in smaller trees physically sanding the infected bark with fine grit sand paper combined with trunk and limb spray of insecticide and oil should be done as soon as identified and then twice again within 2 to 3 weeks. A preventative schedule of dormant oil should be applied November, December and February. Ensure ample amounts of water and fertilizer are introduced into cultural habits.

Common California scales and susceptible plant species:

Red gum lerp psylid _Glycaspis brimblecombei: is an insect that attacks only red gum eucalyptus. The small psylid extracts sugars from leaves and excretes it to create a sugar based “shell” which appears somewhat like a cone. Once populations grow, they can completely defoliate a giant eucalyptus within three years and either predispose it to attacks from long horned eucalyptus borer or outright kill it. In the interim they are easily controlled with the systemic insecticides, increased watering, and fertilizer to create balanced new growth of foliage.

Oak Leaf roller­_ Archips semiferanus [Walker]: also known as web worms, is a moth that chews leaves, in the caterpillar stage, creating clusters of webbing which resembles that of a spider. Mostly found in live oak and valley oak, damage is seen as “flagging” of dead branches throughout the crown of the tree.

A foliar application of BT (Bacillus thuringiensis) in March and again in April, during the leaf rollers larval stage, is the best control. BT is a bacterium that produces crystals that are toxic to caterpillars only. Once eaten, these crystals destroy the stomach lining of the caterpillars and they stop eating and soon die. BT should be applied twice within fourteen days of each application.

As caterpillars, Western tussock moth _Orgyia vetusta, chew on the leaves of most oak species but are predominantly found on live oak. The adult moth lays its eggs in the cocoon it molted from. The cocoons are brown and “fuzzy” with exit holes at the apex. They are mostly found on the underside of limbs or places protected from weather. The tussock moth is relatively harmless but a large population is easily capable of defoliating trees.  Natural predators of the moth generally prevent such large out breaks. Bt should be applied during the caterpillar stage. For best results two applications should be made, within fourteen days of each other.

Thrips that are pest are often in the Family: Thripidae

Lace bugs Family: Tingidae

Thrips and lace bug Thrips feed on both the upper and lower sides of leaves. Lace bugs only from the underside. You can identify lace bugs by stippling on the top of the leaves and excrement on the bottom. Both sides of the leaf have stippling and excrement in the case of Thrips.  Common species infected are Photinia, Azalea, and Rhododendron. Possible treatments include a systemic soil drench or foliar spray with a contact insecticide.



Borers the Order coleoptra contains 1/5 of all described species in the animal kingdom. There are hundreds of thousands of species and they discover more every year. Beetles are extremely important to ecosystems because of their proficiency at facilitating decomposition of organic matter. However this is also why they can be pests. Some species feed and/or breed inside living trees. They are opportunistic and generally a secondary illness, which means they are infesting your tree because your tree has a primary contagion and is not healthy. The most damage is done during the larval stage of the beetle. Many beetles are attracted to one or two genus of tree and thus are named after what they infect.

The following are some common borer in the Sacramento valley and their habits. Keep in mind that a healthy tree that is fertilized and receives proper watering and mulching is far less likely to be infected if at all.

Bronze birch borer (BBB) _ Agrilus anxius: Unlike most other borers, the BBB is not organ specific it may enter any part of the tree and begin breeding. Often aerial inspections are necessary to identify, exit holes have very little Fras but are distinctively “D” shaped with one flat side. Infection is not a death sentence. Populations can be managed with proper fertilization, pruning of infested limbs, and spraying with a systemic insecticide twice annually during breeding season.

Trunk/bark applications are done to poison the beetle as it enters bark and exits. They should be administered during the breeding season and combined with low nitrogen fertilization.

Pacific flat headed borer­­ _Chrysobothris malis: is also referred to as flat headed apple borer or flat headed borer. These beetles prefer damaged and or weak trees/limbs to infest and lay eggs.  The larvae feed on the cambium tissue and can girdle a young tree in a short span of time. This pest is generally not the primary cause of decline but a secondary cause. Fertilize with a low nitrogen amendment, maintain good cultural habits, and apply foliar systemic insecticide throughout spring and summer months.

Long horned eucalyptus borer­­ _Phoracantha semipunctata & Phoracantha recurva: is a rare insect to find in the adult stage, however they can be devastating. Adult borers are speckled gray with antennae that are 1 1/2 to 2 times the size of the body. Long horned borers are attracted to freshly cut wood, dying limbs, and trees suffering from stress, especially drought stress. Control starts with the removal of all dead wood including surrounding firewood. The next step is a sequence of applications including foliar sprays of oil in conjunction with wedgle injections of a systemic insecticide, and soil injections of merit. This should all be followed by fertilization for two or more seasons.

Sequoia pitch moth_ Synanthedon sequoiae : often confused for a borer pitch moth is identified by a large pitch mass found on pines throughout the bole and trunk of the tree. The adult stage of the moth lays its eggs in the fissures of the bark.  As the eggs pupate they agitate the cambium creating a pitch cocoon that protects them until their adult stage when they fly away. They do limited damage to the tree but may draw the attention of red turpentine beetle (Dendroctonus valens LeConte), especially in susceptible species. Treatment is good cultural habits along with fertilizing and monitoring for RTB.



Anthracnose results from infection by any of several different, host specific, fungi species. Infections worsen when prolonged rain occurs after new shoot growth. Two applications of a contact and systemic fungicide mix should be sprayed at bud break and then ten to fourteen days after. Or a wedgle application in November can be used for large hard to reach trees. Trees should be fertilized to help strengthen their own defenses.

Wilts should be pruned out by sterile tools and either root infusion or trunk application of Pentrabark and Agraphos.

Armillaria _ Armillaria mellea: is a soil borne fungus that causes root rot in many species of tree. Mycelium can persist for year in infected soil, roots, and wood. Presence of armilleria should receive 42lbs of Ca per 1000sqft along with water management and a TGR mixed with beneficial bacteria. This is not a cure only a management plan.

Verticillium wilt _Verticillium dahliae and V. albo-atrum:  is a wilt that can be spread through pruning tools and root graphs. If infection starts from root graphs or infected soil you may see lowest limb from the ground affected first and then others successively up trunk. In some instances you may need to remove a tree to prevent the spread to others if planted in line or a row (remove a link in the chain.) Cures include sterile pruning of infected wood and balancing cultural habits in combination with soil sampling and prescription fertilization. Some cases may require fertilization with PHC while waiting for results.

Phytophthora _Phytophthora spp.: can be treated in all stages except the resting stage (near death) with trunk sprays of pentrabark and Agraphos, applications should be done at leaf set, on deciduous trees.

Fusarium canariensis­: The fungi that cause Fusarium wilt diseases are composed of a group of host-specific forms (forma specialis) abbreviated f. sp. Thus, the fungus that causes wilt of canary island palms is Fusarium oxysporum f. sp. canariensis. It can be spread through contact with infected pruning tools. Infected trees should be removed using tarps to collect dust and protect other palms around them. Responsible cleaning of all tools used during the process should be done immediately after job completion.

Botrysphaeria spp.: Several species of the fungus Botryosphaeria cause cankers on branches and stems of trees and shrubs with low vitality. Branches turn reddish brown and die back and often exude drops of yellowish pitch. Giant Sequoias are common host species for this fungus. Often you will find one dead branch on a redwood that is associated with a canker, a sunken area containing dead tissue. This should be removed back to the trunk with sterile tools and the tree should be fertilized and monitored.

Entomosporium spp.: is a fungal disease that spots the leaves of apple, flowering crab apple, evergreen pear, hawthorn, pear, photinia, pyracantha, quince, Rhaphiolepis, and toyon. Infected trees can be treated with high nitrogen fertilizer combined with two applications of a contact and systemic fungicide mix in November and again at bud break.

Diamond scale _Sphaerodothis neowashingtoniae: primarily affects California fan palm, Washingtonia filifera. It causes yellowing, dark mottling, and necrosis of fronds. Black or greasy, diamond-shaped or elongate fungal bodies occur on fronds, resembling scale insects. It can cause considerable damage when its population grows large enough. Treatment is a foliar application of a contact fungicide. Proper management May require 3 to 4 applications.



Bacterial infections- should be treated by addressing the primary causes such as watering habits, soil fertility, and available organics. A half rate fertilization and bio package combined with vertical/horizontal mulching for soil peculation should be considered

Fluxing/Wet wood– Bacterial fluxing is normally not serious in most trees but as a chronic disease it can contribute to a general decline in tree vigor, especially of older trees growing under adverse conditions. Bacterial fluxing in landscape trees is usually unimportant except for the disfiguring appearance of light or dark streaks where liquid seeps out of cracks and wounds and flows down the bark. The liquid on the bark surface becomes contaminated with mixed populations of many different kinds of airborne bacteria, yeasts, and filamentous fungi that give it a slimy texture and often a fetid odor. Bacterial fluxing should be treated with fertilization and incorporation of fungal based mulch.

Fire blight _caused by bacteria Erwinia amylovora: Prune branches showing dieback and severe blight. Space plants to provide good air circulation. Prune during the dry season when infection is less likely to occur. Do not wet foliage with overhead irrigation; do not over fertilize. For severe cases you may soil drench with table salt at rate of 1lb per 8”diameter and water heavily.

Bacterial knots- several bacterial knots exist in such species as olive, oleander, sycamore, and Hollywood juniper. Pruning should be done, using a sterile tool, at least 18” before canker or between the tree and the canker. Fertilize, irrigate without broadcasting water so as not to transmit bacteria, and mulch to at least 3” depth.

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