The most important objective of my pruning is usually structural improvement of the tree (in some cases I may also be trying to remove diseased wood, or the objective may be aesthetic improvement only). Much of what I am attempting to achieve with the pruning is to reduce the likelihood of trunk or limb failure for those parts which I deem to have any or all of the following: a poor attachment, defect and/or excessive length, weight or exposure.

For trunks or limbs I wish to prune for structural improvement, I either use reduction cuts (reducing limb length by removing the terminal portion back to a lateral branch of at least ½ the diameter of the cut stem) or I selectively remove limbs (thinning cuts) near their ends (Figure 1). In this way, I reduce the lever arm, sail and stress on the trunk or limb and its attachment as well as suppress its growth by reducing leaf area and photosynthetic capacity. This suppression is especially effective if limbs above the pruned limb grow over it and shade it out. Unless I have trained the tree from a young age, the process of trunk or limb suppression takes a few to several prunings.

The following images illustrate how I have suppressed a large limb with a defect (Figures 2-4) and one of two codominant (similar sized) trunks over several prunings.

Normally I would not recommend irrigation during the winter; however, there had been such a long rainless period I started to wonder.

For sure, newly planted trees, should be watered to maintain ideal soil moisture conditions in the container media (which dries out much more rapidly than the native soil; this time of year within a few days) and the surrounding soil.  Most soils will take much longer to dry out – perhaps 7-14 days at 6″ depth depending upon soil type, mulching and other factors.

For established trees, whether or not the tree experiences stress would be dependent upon water use and rooting depth. Deciduous trees should need little water this time of year (however, having a fully (water) charged soil to the depth of the root system would be important in the spring) and evergreen trees are going to use water slowly this time of year. One could monitor for wilting of evergreens. However, it might be good insurance to irrigate once for peace of mind if nothing else.

UCD Professor Alison Berry irrigated her redwoods.  UC Davis Irrigation Specialist Larry Schwankl provided the following response when I asked if we should water our trees:

“I would definitely recommend folks with trees put on an irrigation.  It is important that there be sufficient moisture in the soil when the first root activity (some folks call it a root flush) begins.  For some trees this happens before you see activity above-ground.  You are correct that this time of year there is low ET but what irrigations now are doing is setting the tree up for healthy growth once activity starts.  Agricultural tree growers are starting to irrigate now also.  Not something they want to do because it costs them money but they are certainly concerned about the lack of rain and its implications on tree health and production.

Hopefully we will still get lots of rain, but things don’t look promising for the near future.  Unless February and March are unusually wet, it is going to end up a dry winter.

What kind of irrigation?  The approach should be to refill the water in the root zone of the tree.  That means a long, soaking irrigation which refills the root zone is probably best.  A good soaking now and then shut the water off again would work.  If we don’t get any serious rain soon and the soil profile isn’t refilled (soil moisture monitoring – soil sampling or one of the many devices available –  really helps there) , then more irrigation later would be warranted.”

So hopefully, you have now read this post and the storm has produced significant rain and irrigating your trees is not necessary.  However, if we have received less than a couple of inches of rain, I would suggest one irrigation for peace of mind.

• The root system is visible and girdling roots (circling roots near the trunk) are not common (Figure 1).
  • Girdling roots, which are very common in containerized stock, can prevent adequate anchorage and lead to trunk breakage and/or low tree vigor (Figures 4-6).
• The root system can be inspected and spread out during planting (Figure 2).
  • This can speed establishment and root anchorage.
• There is no container medium, which simplifies irrigation management.
  • There is no need to consider the irrigation needs of container media (which dry out more quickly than native soils and therefore needs to be irrigated more frequently) separately from the native soil during establishment.
• Bare root trees often have sturdier trunks and do not require staking for support (Figure 3).
  • However, they may require staking low on the trunk to anchor the roots until established.
• You can generally buy larger trees for the money.
  • While bare root trees of up to 2 inches are commonly available, larger trees (up to 5” diameter) are being harvested and transplanted bare root using a new methods including 1) the Missouri Gravel Bed (MGB) method where trees are planted into a pea gravel and sand matrix, and 2) pneumatically excavated trees in which the roots are exposed and sprayed with a hydro gel (Urban, 2011).
• Bare root trees are light and easier to transport.
• Producing bare root trees uses less energy and costs less compared with containerized tree production.

There are some limitations associated with the use of bare root stock which include:

• The number of species/varieties of trees available in nurseries is limited.
• Typically, bare root trees are only available during the dormant season.
  • However, hydro gel dips have been used to slightly extend the planting season.
  • The MGB method described above allows trees to be moved in mid-summer.
• Bare root trees need to planted right away or heeled in (roots covered with soil or other medium and kept moist).

I want to share my observations concerning a malady that recently has caused widespread dieback and decline of coast redwood in Northern California. Since the pathogens associated with this malady seem to work in concert with environmental stresses affecting redwood trees, this is also a good opportunity to discuss management practices that encourage healthy tree growth. In this article, I will discuss the above issues and provide recommendations which should help to ensure that these beautiful trees will serve their desired function and thrive for as many years as possible.

Figure 2. The roots of one of the trees in Figure 1 had damaged the fence, pathway, and garage foundation (path was removed).

Figure 1. Healthy redwood trees located within a few feet of two driveways and foundations in Davis. Note the trees are not close to full size at approximately 20-24 inches in diameter.

Coast redwood is one of the most common landscape trees in Northern California. In its native California coastal fog belt environment it can grow to nearly 400 feet tall, with a trunk diameter greater than 20 feet. They are typically fast-growing and often long-lived in our landscapes, relative to other species. However, these trees rarely exceed five feet in diameter and 120 feet in height in most landscapes. In part, their smaller stature is due to the relatively young age of redwoods in landscapes, compared with the age of the giants in redwood forests. It is also due to growth restrictions caused by above- and below-ground environmental conditions.

Since redwoods are such large trees, providing adequate above- and below-ground space for their spread will help to avoid property damage and difficult management choices down the road (Figures 1 and 2). I recommend planting redwoods at least 15 feet from any hardscape (foundation, patio, walkway, etc.) to avoid future damage caused by roots. In addition, I also recommend spacing trees at least 15 feet apart to allow for adequate root growth and support for each tree. Obviously, the spacing between redwoods and other trees will influence aesthetics, as well as shading and the ability to grow plants within their shade.

Figure 3. Upper crown of healthy coast redwood in Davis, CA. Note foliage browning and loss in interior of crown. Loss of older foliage is normal. The timing of this leaf loss is variable.

In their native environment, redwoods receive enough moisture through rainfall and fog drip for their survival. However, outside of their native range, they require regular irrigation to avoid drought stress and withstand attacks by opportunistic pathogens (see below). Root injury from demolition and construction can also lead to drought stress or destabilization. Drought stress results in smaller foliage size, needle browning and drop, twig and branch dieback and, eventually, death. The amount of irrigation required will vary according to tree size, soil characteristics and environmental conditions. In the Central Valley, a weekly irrigation to a depth of two feet, spread uniformly under and beyond the tree canopy, is generally sufficient.

Figure 4. Redwood in Davis apartment complex suffering from excessive irrigation and poor drainage. Note relatively uniform yellowing of foliage.

While regular irrigation is required, irrigation that is too frequent, particularly in a slowly drained soil, can stress these trees, leading to foliar yellowing (chlorosis), root dieback, twig and branch dieback, and tree death. I do not recommend irrigating redwoods more frequently than three times per week. Further information regarding irrigating trees can be found in our previously published article, “Practical Guidance for Effective Tree Irrigation.”

Figure 5. Upper crown of redwood in Gold River, CA with flagging (dying) limb, likely caused by Botryosphaeria dothidea. In some cases, the leaders will die as a result of infection.

In the late 1980’s, almost the only cause of redwood decline involved root injury, inappropriate soil moisture (water deficit or excess), or soil chemical limitations (salinity, toxic ions) (Figure 4). Other issues which were problematic for redwoods included insufficient above- and below-ground space, and poor structure. However, in the 1990’s, I started to see an increased incidence of top and limb dieback caused by the redwood canker fungus (Botryosphaeria dothidea), especially with trees which had undergone drought stress and/or root injury (Figure 5).

Redwood canker is an opportunistic fungus, meaning that it attacks trees that are under stress from drought or root loss. Once the fungus gets established in a tree, its spores are spread by rain. If untreated, the fungus usually spreads through the canopy, causing disfigurement and/or death of the tree. Management of the disease includes providing for the horticultural needs of the tree (appropriate irrigation and soil chemical modification, if necessary) and removing diseased wood well before the onset of winter rains. Other treatments to improve tree health, such as mulching, or loosening compacted soil, may assist in a tree’s recovery.

Figure 7. Close-up of tree seen in figure 3. The branches are sparsely covered with yellow (chlorotic) and necrotic foliage.

Figure 6. View of Davis, CA redwood with severe foliage necrosis (death) and drop resulting in very thin crown. These are symptoms we have been seeing more recently.

In the last several years, I have also seen a greater incidence of foliar dieback in coast redwood, either with or without top or primary limb dieback (Figure 6,7). I recently spoke about this issue with Suzanne Latham, Plant Pathologist with the California Department of Food and Agriculture Plant Pest Diagnostics Lab. Suzanne indicated that there are different species of the fungus that cause redwood canker (e.g., Botryosphaeria lutea) which infect coast redwoods, one of which causes a needle blight. This can be quite damaging to the tree, as often a large percentage of the foliage is killed, rendering the tree very unsightly. Unfortunately, removing infected wood is impractical because of the multitude of infection sites on the tree. The prognosis is poor for redwoods that are badly infected with redwood canker, or that exhibit needle blight caused by another species of Botryosphaeria.

In conclusion, we can provide our redwoods the best chance for a long, healthy, disease-free life by providing for their horticultural needs. These needs include adequate soil chemical properties, moisture and volume; avoiding root injury; and, if feasible, removing canker-infected wood.

Chlorosis often is a symptom of a nutritional deficiency, for which there may be many causes. The most common cause in our region is insufficient uptake of iron, although sometimes manganese or zinc is the culprit. The discussion and recommendations that follow focus on iron, but manganese and zinc deficiencies can be treated similarly.

Plants require tiny amounts of iron, and it is one of the most abundant elements in soil, so iron deficiency usually is the result of a plant’s inability to extract the element despite its abundance. Plants can take up only the iron that has dissolved in the soil water, and this poses two big problems. One is that the solubility of iron in soil water is very low. The other is that iron solubility is strongly affected by soil pH: for each unit increase in pH, the soluble iron concentration decreases 100-fold to 1000-fold.

The alkaline soils (those with a pH greater than 7) that are common in the Central Valley would not have enough soluble iron, manganese, or zinc to meet the needs of any plants if it weren’t for the existence of chelates. These are soluble organic compounds whose interactions with metals such as iron increase their solubility and their availability to plants. Many of the naturally-occurring organic compounds in soils act as chelates. In addition, plants that are adapted to alkaline soils produce root exudates that act as chelates. In our landscapes on alkaline soils, chronic chlorosis usually occurs in plants that are adapted to more acidic soils. Landscape trees that are particularly susceptible to pH-induced chlorosis include Liquidambar species (sweetgum), Liriodendron tulipifera (tulip tree), Magnolia grandiflora (southern magnolia), and Acer saccharinum (silver maple). These plants don’t produce many organic chelates and, therefore, cannot absorb adequate amounts of iron, manganese, or zinc from high-pH soils. The characteristic symptom of the ensuing deficiency is yellowing of leaf tissue between the veins (termed interveinal chlorosis), especially on developing or recently-expanded leaves.

Figure 1. Iron, manganese, and zinc deficiencies usually cause yellowing of leaf tissue between the veins.

The most effective remedy may be to lower the soil pH by adding elemental sulfur. In warm, moist soil, bacteria can convert sulfur to sulfuric acid within a couple of months. Unfortunately these reactions proceed slowly, if at all, when sulfur is not incorporated into the soil, so it can be difficult to acidify soil in established landscapes. We recommend using a water jet to create 2-foot-deep holes on a five-foot spacing under the canopy of mature trees, and filling the holes with a mixture of 25% ammonium sulfate and 75% granular soil sulfur. Even with this method of sulfur treatment, it can take up to two growing seasons before an effect color is noticeable.

Calcareous soils — those that contain calcium carbonate (limestone) — are almost impossible to acidify. No decrease in pH occurs until all of the calcium carbonate has reacted with the sulfuric acid. For some calcareous soils, a pound of sulfur per square foot of soil may be needed just to neutralize the calcium carbonate. Fortunately, these soils are relatively uncommon in the Sacramento Valley and the Bay Area. They typically are found where annual rainfall is less than 10 inches or, in some cases, where topsoil has been removed or has eroded away. The seat-of-the-pants test for the presence of free calcium carbonate in soil is to add hydrochloric acid and see if it fizzes. A better approach, if a soil is thought to be calcareous, is to have it analyzed by a soil testing lab. Correcting chlorosis of plants in calcareous soils is not a sure thing. There have been reports that use of chelated fertilizers (described below) is effective when applied every year, but this practice is not always successful. Calcareous soils also introduce other problems for plants, particularly phosphorus deficiency.

Figure 2. Micronutrient deficiencies due to soil alkilinity usually affect the youngest leaves, as in this Chinese tallow tree.

Another way to treat chlorosis is to add the fertilizers in a chelated form. Several types are available, but products containing the chelate EDDHA (ethylenediaminedi-o-hydroxyphenylacetic acid) are most effective in alkaline soils. Several products provide this form of chelated iron, among them Sequestar and Sprint 138. Both products contain 6% iron by weight. They should be broadcast on the soil surface under the tree canopy in the spring at rates of 2 oz. per inch of trunk diameter or 2 pounds per 1000 square feet (where did you find this info? For my knowledge; I haven’t seen this recommendation), then watered in to the soil. Mixing the product with sand or dry soil prior to broadcasting helps to achieve uniform distribution. This practice should be repeated annually.

In our experience, irrigation is the most important management concern for trees in most of California. Unfortunately, myths and misinformation abound in this area, leading to much confusion regarding how to effectively irrigate trees. The following series of questions and answers aims to provide practical, horticulture-based knowledge to ensure healthy trees.

Figure 1. Drought stressed magnolia.

What are the symptoms of drought stress?

Acute drought stress causes wilting, leaf burn, leaf drop, twig and branch dieback and, eventually, tree death (Figure 1). Chronic drought stress may be less apparent. Reduced growth rate may be the only observable symptom of chronic drought stress, or it may be accompanied by reduced leaf size, leaf drop and changes in leaf color or early fall color. Sometimes the only way to determine that a tree is suffering from chronic drought stress is to compare its growth rate, leaf size and color to that of healthy, vigorously growing trees of the same species (Figure 2). Trees suffering from drought stress are also more susceptible to many debilitating insects and diseases.

What are the symptoms of excessive irrigation/aeration deficit?

Figure 2. Coast redwood foliage from drought stressed (left) and healthy tree (right). Note the difference in foliage size and color.

Excessively frequent irrigation can lead to soils which are anaerobic. Aeration deficits can cause wilting, slowed growth, reduced leaf size, yellow leaves, leaf drop, stem lesions, bleeding, twig and branch dieback, disease susceptibility and eventually, tree death. Anaerobic soils often are gray in color and have a putrid smell (Figure 3).

Figure 3. Grey soil and a putrid odor are often associated with saturated soil conditions.

How should we deliver water to trees?

There are many methods to deliver water to landscape trees, including: hand watering, bubblers, sprinklers and drip irrigation systems. The most important consideration is to use a system which delivers water to as much of a tree’s root system as possible.

While the size and depth of root systems vary, the roots of established trees typically are less than 3 feet deep and extend to twice the distance from the trunk to the edge of their canopy (dripline) (Figure 4, 5).

Figure 4. Drawing of typical tree root system. Typically, tree roots extend well beyond the branches while the majority of tree roots are in the top 2‐3 feet of soil.

We generally do not recommend hand watering of established trees. Leaving the hose on at a trickle near the tree’s trunk or watering with a “root feeder” will only deliver water to a small portion of the root system and may waste water as it percolates below tree roots.

Installing a hose-end sprinkler or soaker hose to spread water over a large portion of the root system can be an adequate means of irrigating trees. However, this manual method requires a level of attention that many clients do not maintain, and it often results in insufficient or excessive irrigation.

Figure 5. Top down view of tree canopy, root system and location to irrigate.

If the tree is located on a slope and/or if the soil is clayey and/or compacted, the rate at which water enters the soil (infiltration rate) will be low. In these cases, there are few sprinklers that will apply water slowly enough to avoid runoff. A drip system will apply water at a slow enough rate to allow the percolation of water to the desired depth. We recommend two foot spacing between emitters within the root zone to achieve relatively uniform coverage. Using in-line drip emitters laid out on a grid or spiral on the soil surface can simplify installation and maintenance (Figure 6).

Figure 6. Illustration of spiral arrangement of drip emitters around a tree. We recommend a spacing of two feet between emitters to create a uniform wetting front. The system should be expanded as the tree grows.

In most cases, I do not recommend using bubblers to irrigate trees, unless they are to be used temporarily to fill a watering basin surrounding a young tree. Bubblers installed in tubes will generally not result in uniform wetting of tree root zones (Figure 7). The use of bubblers installed without tubes or a basin will lead to runoff because the application rate of the bubbler far exceeds the infiltration rate of the majority of soils. In some landscapes, there is an overlap between spray and bubbler irrigation patterns. In these cases, the risk of overwatering is elevated.

How often should trees be irrigated?

Figure 7. Bubblers are often used to irrigate trees. Bubblers installed in tubes (as shown) will wet a limited portion of the tree’s root zone. Bubblers can be used to fill created basins around trees which may be adequate for young trees. Without a basin, runoff and non-uniform wetting of tree root zones is likely.

In the Central Valley and Bay Area, most turf species can tolerate irrigation of two times per week or less and most trees or shrubs can tolerate irrigation of once per week or less during the summer months. These general guidelines could be used as a starting point for developing an irrigation schedule.

There are several methods to determine when to irrigate trees, including feeling the soil, using soil moisture sensors and using measured or estimated amounts of water lost from plants and soil (evapotranspiration, or ET). Unless the need for water conservation outweighs plant needs, plants should be irrigated before they suffer from chronic drought stress and its attendant effects on growth, appearance, and susceptibility to insects and diseases.

Estimating Soil Moisture Content by Feel

Using the feel of the soil to develop an irrigation schedule is not commonly performed. Although simple and cheap, it is also inexact. The following instructions are for the application of this method in landscapes. During a rain-free period just before a scheduled irrigation, dig or auger a few holes per landscape irrigation zone. Dig to a depth of 6 inches for turf or groundcover and 12 inches for shrubs and trees. Is the soil saturated with free water visible? If so, decrease the frequency of irrigation and repeat the test after a couple of irrigation cycles. If the soil can be molded into a ball which crumbles when rubbed, soil moisture is adequate and you may be able to decrease the irrigation frequency somewhat. If the soil is dry enough that it cannot be formed into a ball, you should increase the irrigation frequency and re-test.

Using Soil Moisture Sensors

Soil moisture sensors can provide a more accurate assessment of irrigation needs. Sensors buried in the root zone can accurately measure the depth of irrigation penetration as well as the extent of drying between irrigations (Figure 8). We often install sensors at two or three depths for this purpose. Data can be collected manually or with electronic dataloggers that automatically accumulate data, which can then be graphed and analyzed to determine whether or not the irrigation run time or frequency should be modified.

Figure 8. Soil moisture sensors can be installed within the tree's root zone to monitor soil moisture conditions. This information can be used to determine optimal irrigation frequency and run times.

Using Evapotranspiration (ET)

Using the evapotranspiration rate (or ET) to schedule irrigations is becoming more common. ET data (based on the rate of water loss for a large stand of grass) are available in many areas and coefficients have been developed for adjusting these data to match the water use of specific crops. ET-based irrigation scheduling works well for the relatively uniform conditions under which field and orchard crops are grown. Landscapes, on the other hand, are complex mixtures of species, sizes, planting densities, and environmental conditions, so estimation of their water use in relation to ET data is more difficult. Landscape coefficients have been developed for estimating the approximate water needs of plants over the course of a growing season. However, given the complex nature of landscapes, we do not recommend relying on the ET method unless it is used in conjunction with the soil feel method or soil moisture sensors. More information regarding the use of ET for estimating landscape water needs can be found at University of California Center for Landscape and Urban Horticulture’s website.

How long should I irrigate my trees?

The appropriate run time for a given irrigation system will depend on the application rate, distribution uniformity of the irrigation system, environmental conditions, rooting depth and soil. To determine whether or not the desired depth of irrigation penetration has been reached, one can utilize soil moisture sensors as described above; use a tile probe (Figure 9), which easily can be inserted to the depth of moist soil (in the absence of rocks or roots); or dig a hole and feel the soil’s moisture content within the root zone.

Figure 9. A tile probe can be inserted into the soil to determine the depth of irrigation penetration. The probe should be easily inserted to the depth of moist soil. Probe in several areas within the root zone.

How often should I inspect my irrigation system?

The answer to this question depends upon the characteristics of each individual irrigation system. In our experience, sprinkler and bubbler systems are more dependable than drip systems. Some of the possible malfunctions are valves sticking open and plugged emitters (which are usually problems caused by sand or other solids in the irrigation lines), blocked sprinkler heads, and animal damage. The excessive or uneven irrigation caused by these malfunctions could lead to anaerobic soil conditions and/or drought stress. I recommend inspecting the irrigation system once every two weeks during the growing season.

We hope this article has helped you to understand the relationship between irrigation and tree health and how to effectively accommodate your trees irrigation needs. We are currently preparing a video to accompany this article. Stay tuned and happy watering.

This table is a compilation of information from several references (see below) which we then modified based on our own experience with the species.  Trees which we would seldom recommend were removed from the list.

A tree’s mature size should be remembered when fitting a species to a planting site.  Distances to structures, roadways, signs, solar collectors and other trees are key considerations.  In addition, sufficient below ground space should be provided for trees to avoid conflict between tree roots and hardscape.   Avoid planting large trees within 15 feet of foundations, roads or pathways.

Landscape irrigation practices greatly affect tree performance.  Species that are drought tolerant and those which tolerate frequent lawn irrigation are indicated.  In certain locations, especially those in the western portion of the Central Valley, alkaline soils or those with high levels of boron (> 0.5 ppm for sensitive species) can slow growth and/or discolor foliage.  Therefore, species which tolerate alkaline or high boron soils are noted where known.

In cases where trees infected by debilitating diseases such as oak root fungus (Armillaria mellea), water mold (Phytopthora sp.) or Verticillium wilt are to be replaced, planting resistant species is recommended.  The list provides an indication of species’ susceptibility to these diseases.

The final column was checked in cases where we have little experience with species performance.  The use of trees so indicated is encouraged with caution so that we may learn more about them and increase the palate of trees in our landscapes.  You may wish to avoid planting these trees in areas where the trees will play a vital functional or aesthetic role.

U.C. Davis and C.S.U. Sacramento are excellent locations to view mature specimens of many of the trees on this list.  Prior to your visit, go to either the University Arboretum website or the Grounds Division website, or Sacramento State’s Arboretum Tree Map for information on specific tree locations, maps and tree walks.  The non-profit TreeDavis has a tree tour in downtown Davis and is currently working on a “top 20” tree list.  The Sacramento Tree Foundation also has a list of trees with their characteristics and images.

Please view our Tree Species List that we have compiled.  We welcome comments regarding this list and we plan to periodically update it based on these and our own experience.

-John, Kelly, and Richard