By Nathan Lawrence,
Owner of the Good News Tree Service, Inc. in Wilsonville, OR since 1985 — ISA Certified Arborist • ISA Tree Risk Assessment Qualified • State of Oregon Licensed Commercial Pesticide Applicator • OSU Master Gardener

The following information and recommendations pertain to the northern Willamette Valley areas of western Oregon.

Planting the right tree in the right location will help to minimize plant stress (both biotic and abiotic), pests and mortality, thus ensuring better tree performance for years to come. This is a wise use of resources, economical, good for the well-being of the community and for the local environment and the earth in general. This also means that fewer chemical pesticides will be required in caring for the tree, which is a good thing for everyone and everything.

To accomplish these goals requires intentional planning and tree planting strategies. Prevention is the best medicine! Property owners cannot afford to pay someone simply dig a hole and drop a tree in it, and then  walk away after the tree planter has collected their money and moved on. Garbage in garbage out! If the tree was not planted with intentional forethought with an eye on long term tree survival, it is likely the tree will under-perform, require expensive (often chemical) treatments and may even die. If the tree is planted with strategic and intentional forethought, and then properly cared for subsequently, it will be more likely to perform healthily for generations to come

The following are some things to consider before planting a tree on a site.

Site Evaluation

It is essential to determine the soil volume of the planting area, so that the appropriate tree can be chosen for that specific site. The smaller the soil volume, the smaller the tree (at mature size) that can be planted in that area. Conversely, the larger the soil volume, the larger the tree (at mature size) that can be planted. I recommend that the tree size–soil volume ratio be based on various studies of a tree’s soil requirement at its mature size. If a tree doesn’t have the proper soil volume, its roots will not be able to uptake the amount of soil moisture and nutrients they need to be healthy, nor will they be able to anchor themselves sufficiently against wind storms.

Here is a list from based on the above mentioned soil volume study for urban trees of approximate tree size and suggested soil volume requirements. These numbers are based on the tree’s crown spread and diameter at breast height (or dbh) at the tree’s mature size. (One online calculator for determining the soil volume needed for a tree can be found at https://greenblue.com/gb/resources/soil-calculator/.)

• 14 foot crown spread by 4 inches dbh needs 100 to 307 cubic feet of soil volume.
• 21 foot crown spread by 8 inches dbh needs 200 to 698 cubic feet of soil volume.
• 27 foot crown spread by 12 inches dbh needs 300 to 1144 cubic feet of soil volume.
• 32 foot crown spread by 16 inches dbh: needs 1000 to 1607 cubic feet of soil volume.
• 36 foot crown spread by 20 inches dbh needs 1300 to 2034 cubic feet of soil volume.
• 39 foot crown spread by 24 inches dbh needs 1400 to 2512 cubic feet of soil volume.

To determine the average soil volume of a street tree planting strip, first measure the width of the strip (from sidewalk to street curb), then the average depth of the soil (two feet or less), then the length of strip. For example,

• If the tree’s roots spread to a radius from the trunk of 10 feet with 2 feet of soil depth in a 5.5 foot wide planting strip, then the soil volume in that area is 220 square feet.
• If the tree’s roots spread to a radius from the trunk of 20 feet with 2 feet of soil depth in a 5.5 foot wide planting strip, then the soil volume increase to 440 square feet.
• If the planting strip is 8 feet wide and the tree’s roots spread to a radius from the trunk of 10 feet with 2 feet of soil depth, then our soil volume is 320 square feet.
• If the planting strip is 8 feet wide and the tree’s roots spread to a radius from the trunk of 20 feet with 2 feet of soil depth, then our soil volume increase to 640 square feet. 

Another way of determining the soil volume is to calculate the soil volume based on the size of tree’s canopy. One to three cubic feet of soil is needed for every square foot of crown projection. This is the anticipated area under the drip line of the tree at expected maturity. This formula does not apply to fastigiate and columnar habit trees. The soil volume requirement can be calculated for narrow form trees by basing the canopy diameter on the parent tree’s natural growth form.

The volumes generated in the above lists is an estimate and should only be used to generate an order of magnitude as a guide or target volume. Furthermore, there is a wide range of soil volumes recommended by soil scientists, botanists and landscape architects. The bottom line? The smaller the tree the better for most street tree situation, since recommended soil volumes are seldom high enough to support healthy trees long term.  

Please note that if the soil is only 18 inches deep, then the soil volume will be even less. In many cases in planting strips along roads and sidewalks, it is not uncommon to hit compacted, impermeable gravel in various spots in the tree’s root zone at only a depth of 6 to 12 inches. 

In light of the reality that street trees are seldom planted with the optimal soil volume and soil fertility they would prefer, the manager of the street tree will need to monitor the tree to ensure that it has the proper nutrients needed to sustain maximum tree performance. A planting site that sub par to the tree’s optimal needs may necessitate occasionally feeding the tree if it begins to show signs of stress. This needs to be factored into the long term care of street trees.

Soil Type, Texture and Structure

Next, we must conduct a soil test to 12 inches deep to determine soil type, texture and structure, so that the right tree is planted in the soil type that it prefers. This involves determining the approximate proportion of sand, clay, silt (and loam, which is a combination of the previous three soil types).

An excellent starting place for conducting soil type analysis is to download the free SoilWeb app,which has been produced and updated over the decades by U.S. Department of Agriculture (or USDA) and the Natural Resources Conservation Service (of the USDA). This app allows one to type in any address or location in the U.S. and the soil type will be identified to various depths. Admittedly, this information may be only marginally helpful in urban areas where the soil has been disturbed and compacted. However, this data will give the arborist and basic understanding of the soil type with which they are working.

A soil test can be done via the standard jar test. This involves placing a soil sample with water into a Mason jar, shaking and then letting everything settle out in layers overnight or longer to determine the percentage of sand, clay and silt. Since sand is heavier, it will be at the bottom followed by silt and then by clay on top. Soil structure involves knowing soil particle size to be able to determine moisture retention and the permeability qualities of the soil in question. Also, if soil is compacted, air space may need to be added through air spading, loosening the soil mechanically or by hand. Soil compaction can be assessed by how easily a pointed rod or soil sampling tool penetrates the soil down at least six to eight inches. If soil is compacted, roots will have a harder time penetrating, because pore space is limited, and so will be the air and water that will fill those pores. This will not only hinder root growth, but limit air to the roots, which is necessary for respiration or turning carbohydrates into sugar or energy for root growth. Compacted soil also hinders the roots from uptaking water and mineral. Compacted soil also reduces soil pore space, which hinders microbial activity, which depend on soil and water for their metabolic processes All of these functions are necessary for healthy soil, which in turn promote plants.

Soil Fertility

The overall fertility of the soil helps to determine the health of the tree. Not only is it important to determine soil type, but soil tests may need to be obtained to determine the amount of nutrients therein. Are they excessive or deficient? Does the soil need to be amended? If so, how much and with what? The color of the soil can indicate fertility as well. Brown indicates the presence of organic matter. Black soils—especially with a rotten odor—can indicate too much organic matter. A light brown color indicates low levels of organic matter. Gray soils can indicate too much long term water, which may be accompanied by a foul, sulfur odor.  

Drainage or Rate of Water Leaching 

Evaluate the soil type, slope of the site’s topography, and amount of water that the site receives (e.g., rain, irrigation or runoff from upslope areas) to determine the amount of water that remains on the site versus what runs off or leaches into the subsoil. To test for soil drainage, dig a hole 12 to 18 inches deep, fill it with water, let it drain, then fill it again. Then measure the speed at which the water level drops. If it is an inch or greater per hour, then you have good soil drainage. If it is less than a half inch per hour, then it is best to plant a tree that likes “wet feet” or swampy conditions. Planting trees that don’t like chronically wet soil conditions in soil that has poor drainage is asking for root rot fungal pathogens to invade and eventually kill the tree. 

Knowing drainage rates is important, since the amount of water a site receives will determine the type of tree suitable for that site. Excessive water or the lack thereof is a major source of tree stress causing trees to under perform, decline in vigor or die. Some trees can handle more water than others. Choosing the right tree for the site as it relates to water is essential to keeping healthy, stress-free and long-lived trees. In the long run, this saves everyone time and money, and is a wise use of resources and is good for the environment.

Soil Compaction

Soil fertility and its ability a soil’s drain water is directly tied to soil compaction. Native, fertile soil contains about 25 percent air space, which allows water to flow through the soil and rain enriched nitrogen to percolate down into the soil carrying with it oxygen and nitrogen. Air space in the soil allows roots to grow more easily, microbial activity to occur, and nutrients and minerals to move freely through the soil thus allowing their access to roots. All living organisms need oxygen to sustain life and to produce food. This includes a whole range of living organism both large and small as well as tree roots. These all inhabit the soil in a complex, mutualistic dependency. 

Compacted soil (often due to human activity such as constructions, vehicles and equipment and even foot traffic) literally squeezes the life out of the soil thus killing soil micro-organisms, as well as inhibiting water from draining away from tree roots thus water-logging them. This stresses tree roots making them targets for predatory, parasitic and biodegrading fungal pathogens often leading to the decline or death of the tree. 

Therefore, before planting trees in an urban site where the soils have been disturbed and usually compacted, it is needful to determine how compacted the soil is with the help of a device called a densometer or soil compaction tester. This gives one additional information about the soil structure, the type of tree that will grow in these conditions, and what measures may need to be taken to aerate the soil, thus improving the soil’s fertility and a ability tree to survive. 

Sunlight Exposure 

Be certain that the proper tree is planted in accordance with the amount of sunlight it needs. For example, many people plant dogwoods and vine maples in full sun, yet natively they are understory trees and perform best in partial shade areas and not full sun, where they are more susceptible to stress and under-performance. Conversely, most confers prefer sunny areas. In shade conditions, they will under-perform.

Irrigation 

 Most new trees require copious amounts of deep watering until their roots are established in the surrounding “native” soil. This extra watering may be required for several years during warm, hot dry weather. This requires collaboration with those who manage the irritation systems. For example, in lawn areas, the irrigation applied may be sufficient for the shallow roots of grass, but may not reach the deeper roots of trees. Therefore, trees may need extra water until their roots are established  even up to several years after their planting. If one is not sure whether the tree is receiving adequate water, then during the dry season, the soil should be evaluated to a depth of at least 12 inches to determine whether the amount of water reaching the roots is sufficient to keep the tree hydrated. If it is not, then the tree will need extra water whether that means installing a drip system to that tree, or placing a tree gater around the tree during hot weather. If the later, then someone must be chosen who will ensure that the tree gater is kept full of water. A soil tensiometer is a helpful devise that measures soil in water and helps to determine whether a tree is getting enough water.

Climate Change Considerations 

Recent changes in the climate have necessitated that tree planters and managers reconsider the old ways of planting and caring for trees. The last few years, for example, the summers have been longer and hotter. This requires that newly planted trees have more water longer, otherwise they are likely to stress and die. Also, we have had a few years recently where the rainfall during the normally rainy months has been erratic or intermittent. There have been times in the fall, winter and spring where the trees have gone for weeks at a time with little or no rainfall. Imagine your having to survive without a single drink of water for a few days or weeks? Sometimes trees have to go thirsty for weeks or a two or three months. This puts unusual stress on many trees. If long periods of dry weather occurs, some newly planted trees may need some irrigation, especially starting in the late spring when temperatures begin to warm and a tree’s water needs increase all the way through to the fall when the rains return. Ascertaining the needs of the tree will be up to the manager of the trees, who will need to monitor them during these critical times. If the irrigation managers have concerns, they need to talk to a qualified ISA Certified Arborist, who is also a plant health care specialist, who can then advise them on what to do in caring for the trees.

Pest (Diseases, Fungi, Insects and Other Pests) Considerations 

When choosing a tree, it is imperative to know what kinds of pests are prevalent in that area, so that tree species that are susceptible to those pests will not be planted on that site. The importance of this cannot be stressed enough. 

A qualified arborist who has experience and expertise in plant pathology and has a working knowledge of the needs of various tree species will know these things and will not recommend plopping any old tree anywhere just to make a buck. There is a right and ethical way to do things with an eye the client’s long term needs and the future health and well-being of the tree.

Some trees are more resistant to root rots than other trees. For example, Oregon State University publishes a list of trees that are resistant and susceptible to the common armillaria root rot as well has an online guide that helps one to determine the types and causes of regional plant pests. Similarly, the British Royal Horticultural Society publishes a list of trees that are resistant to the common fungal pathogen known as phytophthora spp..

Replacing a Tree That Has Died

When replacing a tree that has died, it is essential to ascertain why the tree died, so that the next tree will not die as well. This is where diagnostics and plant pathology comes in to play. A “forensic” analysis of the causal factors resulting in the tree’s death must be determined. This may involve sending plant tissue samples (including leaves, roots, bark) to a plant diagnostic lab such as the one at Oregon State University or obtaining soil samples to determine mineral and fertilizer needs.

If it is suspected that a root rot fungal pathogen caused the death of a tree, and a new tree needs to bed planted in the same spot, some steps can be taken to protect the new tree from succumbing to the same fungal pathogen that killed the former tree. Most notably, before the new tree is planted, the soil area of the former tree can be drenched with a soil fungus controlling or inhibiting pesticide. As discussed below, the roots of the newly planted tree should be drenched in soil pathogen controlling agents as well. 

It must be stated that soil borne fungal pathogens technically can’t be eliminated, only controlled. This is done by using fungistats or by strengthening the tree’s health and vigor and thus its ability to fight fungal pathogens internally through its own natural defense mechanisms. Even then, many fungal pathogens that are ubiquitously present in the soil profile in time will likely be carried by soil water movement back into the root zone of the tree, so continuous monitoring and on-going treatments, as necessary, may need to be applied to the tree.

Tree Planting

Selecting Trees

There are several factors to consider when selecting trees from a nursery.

• Does the nursery have a good reputation for producing healthy plant stock?
• Inspect the health of the tree including leaves, branches, trunk, root crown and roots for mechanical as well as for pests damage. Also check for other abiotic and biotic issues of concern.
• Look for girdling or pot bound roots and reject the tree if these are found.
• Inspect the roots for good health. Reject trees that have poor root structures or discolored roots.

Prepping the Tree With a Fungicidal Drench 

Drench the roots of the tree (either in the pot or once it is planted in the ground) with a broad-spectrum chemical fungicide such as phosphorous acid. If possible, also add into the drench a biological fungicide containing soil micro-organisms that will help the tree to overcome transplant shock and to establish beneficial micro biota into the soil profile. Add these fungicides into the drench mix and pour over the top of the tree’s root ball. 

Planting

Depending on the site condition, make a planting hole that is larger than the tree’s root ball. If planting in loamy topsoil with plenty of organic matter, a smaller diameter hole is sufficient. Most urban soils are disturbed, compacted, and have inadequate drainage, so making a planting hole two to three times larger than the root ball maybe necessary. Situate the root ball at the bottom of the hole so that the top of the root ball is about one or more above soil grade, so that two or three inches of an organic matter mulch can be added (discussed in more detail below).

Addition of Organic Matter. 

For decades, many of us have been adding organic amendments into the soil mix with which we backfill our planting holes. This has worked, and trees have survived and thrived. Some soil scientists still recommend this . However, other soil scientists are telling us that this is not necessary. Instead, they say, at the time of planting add several inches of mulch over the top of the tree’s roots and let the nutrients therefrom gradually and naturally work their way downward into the soil profile. This is an excellent practice, since it reflects the natural process of nature in a forest setting, but it takes a while for this process to occur. In the non-forest and non-ideal setting of urban soils, however, it seems that the process of incorporating a small quantity of organic matter into the tree’s root zone to speed up the process may be a good thing. 

One thing that soil scientists everywhere agrees on is that in an ideal soil profile, organic matter should make up three to ten percent of the soil volume. So here is my key take-way from this debate. If the soil has no organic matter or is sandy (not common in this area of the Willamette Valley), then add organic matter such as potting soil, peat moss, rotted manure or garden mulch, and also add some mulch over the top of the soil as discussed below. This is my middle of the road approach between the two opposing viewpoints of the soil scientists. After more than 50 years of planting and caring for trees and shrubs in this region, I have yet to see the harm in adding organic material to any soil type, although, admittedly, it may not always be necessary.

Watering

As the soil is being back-filled, do so while drenching the soil mix with water. This fills in airspaces around the roots and helps to hydrate the tree thus easing transplant shock. Afterwards, gently tamp the soil down around the top of the root ball with your foot.

Mulching

By leaving the top of the root ball slightly higher (one to two inches depending on the size of the root ball) than the grade of the surrounding ground, this will allow for the addition of a couple of inches of mulch (preferably wood chips or decomposed garden mulch, and preferably not barkdust, which does nothing to feed the soil) to be placed over the top of the root ball, thus leaving no roots exposed. Do not pile up mulch around the trunk of the tree so that it is touching the trunk, thus preventing trunk decay from occurring.

Staking

Some people stake newly planted trees every time no matter what. In my decades of experience in planting trees, if the site is not windy and the root ball is heavy, staking is not necessary if the tree is small. If the tree is taller than eight feet or so, staking may be advisable, since there is a higher crown and more foliage to catch the wind and destabilize the tree. If the site is windy or the tree is planted barefoot, staking is advisable. When staking, use at least two wooden stakes with appropriate tree tying material and place the stakes at right angles to the prevailing wind (which in the northern Willamette Valley around Wilsonville) is from the southwest. That means the stakes should be on the SE and NW sides of the tree. Why is this? This is to insure that the tree can move,twist and torque in the wind without blowing over. Wind movement on a tree’s trunk makes for a stronger tree trunk, and thus better able to withstand the wind as it grows larger. So that movement can occur, tree ties should be slightly loose, so the tree can move slightly in the wind without being blown over.

Timing of Planting

Trees can be planted any time of the year, although the best time is after their leaves have fallen off in the fall and before bud swell occurs in the spring. These times of planting are less stressful on a tree. If planted during the growing season or in the heat of late spring and summer, extra care needs to be taken to reduce plant stress and to ensure it has plenty of water. If possible, plant trees during cool weather or at cool times of the day.

Irrigation

After the tree has been planted, make certain that a plan is in place to water the tree as it needs as determined by the arborist. Then a designated person needs to monitor the tree to ensure that its water needs are being met. As noted above, it is vital to the tree’s survival that it be watered—especially during the late spring, summer and early fall—for the first few years until its roots are established.

Grass-free Planting Circles

The bigger the mulch-covered planting circle the better is the basic rule here. Lawn grass competes with a tree’s roots for water and fertilizer. Ideally, the tree would prefer that a grass-free area extend, at the minimum, out to its drip zone of the tree. Whether the owner or manager of the tree will agree to the aesthetics of this look is another issue.

Management of Trees Planted

Monitoring and Record-Keeping

Once the proper tree has been selected and planted in the appropriate site, it is essential to monitor that tree for its health and well-being. I will now discuss a plan to accomplish this.

The owner of the tree, or its manager (if it is a home owners association tree, or is a tree located on commercial or public property) needs to be instructed by the arborist on how to care for the tree, and how to watch for signs and symptoms of tree stress. The tree’s owner or manager plays an integral role in caring for a tree, since they are the early warning system when issues of concern pertaining to the tree’s health arise. It is the arborist’s job to educate the tree’s manger in this area. At the first sign of stress or the incursion of pests, the owner or manager needs to contact the arborist in charge so that an inspection can be made.

Someone, most preferably the arborist, needs to keep a written record (for example, on a computer spreadsheet involving a numbered rating or some sort of a ranking system) of the pre-planting site conditions, if modifications were made, pre-existing soil diseases, when the tree was planted, irrigations issues, and any other concerns relating to the tree’s status and well-being. This could be accompanied by photos documenting the tree’s health and attached to the spread sheet. This will be an on-going a written record to help manage the trees planted, to ensure their proper care, and to improve the rate of the trees’ survival. This record can be then transmitted to subsequent tree managers. 

The process of monitoring and keeping records may seem laborious, but if a system is put into place using a spread sheet and portable computer tablet, in many cases, trees can be managed simply by driving by the tree and making visual observations from a vehicle. 

Conclusion

With some practice and a system put in place to ensure that the right tree planted in the right site and subsequently cared for, efficiencies will improve and such a process will contribute to the longevity of a well-performing tree. This will also save the tree’s owner money by not having to remove and replace large, valuable trees that have become stressed or have died. This is also the right thing to do for the greening of the Earth and the increased sustainability of life on our one and only planet.