Trees have a few basic parts: Roots, Shoots, Flowers, and Fruits.

The primary part is the root system. Roots are primary in two ways: the first part of a new plant emerging from seed is the root, which shows how critical the root-soil connection is for plants. In the landscape, the root function is critical. Roots makeup roughly 25% of the mass of a tree, and have the following functions:

• They anchor the tree to the ground. Trees have to stand up against gravity and wind, so the roots spread widely (NOT deeply) to hold up the crown (see figure below).
• They take in and distribute water and nutrient elements from the soil ecosystem, both of which are used throughout the tree for many purposes. In order to do this, roots need to be able to get fresh oxygen from the soil.

Tree roots grow in a shallow, wide form. Most roots will be less than three feet deep. However wide the tree’s canopy is (x), the roots could grow 2-5 times that diameter, depending on site conditions and available soil and water resources. Image: Joran Viers.

Everything above ground is part of the shoot system. Roughly 75% of the tree mass is in the shoot system. Much of that is in the form of wood. Sapwood has active water vessels (xylem), the heartwood is old sapwood that no longer has active xylem. Older wood can be thought of as being dead, but healthy dead. The cells don’t have living contents, but they are not being decayed. This kind of wood is very strong yet flexible.

• Landscape trees should have strong trunks to support their branch systems. Poor structure and previous damage are indicators of a greater chance of failure in the trunk system.
• The branches grow out of the trunk. They start out as new twigs, and over the years certain ones keep growing, becoming the woody scaffold holding up the leaves. Branches can have a strong or weak attachment to their parent stem (larger branch or trunk); weakly attached branches fail more often.
• The leaves are where photosynthesis occurs and they are very important for the tree. Leaves are also expendable; most trees lose their leaves every year. The loss of a small percentage of a tree’s leaves usually doesn’t hurt a tree in any real way.

Most trees will also produce flowers and fruits. These are the reproductive parts, leading to new individual trees formed in the seeds found in the fruit. Trees with showy and/or sweet-smelling flowers can add a nice dimension to a landscape. However, many fruits might be considered a nuisance in a typical urban landscape setting. Fruits can be fleshy (like an apple or peach) or dry (like a locust pod or an acorn).

Trees grow differently than animals and humans. We get larger in all of our parts, but we don’t add new parts (two legs, two feet, two arms, two hands...). Trees add new parts on older parts, small parts on large parts (twigs on branches), and the new parts also get larger over time (twigs become branches). A little branch that is five feet above the ground today will be a large branch five feet above the ground in twenty years. There is no set number of parts a tree can have; that number is determined by the combination of genetics and environment. Trees can overcome the loss of some of the parts of a given type (leaves, branches, roots), as long as not too much was lost.

New parts will grow to replace the lost parts. Tree roots, trunks, and branches will slowly get thicker around over the years. This is because new layers of growth are added to the outside of the accumulated mass of previous growth. If the tree, or that part of the tree, can’t continue to expand its girth it can become stressed and even die. This happens because the little tube-like cells that allow water to move up from the roots into the leaves become crushed and the water movement stops.

Trees are plants, and plants grow where they can. They make their own food using the energy in sunlight, carbon from the air, and water from the soil. This process is called photosynthesis. In order to grow and thrive, trees need to be able to carry on the process. Plants are also sedentary organisms; they don’t move around in their environments. Wherever they are growing is where they have to grow, or die trying. Plants get the things they need to grow (water, soil, sunlight, air) from their own immediate surroundings.

The first step in selecting a new tree is to study the site where it will be planted. How much space will the tree have, both above ground (look for overhead utility lines, etc.) and below ground? Don’t try to put a large-maturing tree in a small space, it will ultimately be a disappointment. Soil type matters to some degree, as some species tolerate poor drainage (like in clay soil) better than others; of course, soil compaction matters as well. If the area is compacted, break up the compaction physically (pick and shovel, backhoe, etc.). There are no chemicals that will reverse compaction, it has to be done physically.

Once you have assessed the site, consider species choices. Look for one that matures to a size that fits the site. Take into account any preferences you have for overall shape, as well – do you want a tall, narrow accent tree, or a low, broad shade tree? Are there other aesthetic aspects you want – spring or summer flowers, good fall color, etc.? You can review some species information by studying the City of Albuquerque Master Tree and Plant List [PDF]. There are also some very informative plant books available, some with a very local focus. Many of the plant nurseries can make good recommendations; however, they may carry species that simply aren’t well-adapted here, so check their suggestions against the Master List and/or books. 

Book resources:

• New Mexico Gardener’s Guide, by Judith Phillips
• Best Plants for New Mexico Gardens and Landscapes, by Baker Morrow
• The New Sunset Western Garden Book, by Sunset Magazine

Once you know what species you want, the next step is selecting an actual tree to plant. Not all specimens are equal! Trees in nurseries can be subject to neglect and abuse, resulting in trees that may fail to thrive, or even die. Take your time to inspect the choices at the nurseries.

Small is better. Small trees establish more quickly and grow faster than larger trees. They are also cheaper and easier to handle. We plant larger trees in the City Parks due to concerns about accidental damage or vandalism, but in a home setting, you can choose a smaller specimen.

The health and shape of the root system are critical, yet it is hard to see. Ask the nursery to pull the root ball out of the pot so you can look at the roots. If there are a lot of roots circling around the outside of the root ball, these will need to be cut off to prevent problems later in the landscape.

The condition of the trunk and branches if important, too. Select trees with a single stem and small side branches. IT WILL NOT LOOK LIKE A MATURE TREE!! This is VERY IMPORTANT! Trees grow by adding new mass on top of the old mass, so a young tree with a branch three feet above ground will become an old tree with that same branch still three feet above the ground. Inspect the trunk and branches for damaged and dead pieces – a few of these can be pruned out, but a lot of damage suggests neglect that may have caused more problems than you can easily see.

Trees are available in different forms for planting purposes. The most common are bare-root, container-grown, and balled-and-burlapped.

Bare root: common with mail-order fruit trees. They will be relatively small and the roots will not have soil around them. They are shipped late in the winter for early spring planting. These tend to do very well, as the bare root package leads to great root-to-native-soil contact, a very important thing. Being small, they tend to establish easily and grow quickly.

Container grown: common in local nursery stock. These trees have lived their entire lives growing in containers of different sizes. It is common for these trees to have circling roots due to the size limitations of the container system. These circling roots need to be cut at a point before the circling starts; new growth from the cut surface will grow out away from the trunk, as it should.

Balled-and-burlapped (B&B): Most larger specimens will have this type of root package; it is what the City of Albuquerque typically plants, due to our need for larger initial size. These trees are heavy, a few hundred pounds usually, and should be handled and planted by professionals. While they provide a quicker initial presence, they usually take so much longer to establish and begin strong growth that a smaller specimen will overtake and surpass them within a decade or so.

Trees can grow to a large size or be stunted, depending on the species and environmental conditions. Consult your favorite tree or plant reference guide [PDF] and remember that the size listed is usually a maximum. If the species has the genetic potential to be big, and the site conditions are adequate, the growth accumulates over decades and the plant can become quite large. Large trees can provide greater benefits than small trees, but they can also become larger problems. Wood is very heavy, and falling branches and toppling trees can do considerable damage to people and their homes, cars and other belongings.

Trees can be very long lived. In a wild state, some trees can live for a few thousand years. Several hundred years is not uncommon in many forest species. However, the average age of human-planted, urban trees is likely less than ten years. This is true because a very high number of newly planted trees don’t survive the early establishment years. Reasons for this will be discussed below. Given good conditions, urban trees in Albuquerque should be expected to live 50-100 years, depending on species and location. Most of Albuquerque is not naturally a good environment for trees, so we need to care for them appropriately to get them to live longer. Older trees are larger trees, and larger trees provide more of the benefits that we get from urban trees.

Trees need space. Above ground, they need enough space for the trunk and branches to grow to the limit set by the combination of the tree’s genetic programing, and the specific site conditions. People often make the mistake of planting a large tree in a small space.

Overhead obstacles: make sure there are no overhead utility lines, overhanging roofs, or other things that can come into conflict with the growing tree. Trees are often pruned to poor structure to get them out of utility lines; a better choice is not to plant a tree under the line that will grow tall enough to become a problem, or to plant a shorter tree.

Underground obstacles: be aware of underground water and sewage lines, as well as other buried utility lines. Roots will follow any of these because it is easier for roots to grow in the trench made to lay these lines. Building foundations, stem walls for perimeter fencing, and even sidewalk underlay materials can all be impacted by trees. Don’t plant trees close to known obstacles.

Microclimates and building effects: large differences exist in the amount of sunlight, the temperature, and the wind effects that plants experience near buildings. Southern exposures are warmer, northern exposures colder. These kinds of differences can impact when trees leaf out, when they lose their leaves, and many other growth parameters.

The size and shape of the hole are important. We want to encourage roots to grow out horizontally, where they will find the resources they want. The ideal planting hole is only as deep as the root system, measured from the top of the highest root to the bottom of the root ball. The hole should be much wider, however – two to three times as wide as the root ball (see figure below). Carefully place and align the tree in the hole, and backfill with the same soil you took out to make the hole. When the hole is halfway backfilled with soil, add enough water to fill the hole, and let it settle in before adding the rest of the soil. Flatten the soil surface, making a retention berm around the edge of the hole, and flood that basin. This gives the tree a very thorough initial watering.

Tree planting hole. A = native soil; B = un-amended native soil backfill; C = lightly amended native soil backfill; D = mulch. Image: Joran Viers

Most soils will not need any amendments (like compost) added. Trees don’t need rich soils. However, if the native soil on your site is very sandy (like on the west side of Albuquerque) or a heavy, tight clay, adding a small amount of compost can help. Add no more than 10% by volume, and mix it well into the backfill soil before backfilling the hole. Do not add sand to clay soils, or clay to sand soils – it is not easy or practical to change the natural soil texture, and you are more likely to create additional problems than to improve things.

Learn more about tree planting by visiting this website from the University of Florida.

Trees can be planted year-round, if care is taken; however, if possible, avoid planting during the hot summer months.. Fall planted trees generally suffer less transplant shock, as the demand for water is going down and the roots can still grow a bit into the new site. Trees planted in late winter, near the end of dormancy, tend to do well also. Planting during the hotter months requires more care to maintaining appropriate soil moisture through the transition period, and the trees may show more signs of transplant shock (drooping or dropped leaves, for instance).

Trees need soil. They don’t need a rich soil, but they do need it to not be compacted, to have a little bit of organic matter, and to be irrigated. Good soil that is not irrigated is of no use to a tree; terrible soil that is irrigated is of no use to a tree.

Trees need soil for a few reasons. Soil provides the friction on the root system that holds the tree in place. Soil holds the water that trees need to take in. Soil provides many of the nutrients trees need to grow.

• In order for roots to grow through soil, the soil has to be sufficiently moist and sufficiently soft. Many urban soils are very hard due to compaction; roots don’t grow well, if at all, in compacted soils. Moisture in the soil helps soften it to some degree; however, a well irrigated but compacted soil will not support good root growth. Similarly, a sufficiently loose soil that is not irrigated will not support good root growth. Soil and water are two sides of the same coin; the tree cannot use one without the other.

• Roots also need and use oxygen. Compacted soil doesn’t have as much air space, so it has little available oxygen. Plastic sheeting used as a weed barrier material don’t allow gas exchange between the soil and the air above, so those can suffocate roots and lead to poor growth, and even to eventual tree death.

• Roots grow in two ways, by adding new cells and then by making those new cells bigger. This process pushes the tip of the root through the soil. As the things roots need (water, oxygen, nutrients, loose soil) are found close to the soil surface, roots tend to grow shallow. The majority of a trees roots will be found less than three feet deep. Some roots may grow deeper, if soil/water conditions allow, but even then the majority of roots are shallow. They do grow long, however, and can grow well beyond the edge of the branches (again, if soil/water conditions allow). This wide, shallow structure helps support the tree – like the base of a wine glass supporting the body of the glass above.

There are different types of soil. Those differences can have a major impact on tree establishment and growth. Soil types are based on the size of the mineral particles in the soil. Clay soils have very, very small particles, while sand soils have relatively large particles. Silt soils are in between. All soils are a combination of those three size classes (clay, silt and sand), plus some amount of organic matter.

Clay soils are “heavy”. They absorb water slowly, but hold a lot of it; they don’t like to give it up, though. Clay soils easily become waterlogged (too much water, no available oxygen) which can be very harmful to tree roots. Clay soils can have a good nutrient load, and they can hold applied nutrients well. Trees in clay soils need to be irrigated less often, but with more water, compared to other soil types.

Sand soils are “light”. They are easily worked, and harder to compact than other soils (though it is quite possible to do so!). Sand soils absorb water quickly, but they neither hold much, nor for long – it just keeps sinking lower into the soil. Sand soils have very low nutrient levels, but are more easily amended and worked with. Trees in sand soils need to be watered more often, but with less water, compared to other soil types.

Silt soils are in between the clay and sand in size and in water holding/nutrient holding capacities.

Many soils are complex combinations of sand, silt and clay, and may vary substantially over short distances.

Water moves in soil based on two forces. One is gravity, which always pulls water downward. The other is capillary pull. This force moves water into the spaces between soil particles, and it is caused by the electrical charges on the surface of the soil particles interacting with the electrical charges on water molecules. The smaller the space between particles, the stronger capillary pull is. In a sandy soil, gravity overwhelms capillary pull and water moves largely (and quickly) straight down. In a clay soil, capillary pull is very strong, and water moves horizontally about as fast as it moves downward; water will spread out farther in a clay soil. Clay soils hold a lot of water, but they take it in slowly and they don’t give it all up; sand soils don’t hold much water, but the water they do hold is very plant available.

Planting trees in landscapes

Trees need sufficient soil volume for the root system to grow in. This volume varies a lot with soil type and other site considerations. Compacted soils, soils under pavement, soils under partially permeable covers, and non-irrigated soils will not add much to the tree’s resource base. The Figure below shows the average soil volume needs for trees of various canopy sizes. Because roots grow relatively shallow, soil depth below 3 feet is usually not considered useful to the tree.

The soil and water available to a tree should be considered as two sides of the same coin. Dry soil doesn’t help trees; moist soil of insufficient volume doesn’t help trees. Trees need enough soil, and they need that soil to be moist enough to allow for root function. Plants feed themselves, making sugars from sunlight, water and carbon dioxide. In order to do that, though, they need certain chemicals, which we call plant nutrients. Most of those come from the soil, and the tree takes them in with the soil water it is taking in.

Urban tree soil requirements. From: Up By Roots, by Jim Urban (2009).

Trees use those chemical nutrients to build all sorts of other chemical compounds, and those in turn are used to build cells and all of their contents, including enzymes, hormones and other complex molecules. For instance, both iron and nitrogen are critical to building the chlorophyll molecules that convert the sunlight, water and carbon dioxide into simple sugars.

A healthy soil has millions, even billions, of fungi, bacteria and other microorganism living in it, as well as larger things like mites, nematodes and earthworms. The majority of these life forms contribute in a positive way to the health of the soil, and together they are called the “soil food web”. If the web is intact and strong, plants tend to grow well (assuming no other problems exist...). Many urban soils do not have a vibrant soil food web, for a variety of reasons:

• We don’t let fallen organic matter (leaves, fruit, branches) lie on the soil surface and decompose. If they did, they would supply new fuel to the soil food web, but for aesthetic reasons, we clean up that debris and haul it away.
• Urban soils are often compacted, which makes the soil a less livable environment, even for microscopic organisms.
• Urban soils may have other issues, like water availability, drainage, contamination, etc.

Like all living organisms, trees need water. All of the water they need, they take up from the soil through their root systems.

Trees use water is a number of different ways.

• Water coming into the root system brings along many of the essential elements (nutrients) the tree needs to live and grow.

• Water evaporating from leaf surfaces draws more water up through the vascular tissue (the xylem), at the same time cooling the leaf so that photosynthesis can continue; when leaves get too hot the chemical processes stop working right and no new sugars are made.

• Water is fundamental to processes going on inside each and every cell, both as the internal fluid of the cell and as an “ingredient” in biochemical reactions occurring within and around the cells.

Trees need a lot of water, but how much? Unfortunately, we don’t really know, in terms of gallons per week – good research on this question hasn’t been done, and the answer depends on many variables. Often we suggest trees be watered every other week, but with a lot of water spread over a lot of soil and allowed to soak in.

What is really important to the tree is the combination of a large volume of water into a large volume of soil. While trees can be grown using drip irrigation systems, to adequately water a tree of large size, one would need dozens of emitters spread throughout the root zone. Drip systems are usually set to run too often, but not long enough, to adequately water a tree. Finally, the bigger the tree, the more water it needs. As the years go by and the tree grows, it will need more and more water, so any automated irrigation systems need to be tweaked every couple of years. Add emitters out to the edge of the branch canopy (the drip line) and beyond; old emitters at the base of the trunk can be capped over time.

How often we need to water trees depends on a few things. Recently transplanted trees need to be watered more often than established trees because their root systems haven’t yet grown out of the restricted form they had in the nursery pot. Water these trees a few times a week.

• Dormant trees should be watered about once per month, so they don’t start the growing season already needing water.

• Trees in small soil volumes need watering more often than trees with unrestricted soil volume (assuming the latter is watered across the whole available soil surface).

• Trees in sandy soil will need watering more often, but with less water. Trees in clay soil will need watering less often, but with more water.

Newly planted trees have to make a transition from the limited resources but constant care of the nursery, to the much larger resource base but much less frequent care they will get once established in the landscape. It takes them a bit of time to make this transition. Water very well at planting, and for the first month or so, water a few times per week, making sure the original root ball is getting moistened. Feel this soil prior to watering, however, to avoid adding water to an already wet soil – that could lead to root dieback and rotting. Over time, water less often but with more water at each irrigation event.

Trees are often damaged when new construction cuts through roots, or compacts soil in the root zone. Also, trenching to repair water, sewer and irrigation lines can cut through roots, leading to tree health problems. Sometimes that damage can be avoided by using compressed air excavation tools, which do much less damage to root systems.

Sometimes we are tempted to address the weak soil food web by adding synthetic chemicals to the soil (fertilizers). While these can provide the needed nutrients, it is hard for us to predict the amount needed, the timing of the need, and the possible negative effects. Fertilization tends to throw the soil food web farther out of balance, may lead to salt build up in the root zone (a bad thing!), and may add to pollution in waterways when excess fertilizer moves into surface and subsurface waterways. Use fertilizers very lightly with ornamental trees! An over-fertilized tree may actually attract more pests and have more problems than one on a “lean diet”.

Do not fertilize newly planted trees. Let them get established for a couple of years before specifically fertilizing them. Unless there is some reason to push growth at an unnatural rate, don’t fertilize them at all. Decomposing woodchip mulch will provide slow-release nutrients. Trees in lawns get enough fertilizer from turf applications, so don’t apply any more. Trees do not generally need rich soils.

Trees, like other living things, will sometimes have pest and diseases problems. Like other living things, the healthier they are the fewer problems we would expect them to have, and the less damaging those problems are likely to be.

Most of the insects you will see in your landscape ARE NOT harmful pests. Don’t kill them just because they are icky and have six legs! Find out if they are really a problem, and if so, how bad of a problem, before spraying toxic chemicals into the environment in an attempt to kill them.

Environmental stress underlies many of the pest and/or disease problems our trees have. Trees struggling to grow can’t protect themselves as well as trees growing strongly. The most common environmental stresses in Albuquerque are lack of water, and heat stress; those two often occur together. Likewise, often pests and diseases occur together, with one following the other into a target tree. Discovering and resolving the stress factor may be the most important step in addressing pest or disease problems.

Many companies offer pest and disease control services. Not all of them can good diagnosticians, so getting more than one opinion can be very helpful. Government funded services like the Bernalillo County Cooperative Extension Service (bernalilloextension.nmsu.edu or 505-243-1386) can be very helpful in making accurate identifications and in prescribing appropriate controls, and in helping you know when those controls will be most effective. The life cycle of the pest or disease will have weak points, and those are what we try to target. Because this office is not funded by client payments, they have no incentive to provide anything but the most accurate information.

Pruning our trees can be very beneficial to them, but it can also be very damaging. All pruning wounds the tree, and the tree has to respond to the wound by trying to seal away the damaged area so that decay fungi cannot get at the interior wood. Smaller cuts seal more easily; larger cuts may never seal in time to prevent the onset of decay. Pruning also take away leaf tissue, and that limits how many sugars the tree can make to feed itself (through photosynthesis). Pruning too heavily can lead the tree into a starvation cycle. So – have a good reason before pruning!

Early pruning to guide growth into mechanically strong form is called structural pruning. Much like early childhood education, it is the most beneficial time to intervene in the life of the tree. Young trees pruned to eliminate weakness in the branching habit will not need nearly as much “remedial” pruning later in life. For information on structural pruning, go to: http://hort.ifas.ufl.edu/woody/structural-pruning-flash.shtml . If you are paying to have the work done, make sure the person making the pruning decisions knows these concepts and principles.

Prune initially at planting, and ideally every 2-3 years for the first decade of the tree’s time in your landscape. During the second decade, consider having the tree structurally pruned every 5 years. After this, minimal pruning should be needed if there is no storm damage or similar problem.

Mature trees that did not receive structural pruning may need to have larger branches taken off to avoid mechanical failure. Co-dominant stems with included bark zones are prone to splitting failure, which can be very dangerous if there are targets in the vicinity. For all mature trees, the bulk of pruning should be aimed at managing risk; a strongly structured tree may not need pruning for years and years. Some things to remember:

• DO NOT have a tree pruned solely on the advice of someone selling pruning services; consult with the local Cooperative Extension Service office for unbiased opinions.

• Do not judge the worth of a pruning job on the amount of wood the pruners removed. Less is better! In most cases, do not remove more than about 24% of the living tissue in a given year; much less on older and/or struggling trees.

• Do not have interior foliage and branches removed – the tree put them there for a reason. If you prune them out, they are likely to regrow in a few years anyway, because those interior leaves are very helpful to the tree in a variety of ways.

• Do not use pruning paint or sealant – it doesn’t help.

Sometimes branches need to be pruned off for clearance reasons – maybe they block traffic signs, or hang low over sidewalks. Try to see these developing and remove them early in the life of the tree. Remember that a small wound is much easier for the tree to deal with than a large wound.

Fruit trees and small ornamental trees may also be pruned to emphasize flowering and fruit production. Look up information on the particular species of tree to find the best pruning approach; we recommend looking at sites coming from universities, in particular from the Cooperative Extension Service departments at many land-grant universities.

Bad decisions on what to prune, and poorly executed pruning cuts, can reduce the structural integrity and health of the tree. If you are comfortable with gardening and landscaping, read up on the specific type/age of tree you have and make your pruning decisions based on that. If you are not sure about the pruning needs, consult a qualified, trained and reliable professional service. You can find a listing of local ISA Certified Arborists by going to the ISA website and searching by location.

Like any industry, there are good service providers, and not-so-good ones. It can be difficult to select the right people, but here are a few tips:

• Look for companies that employ ISA Certified Arborists. ISA stands for International Society of Arboriculture. There is a fairly rigorous knowledge test one must pass before obtaining ISA Certification. While it is not a guarantee of excellence, it does strongly suggest that a good base level of tree care knowledge exists in the person holding the Certification. Note that individuals are certified, not companies. Find out how involved the Certified Arborist will be in the actual work being proposed. You can find a listing of local ISA Certified Arborists by going to the ISA website and searching by location. Not all local Certified Arborists are for hire, some work in the public sector.

• Ask for references, and then contact them to see what the previous clients’ experiences with that company were like. Services like Angie’s List may also be helpful.

• Do not believe companies making claims that they can cure any and all tree diseases. Even those of us with a lot of training and education know relatively little about the tree/disease interaction. Miracle rescue chemistry doesn’t exist. The single most important chemical you can provide to your tree is H2O, also known as water.

By now, most of us have heard that the trees in Albuquerque are in some amount of trouble. We are losing our urban forest canopy at an alarmingly high rate. In large part that is due to the combination of urban site conditions in a SW desert city, and drought/heat stress. We also see problems from improper planting, insufficient watering, and poor species choices. Being a tree in Albuquerque is difficult, and though trees are remarkably tough, they still need our understanding, care, and attention.