Woody Plants (Continued)

Selecting Plants for Your Landscape

When selecting plants for a landscape project, one decision that must be made is whether you want native plants or non-native plants that have adapted to the environment conditions of the planting area.  Whatever you choose, the plants should appear to fit naturally into the surroundings.  Care should be taken not to select invasive species.

There are many other things to consider when selecting appropriate plants for a landscape.  Knowing how much time and money the homeowner is willing to spend maintaining the selected plants is important to know so that you match your selection with their expectations and ability to maintain the plants.  It is important to consider susceptibility of possible plant selections to disease and insects so that, if selected, mitigation efforts to prevent damage from disease and insects can be factored into maintenance costs.  You may want to avoid plants and trees like Bradford pears that are susceptible to wind and ice damage due to soft or brittle wood.  Some plants and trees produce fruits and seeds that are messy or smell bad which should also be a consideration.

It is important to  consider the size that each plant or tree will reach at maturity, how fast they grow, and what their typical lifespan is.  You need to consider the potential size of the tree at maturity so that you can ensure enough ground space is allotted in the plan and its mature height will be compatible with structures (homes, buildings, etc.) and features on the property.  Generally, trees that grow tall work well around two story or taller structures but may dwarf one-story buildings throwing off the balance of the design.  For one story buildings, limit trees to those that do not exceed 35 feet or so when fully grown. 

Similar considerations must be made when selecting shrubs.  They should be selected based on their mature size to minimize the need for them to be pruned and to prevent them from hiding windows, encroaching on sidewalks, and crowding out other plants.  Plants should be spaced based on their mature spread so that ample space is left between them.

The life span of plants is another important consideration.  Some trees grow fast yet have a relatively short life span meaning that if selected, they will need to be replaced after a certain period.  Again, this adds to on-going maintenance costs.

How to Choose the Right Tree Size

Once you have chosen a tree to plant in each spot, you must also choose which size trees to purchase and plant.  Choosing the right size tree should be based not only on cost, but also on watering requirements.  All trees bought at a nursery and transplanted in a landscape require frequent watering initially.  For small trees, regular irrigation is often required for several months.  But for larger trees, watering is required even longer.  If you can meet the watering needs and can afford it, then purchasing the larger tree might be appropriate if you want to speed the filling in of certain areas of the landscape.  However, if you cannot meet the watering needs, then select the smaller tree as it will require less maintenance.

How well the planting area drains is also a consideration.  Small trees with small root balls do well in areas with poor drainage.  Their root balls will avoid being submerged in water.  Larger trees with large root balls, however, may have the bottom of the root ball being submerged in water which could stress the tree.  As mentioned in previous chapters, any stressors like this will make the tree more vulnerable to injury from disease, pests, and drought.

How to Choose the Right Shrub Size

The ultimate height and spread of a shrub at full maturity as well as how long it will take to reach maturity should be considered when deciding which size shrub to purchase.  If you need a shrubs that will start out a certain size and basically remain that size, then you may want to consider purchasing a mature dwarf shrub.

It is important to consider the mature spread of shrubs when calculating how many are needed for a space and desired density.  In areas where you want them to give a mass effect, you will want to plant them closer together than you normally would.  This is also true if you are creating a hedgerow with shrubs.  But if you want each plant to take on its on natural form (e.g. a stand alone azalea bush) then you will want to plant the shrubs far enough apart that they do not encroach on other plants when all have reached maturity.

Selection Based on Production Method

If the site where a tree is to be planted is well drained and can be irrigated regularly, then the tree production method used to grow and harvest the tree is of little importance.  However, if the soil is poorly drained or it cannot be irrigated regularly, then the tree production method used to grow and harvest the tree can affect its chance of survival after it is transplanted.

Field-Grown Trees

Trees that are grown in the ground at nurseries and then harvested for selling are called field-grown trees.   Ideally, they receive drip irrigation and fertilization the first several years they are in the ground at the nursery so that they develop lots of fine root growth near the trunk.  This dense root system will develop into a healthier root ball that is larger and capable of storing more water once harvested so that it does not dry out as fast compared to trees growing using other methods.

Before being harvested, the roots of field-grown trees should be “hardened off”.  To harden off a tree, its roots are pruned several weeks or even months before harvesting.  During the hardening off period after the roots are severed and before harvesting, the redirects resources by slowing the growth leaf shoots so that its energy can be spent regenerating new roots to replace the severed ones.  The nursery will carefully and frequently irrigate the tree during this period.  The chemical changes that occur because of hardening off the tree makes it more tolerant of being transplanted into dry landscape soil where irrigation may not be available.  These hardened off trees also thrive in landscapes where irrigation is available.  If a tree is not hardened before harvesting, it should only be transplanted in landscapes where frequent irrigation is available for many months.

When the nursery harvests a field-grown tree, they wrap burlap around the root ball and secure it in place using nails, twine, or wire.  The root ball on field-grown trees are significantly larger and heavier than those of container-grown trees which makes them difficult to handle.  Harvested trees with ball-in-burlap root systems are durable, but you should be careful not to break or crush additional roots when transporting them. 

Container-Grown Trees and Shrubs

Most container-grown trees and shrubs are grown in plastic containers.  The tree or shrub container can either be placed above ground or below ground.  In recent years, nurseries have started using the pot-in-pot method where the containers containing the plant are placed below the ground inside another larger permanently installed container with specialized drainage holes.  This insulates the root system and more evenly distributes the roots compared to placing the container above the ground.

With container-grown trees and shrubs, the container is usually filled with some type of artificial or soilless growing medium.  This growing medium is often composed of some combination of coarse materials like peat moss, compost, sand, or bark which drains quickly preventing root rot.  This rapid drainage means containerized trees need to be irrigated one or more times per day during the summer months.

When container-grown trees are transplanted, the coarse growing medium allows moisture to be drawn out of the root ball quickly into the more fine landscape soil causing the root ball to dry out faster than it did in the container.   For this reason, they must be watered at least as often as they were in the nursery.  During summer months, they should be watered daily for several weeks or months if the soil is sandy and well-drained.  This is especially true for trees whose trunks are over two inches in diameter.  As the tree roots begin to grow out into the landscape soil, the can be watered less and less.

Roots of trees grown in standard plastic containers typically grow in a circle along the outside of the root ball and inside edge of the container.  When the tree is transplanted, the encircled roots should either be separated and straightened out or trimmed off the root ball to prevent them from eventually girdling and strangling the tree trunk.

Some nurseries grow trees in low-profile containers that are shorter and wider than standard plastic containers.  These low-profile containers create root balls that are great for poorly drained sites and compacted soils.  Circling roots are less of a problem with these types of containers since they are farther away from the trunk.

Air-pruned containers have lots of holes in the sides and/or bottom of the container.  They use the air entering the holes to naturally kill the tips of the outward growing roots thereby eliminating circular growing roots and forcing all tip growth inward.  This creates a better root system than trees grown in standard containers.

Nurseries have also started growing trees and shrubs in copper-coated containers. The inside of the container is coated with a copper compound that prevents circling roots from forming.  These copper-coated containers produce a superior root ball.

Bare-Root Shrubs and Trees

Bare-root shrubs and trees are field -grown and hardened off.  They are sold with literally bare roots, no soil.  If they are kept in a cool, moist place out of the sun until they are planted, they survive and thrive equally as well as field-grown and container-grown plants once they are planted.

How to Inspect Root Balls for Defects

To inspect a tree or shrub’s root ball for defects, one must often remove it from its container.  Before doing so, be sure to get permission from the nursery.  It is best to work with nurseries which know about the cultivation methods used to grow their plants.

One test for trees is easily performed without removing any growing medium from the root ball.  If the tree is staked, then remove all stakes.  Then while holding the root ball still, push the tree trunk back and forth a couple of times.  The trunk should bend but should not move at the ball.  If the trunk moves at its based before it bends or appears loose in the root ball, then it has a defective root system which may lead to instability when the tree matures.

Next inspect the tree for the location of the root flare – where the trunk begins to flare out at the roots.  If you cannot see the root flare, then run your fingers along the trunk into the soil to find the first root growing from the trunk.  If the root flare and first roots are below the surface of the soil, then the tree plant was either planted too deep or the nurseries equipment has thrown soil around the trunk (if field-grown) raising the level of the soil.  This can become an issue later as the tree matures if the excess soil is not removed to expose the root flare and top most root of the root ball before planting it with the root just below the surface of the soil.

You should check the root ball of trees for circling roots.  Clear the top 4 inches of soil or growing medium and about 3 inches out from around the trunk.  This can be done with your fingers or using a garden hose.  Inspect the exposed trunk and roots for circling or entangled roots.  Do not purchase the tree if you notice circling roots close to the trunk.  Circling roots farther out can be cut at the point where they begin to circle if they are less than one third the diameter of the trunk.  Similar to hardening off a tree’s roots, this may slow the growth of the tree temporarily but should help with the future survivability and growth of the tree.

If the tree was container grown, then check the bottom of the container.  While some roots escaping through holes in the bottom is normal, if roots larger than one fifth the trunk size are growing out of the bottom of the container, then cutting them to remove the tree from the container could damage the tree and reduce its chance of survival.  Smaller roots can be cut off with little chance of damaging the tree.  If you attempt to pick up the container and roots have grown into the ground under it, simply pick a different plant.

Next, lay the tree or shrub on its side and attempt to pull the container off the root ball.  If it does not come off, then press on the middle of the bottom of the container while holding the rim of the container.  If the root ball still does not break free, then it may be pot bound.

If you manage to get the tree or shrub out of its container, then the root ball should stay intact.  You should be able to easily return the plant to its container without losing much of the root ball.  If the root ball falls apart, then it could have just been transplanted into a larger container.  In this case you might be paying for a larger plant while getting a small plant in a large container.

If you notice lots of roots circling around the outside of the root ball or if the root ball is hard, then the plant is likely pot-bound or root-bound.  This can prevent the roots from penetrating into the landscape soil after you plant it or can choke (and possibly kill) the tree as it matures.  If you notice this condition, then do not buy the plant.

Healthy roots are never black.  If the roots on the surface of the root ball appear black then do not buy the plant as it was likely damaged from heat, freezing temperatures, or too much water.  If you are purchasing a bare-root tree, then make sure the roots have small roots with moist, whitish insides growing from them.

Evaluating Trunk Form and Structure

Trunks come in many shapes and sizes.  Knowing what to look for can assist you in selecting healthy trees and shrubs for your landscapes.

Strong healthy trunks are thicker at the bottom and get smaller in diameter as you go up the tree.  There should be no need for stakes to help support them.  Some nurseries will stake their trees.  If a tree has been staked for a long time, then they may be weakened.  Inspect the trunk to be sure it has a larger diameter at the ground than it does several feet up the trunk.

Next, move the tree to an open space and remove any stakes.  If it has leaves and stands straight, then it is probably strong.  You can also wet the leaves with a garden hose is if it were raining.  Make sure the trunk remains erect even after there is water weighing down the leaves.

Trees with One Trunk

Single-trunk trees are generally stronger than multi-trunk trees and more desirable for landscape design.  A few smaller trees like crape myrtles and wax myrtles are exceptions and usually have multiple trunks.  But even these can be trained to have a single trunk at the nursery.

Large trees that mature to 40 or more feet in height should have a single trunk far up into its canopy.  If the trunk bends a bit, this is not a problem.  When buying a tree, look for forks in the trunk.  If the tree forks in the top half, it can be okay if you remove one of the two branches before planting it.  However, if the tree trunk forks in the bottom half, then do not buy it.  Also, if a single trunk tree is extremely bent or has a severe dog-leg, then do not buy it.

You should use single trunk trees along streets and sidewalks as they are easy prune or umbrella so that they do not interfere with vehicles and pedestrians.  However, if you plant multi-trunk trees in these areas, they will likely become a problem as the tree matures requiring that some of the trunks be removed which could leave the tree disfigured.

Trees with Multiple Trunks

As mentioned previously, trees with multiple trunks are generally weaker than single-trunk trees.  But if the trees are smaller than 25 or 30 feet in height, multi-trunk trees are useful in landscape design and can add a lot aesthetically to the landscape.

Trees that have multiple trunks frequently suffer from embedded bark in the crotches.  Embedded bark occurs when a long crease in the bark develops which can run several inches or feet down from the crotch.  The bark gest “pinched” in the crease and can weaken the tree causing a trunk to split from the rest of the tree on a windy or stormy day.

Avoid buying multi-trunk trees with signs of embedded bark.  Also, avoid buying multi-point trees with narrow branch angles or branches that are more than half the diameter of the trunk to which they are attached.

Size and Arrangement of Branches

Branches should exist all along the trunk rather than grouped only near the top of the trunk.  This helps to distribute stress from wind throughout the length of the trunk.  A good rule of thumb is that at least half of the leaves should come from branches on the bottom two thirds of the tree’s trunk.  Branches should have diameters less than half the diameter of the trunk.  The crotches should be “U” shaped instead of “V” shaped since “U” shaped crotches are stronger.

Spacing and arrangement of branches is important on large trees that will be 40 or more feet high when they have fully matured.  The larger, main branches on saplings with trunks less than 2 inches in diameter should be about 6 inches apart.  Trees with trunks between 2 and 4 inches in diameter may have developed one or two permanent branches which should be at least 18 inches apart and not have embedded bark in the branch crotches.  Trees with trunks more than 4 inches in diameter will have several permanent branches which should be at least 18 inches apart.

For trees that will be smaller once they mature, spacing and arrangement of branches is less important.  Just look for with an aesthetically pleasing branch arrangement that will look good where you plan to use it on the planting site.

Inspecting for Damage

It is important that you inspect all trees for signs of pests, diseases, or physical damage before purchasing them for planting.  Not only do you want the trees you purchase to have the best chance of survival, but you also do not want to infect other trees at the planting site by introducing a new plant with a pest infestation or disease.  Inspect both sides of all foliage to see if mites, scales, lace bug, or other pests exist, especially if the leaves are speckled.  Speckling can be a sign of relatively harmless leafspot or other issues like sunburn or chemical damage.  If pests of any kind are detected, do not purchase the plant.

Scale insects are particularly hard to detect since their color looks a lot like that of the twigs and branches.  If you notice raised bumps on the tree’s twigs, attempt to pick or scratch them off with your fingernail.  If the bumps are scale insects, then the place where the bump was previously should remain relatively unchanged.  However, if they were not scale insects, then you should see green or white tissue that would normally appear below the bark.  These insects are much easier to see on leaves because their color will stand out.  If you detect scale insects, do not buy the tree.

Trees at the nursery should have leaves all the way to the end of their branches year-round unless they are deciduous, and it is their dormant season.  If the tree has leaves but only part of the way from the trunk to the end of the branches, then the tips of the branches could be dead.  If the tree is deciduous and dormant, you can still test for dead tips be scratching some of the twigs with your fingernail.  You should see green or white tissue below the bark indicating the twigs are still alive.  If you see dry, brown tissue instead, then the twig or branch is dead at least from that point to the tip.  This condition is called dieback, and trees in such condition should not be purchases.

You should inspect the trees for scars and open wounds along the trunk, and if present it is recommended that you avoid purchasing them.  Remove any trunk wrap if found (with permission from the nursery) and inspect the wounds beneath it.  If they are small pruning wounds, then the tree should be okay. Otherwise, do not buy the tree.  If the tree has large broken branches, then do not buy it.  If it has small broken branches, then they can be trimmed back to healthy tissue.  Inspect the tree for other types of wounds like those from stakes or ties.  Be sure the leaves on the tree are the same size and color as other trees there of the same species.  If they are not, then the tree may not grow well when it is planted.