Woody Plants

Before selecting and planting woody plants for your landscape, much must be done.  You will need to start by understanding the landscape and associated conditions in order to select plants appropriate for the various parts of the property.  So, perform a site analysis to document everything about the landscape site prior to plant selection.

Site Analysis

Many things go into creating a beautiful, yet functional, landscape.  Before such an endeavor is undertaken, you must assess and understand the site’s conditions.  These include climate, sunlight received by various parts of the property, exposure to wind, and many above and below ground features.  It is also important to select the correct combination of plants that will provide beauty while coexisting with each other. Consider what the landscape will look like during the different seasons of the year.

Climate and Microclimates

The climate and microclimates should be a major consideration when selecting plants for a landscape.  The USDA developed a plant hardiness zone map based on the average annual minimum winter temperature divided into 10° F zones.  This is useful in determining the types of plants that are likely to thrive in any location in the US.  In the South it is also important to consider heat hardiness since plants that are capable of surviving winter might succumb to the extreme heat and humidity.  A “Heat Zone Map” was created by the American Horticultural Society for just this reason.  It has 12 zones based on the average number of days per year where temperatures are 86° F or more.  Both maps are useful when selecting plants for your landscape.

In addition to the overall climate of the region where the landscape property exists, specific parts of the property itself can have experience different weather conditions called microclimates simultaneously.  These are influenced by sun exposure, wind exposure, vegetation, buildings, etc.  The northern side of a house can be several degrees colder than the southern side in winter.  Parts of the property with tree canopies will be several degrees warmer in winter because they reduce radiant heat loss. The trees can slow the rate of thaw in the winter mornings often reducing the amount of plant damage from the cold.  Buildings, arbors, and fences can also protect plants in a similar way.

Analysis of Above-Ground Conditions

There are many above-ground conditions that should be considered as part of your site analysis.  Below are just a few.

Sunlight Exposure

All plants need sunlight to create food through photosynthesis.  However, different plants need different amounts.  Some do best in full sun while others do best in partial sun or full shade.  Before selecting plants, it is important to observe and document how much sun each area of the property receives.  Be sure to consider that the angle of the sun can be very different from summer to winter and that there are simply more hours of daylight in the summer than in winter.

Plants that require full sun require at least 6 hours of direct sun per day.  Plants that require partial sun/shade need 3-4 hours of direct sun each day. Shade loving plants can adapt to areas with no more than 2 hours of direct sun or filtered sun/shade.  Shade loving plants may be able to hand direct morning sun but could suffer if exposed to multiple hours of midday or afternoon sun.

Wind Exposure

Wind can increase the amount of water that evaporates from a plant.  If the plant’s roots can grow uninhibited in search of water, this may not be an issue.  But if the soil is compacted or structures like buildings, driveways, sidewalks, streets, etc. confine the plant’s roots, then plants can be very vulnerable to the effects of wind.  Similar issues may arise due to wind exposure if the soil is sandy and well drained.

While such situations may sometimes be solved through proper irrigation, this is not always possible.  It may be best to choose drought tolerant plants.  If the soil is poorly drained as might be the case with clay soils, select a species that can tolerate both dry and wet conditions.

Overhead Power Lines

If you are planting beneath or within 6 feet of power lines, then make sure trees selected are no more than 25 feet at maturity.  If you are planting between 6 feet and 50 feet of power lines, then you will need to consider the size of the selected tree’s canopy at maturity. A tree with a mature canopy of 50 feet in diameter should be planted more than 25 feet from the power lines.

Existing Trees

If you plant trees or shrubs requiring full sun where they will be under or near the canopy of older established trees, then they will grow towards the sunlight and often become deformed.  Not only will such plants “lean” towards the sun, but often the side farthest from the sun will be thinner (i.e. have fewer branches and leaves).  It is typically better to use shade tolerant plants when planting in the shade of existing trees.

Analysis of Below-Ground Conditions

Some example of below-ground conditions that affect plant selection include the pH of the soil, soil texture, soil depth, underground utilities, and more. Often, these things are not considered which can lead to the failure of plants to thrive in the newly designed landscape.

If you are undergoing new construction and you determine the property has good quality soil, then you should work with your contract to save and store the quality soil for use after construction is completed rather than allowing it to be hauled away or buried.  Ask the contractor to limit the compaction of the soil.  Otherwise, it will need to be broken up and mixed with loose soil before planting can begin.

In urban areas you may need to do more extensive soil testing and analysis than in older rural areas.  Urban topsoil is often replaced with lower quality subsoil and may contain all sorts of debris.

Soil pH

The most important component of a soil test is pH.  Soil pH controls the availability of nutrients in the soil for plants to absorb.  It also affects the types of microorganisms and levels of their activity in the soil.  It is important that you test several areas of the landscape where there are distinct differences in soil color and texture.  If soil pH varies too much across the site, different types of plants may have to be used in areas with different pH levels.

Soil Texture

Understanding the soil texture of the different areas of a landscape is important.  If the soil is dense clay, then you will want to determine how well the clay drains so that you can select plants that are well suited for that moisture level.  If the soil is sandy and well-drained, then you may want to select drought-resistant plants for the site.  Sandy soils often lead to leaching of nutrients like nitrogen, potassium, and magnesium causing those nutrients to move below the root zone.  So, knowing the soil is sandy might cause you to modify your fertilization process to use a granular, slow release fertilizer or to select plants that grow well in infertile soils.

Compacted Soils with Low Oxygen

Compacted soils that drain poorly like clay have little room for oxygen between the soil particles which is vital for proper root health and function.  While some plants are tolerant of such soils, most will eventually die from disease or insect problems.

Tests should be run to check the soil around the property for compaction.  A percolation test can easily determine whether the soil is draining properly or not.  Another less scientific way is to simply smell the soil.  If it smells sour, then it is likely deficient in oxygen.  Such soil will often also be gray in color.  You may have to break open a clump of soil and hold it close to your nose to detect the sour smell.  But in some cases, you may be able to smell it while standing near a hole that was recently dug.

There are options for poorly draining soil.  You may be able to build a berm using topsoil that is like that of surrounding areas and place the plants on the berm.  However, you need to make sure the sides have a gentle slow so that runoff from the irrigation system does not erode the soil exposing the roots of your plants.

Another option is till the soil to loosen it.  Care must be taken to not till under the dripline (canopy) of trees or shrubs as you may damage their roots.  An air spade can be used under their canopy if necessary.

Compacted Subsurface Layers

In soils where the top layer is loose while sublayers are compacted, roots will tend to grow only in the loose top layer of soil.  These shallow roots can make trees unstable and even dangerous as they mature and grow larger.  For this reason, it is best to plant only small and medium sized trees in such areas where there is 2 feet or less of loose soil spread over compacted soil.

One way to detect such areas with compacted subsurface layers is to view the property a day or two after a large rainfall.  Those areas that remain wet after a day or two could have compacted subsurface layers and might require that you use moisture tolerant plants.  Such areas might not be detected if the landscape was only viewed during dry times.

Artificial Soil Horizons

Many landscapes are disturbed by construction equipment.  Sometimes this brings poorly draining clay soil to the surface.  Other times, construction debris and soils from other locations may be layered to create an artificial soil horizon or profile leading to poor drainage.  If the soil contains bricks, concrete, gravel, and other debris, then you should consider replacing the soil or at a minimum sifting it to remove the debris.

Salt Levels in the Soil

In states with a coastline, the soil often contains high concentrations of salt.  The level of salt present can easily be determined with a soil test.  When coming up with a landscape design, you should be mindful of the fact that the irrigation water could also contain high levels of salt if supplied by a well.  If you do not have access to good water for irrigation, then select salt tolerant plants that can thrive with your salty soil and irrigation water.

Soil Depth

When planting, it is generally desirable to have at least 5 or 6 feet of soil above the bedrock.  Before planting any large trees, you will want to determine how deep the soil layer is.  This can be done by digging a hole.  If the soil is shallow, then plant only small or medium trees.  Planting a large tree in such cases can lead to large surface roots that damage driveways, sidewalks, and foundations.  Such trees with shallow roots are prone to falling over in large storms.

Distance to the Water Table

The water table is the top of the zone where groundwater saturates spaces between soil particles and cracks in rock.  It is at this point where water pressure and atmospheric pressure are equal.  It is important to understand how deep the water table is.

To determine the how deep the water table is, dig a few holes 2 to 3 feet deep in the areas where you plan to plant. After a couple of hours, check the holes to see if water is visible.   If there is no water in the hole, then there is no need to consider the water table when selecting plants.  If it is water in the hole, then the site has a high water table which means you should select plants that thrive in wet environments.  Never plant large trees in locations where the water table is 18 inches or less from the soil surface as they will develop shallow root systems making them prone to falling over during storms.  The only large trees that might be appropriate for such areas are those that grow with underwater root systems like a bald cypress tree.

Underground Utilities

Before designing for a landscape, it is important to know where all underground electric, telephone, cable, water, sewer, and gas lines are located.  Contact your utility companies to have all such lines marked so that you know their location. 

Never plant trees over utility lines since those lines do sometimes have to be services which could result in damage to or removal of the tree.  Since the root systems of trees are generally as large in diameter as their canopies, it is probably wise to determine the maximum diameter of the canopy at maturity for each type of tree you plan to use in the landscape, divide the diameter by two to determine the radius, and then plant the trees at least a distance equal or greater than the radius away from all underground utility lines.

When planting around septic tanks, it is important to take even more precautions.  Once you determine the diameter of the canopy of a mature tree, plant the tree at a distance from the septic tank equal to or greater than the mature canopy diameter.  In other words, if a mature tree can have a canopy 30 feet in diameter, then plant the tree at least 30 feet from the septic tank to ensure the tree roots do not damage the septic tank.

Preparing a Planting Site and Amending the Soil

Before planting, many sites must undergo all sorts of preparation.  The first step is typically to grade the landscape to achieve the desired landscape form and correctly direct water flow away from structures and landscape beds for proper drainage.  Steep hillsides may need to be terraced to reduce runoff. If it has been determined that the soil is compacted, then it will need to be plowed or tilled to loosen the soil thus improving the ability of roots to grow and penetrate the soil.  Care should be taken not to cultivate under shrub or tree canopies as this could damage their root systems.  Finally, while the entire landscape may not require soil amendments, adding organic matter like compost to large planting beds will improve the structure of the soil and its fertility.