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Site Considerations When Selecting Plants

Planning Before Planting

A beautiful and functional landscape owes its success to many considerations. Chiefly among them is a keen assessment and understanding of the conditions particular to the landscape site. No matter how attractive, healthy, or well-tended the plant, if the plant is not suited to the site, it may not survive or fulfill its intended purpose. When selecting plants for the home landscape, consider the following critical factors:

  • planting site climate and micro-climates
  • direct and indirect sunlight an area receives throughout the day
  • exposure to wind and, in coastal areas, salt
  • various characteristics of the soil and the underground environment
  • man-made components such as utility lines, signs, fences, and buildings

Proper planning helps to ensure a long, healthy life for plants selected for the landscape.

Climate: The United States Department of Agriculture (USDA) Hardiness Zone Map divides the US into zones based on average minimum winter temperatures. Select plants from the correct hardiness zone that can tolerate the average lowest winter temperatures a region experiences. Figure 1 shows the most current USDA Hardiness Zones for South Carolina.

Figure 1. Find USDA Plant Hardiness Zone Maps at

Figure 1. Find USDA Plant Hardiness Zone Maps at https://planthardiness.ars.usda.gov/PHZMWeb/.

Conversely, plants also need to be able to tolerate hot summer temperatures. Plants not adapted to these temperatures may grow poorly and decline over time. Find plants appropriate for South Carolina landscapes by searching the Carolina Yards Plant Database. For questions concerning plants not found in the database, contact the Home & Garden Information Center at 1-888-656-9988.

In addition to the broader climate of the planting site, micro-climates (or pockets of weather specific to some regions of a property) will affect plants. In general, colder winter temperatures occur on the northern side of a house. In contrast, hotter summer temperatures occur on unshaded western exposures. Southern exposures will be the warmest during the winter. However, plants growing on a home’s south side may experience winter damage when mild daytime temperatures stimulate new growth. This new growth is not sufficiently hardened-off and cannot tolerate the sudden drop in temperature at night.

Tree canopies protect neighboring plants by reducing their radiant heat loss. In winter, the microclimate beneath a tree may be several degrees warmer than the surrounding air, and this small increase in temperature could keep some plants alive. Furthermore, the tree’s shade during the early morning slows the rate of thaw and can reduce cold damage to some species. Building overhangs, arbors, and fences may provide similar kinds of protection.

Above-Ground Site Analysis

Exposure to Sunlight: All plants require sunlight, with some species needing several hours of full sun exposure, while others prefer more shade. When possible, observe how many hours of sun and shade the site receives throughout the year before choosing plants for the landscape. Remember that the changing angle of the sun during the year will result in more hours of direct sun in summer than during other seasons.

Plants requiring full sun need at least six hours of direct sunlight daily and often produce the best form and growth when receiving sunlight all day. Plants labeled for full sun to partial sun/partial shade need approximately three to four hours of direct sunlight. Shade trees and shrubs usually perform best in filtered/partial sunlight rather than full shade. Shade-loving plants adapt to sites receiving less than two hours of direct sunlight in the early morning or filtered sun/filtered shade throughout the day. Often, shade-loving perennials and groundcovers are best suited for heavily shaded locations receiving less than two hours of morning sunlight. Be sure to protect plants labeled for partial shade to shade from hot afternoon sun exposure.

Wind: Wind increases the amount of water lost from a plant to the atmosphere. The root systems of plants growing in confined rooting spaces use soil moisture quickly, thus making the plants susceptible to further drought stress from the wind. Plants growing in very sandy, well-drained soils are also vulnerable. Well-managed irrigation can partially overcome such water deficits. However, the best way to manage water loss on a windy site is to select drought-tolerant species. See HGIC 1717, Plants That Tolerate Drought for more information. Choose species tolerant of both dry and wet soil for poorly drained sites.

Salt: Airborne salt can affect plant twigs and leaves or, after being deposited on the ground, their roots. Plants within a quarter of a mile of saltwater coastlines should possess some degree of tolerance to salt spray. Those exposed to direct spray along the dunes will need to be highly salt-tolerant. Though salt-tolerant plants can survive and grow with direct exposure to salty air, they are often deformed. Salt-sensitive plants grow poorly or die when exposed to salty air. For more information on salt-tolerant plants, see HGIC 1730, Salt Tolerant Plants for the South Carolina Coast.

Overhead Power Lines: Only plant trees and shrubs that grow to less than fifteen feet at maturity directly under or within 6 feet of overhead lines. Refer to Figure 2 for planting distance recommendations around utilities. Be aware that even a 50-foot buffer may be too close for trees with a large canopy. Check the mature canopy size of the tree species before planting. Choose appropriate species to avoid damage to overhead power, phone, or cable lines as well as to avoid forcing utility companies to have to prune trees growing into these lines.

Figure 2. Planting guidelines for around above- and below-ground utilities. Excerpted from Avoiding Tree and Utility Conflicts by International Society of Arboriculture.

Figure 2. Planting guidelines for around above- and below-ground utilities. Excerpted from Avoiding Tree and Utility Conflicts by International Society of Arboriculture. https://www.treesaregood.org/portals/0/docs/treecare/Avoiding_Conflicts.pdf

 

Buildings: Plants growing under the eaves of a building will receive little or no rainfall if gutters are present. Regular irrigation is necessary to prevent the soil from becoming too dry to support good root growth. Water running off the roof of a building with no gutters may supply plants under the overhang with several hundred inches of rainfall every year instead of the usual 50 to 60 inches. Locate plants so that water cascading off a roof will not land directly on the plants.

Most perennials and vines can be planted within a foot of a building, as their roots will not grow large enough to damage the foundation. However, trees and shrubs planted near the house foundation should have enough room to reach a mature width while leaving a 12- to 18-inch air gap next to the wall.

An air gap allows adequate air circulation for house foundation vents and helps prevent mildew. Proper spacing also prevents shrubs from becoming one-sided and appearing jammed against the building when mature. See fact sheet HGIC 1702, Foundation Plantings for more information.

Trees are most stable when they develop a uniform root system distributed more or less evenly in every direction from the trunk. The unbalanced root system that develops when roots meet a building wall can reduce the wind firmness of the tree. Large-maturing trees planted within 10 feet of a one- or two-story building can blow over, especially if the wind comes from the building-side of the tree. In most cases, planting a tree less than 15 feet from a building is not a good idea. As a rule, trees develop extensive root systems, spreading two to three times the width of the tree or more.

Existing Trees: Trees or shrubs requiring full sun will bend in the direction of the sunlight, thus becoming deformed when planted under the canopy of established trees. Choose shade-tolerant plants for planting in the shade of established trees. Avoid planting too close to the trunk of an established tree as tree roots spread outward two to three times the width of the canopy. Severing one large lateral root when digging around a tree can cause a loss of 5 to 20 percent of the root system.

Below-Ground Site Analysis

Good-quality topsoil is a precious commodity. If possible, when building on an undisturbed site, identify and preserve topsoil before construction starts. Work with contractors to save and store high-quality soil to use after construction is completed. Do not permit this soil to be taken away or buried. Also, work with contractors to prevent soil compaction in areas where trees will be planted. Isolate these areas with orange safety fencing or silt fencing.

More frequently, construction is already complete, and the soil has been modified and compacted by heavy equipment. Simply layering soil over compacted soil will not promote good plant growth. Break up compacted soil and mix with composted organic matter. See HGIC 1655, Soil Conditioning – Establishing a Successful Gardening Foundation for more information. Examine soil throughout the planting site and test each different type found. Have the soil tested through the local county Extension office. For more information on soil testing, see HGIC 1652, Soil Testing.

Soil pH: Soil pH is the most critical component of a soil test; therefore, never apply lime without testing the soil first. Perform soil tests of several areas of the planting site. The pH may be lower or higher next to a building because of sand, cement, or other materials used near the footings.

Soil Texture: While soil texture is not in itself a growth-limiting factor, it does indicate other soil attributes that influence plant growth. Clay, with its dense texture, may drain poorly if the terrain is flat or the ground has been compacted by heavy equipment. When planting in clayey soil, assess the level of moisture that will prevail when selecting plants. On the other hand, many sandy soils drain quickly. Choose drought-resistant species to eliminate or reduce irrigation needs.

Rainfall and irrigation cause nitrogen, potassium, and other essential plant nutrients to leach more quickly through sandy soils. Use slow-release fertilizers for sandy soil because water-soluble nitrogen fertilizers leach quickly. Choose native plant species or plants adapted to a sandy soil type, as these plants are typically tolerant of soils with lower nutrient content.

Soil Drainage: Soil type, compaction, and distance to water table determine soil drainage. Two types of drainage to inspect at the planting site are surface and internal. Proper surface grading allows water to penetrate the soil evenly and directs runoff away from unwanted areas to prevent pooling. Clay soils and compacted surfaces will cause more water runoff than sandy soils. To redirect flow, construct swales or gentle slopes in the landscape to direct water to the desired location. Rain gardens are an excellent way to collect surface runoff and allow it to infiltrate back into the soil.

Internal soil drainage is an essential factor for optimum root development. Most plants prefer well-drained soil that provides adequate water and oxygen to plant roots. Poor drainage can result in poor root development due to low oxygen levels, as well as root rot diseases.

Compacted Low-Oxygen Soils: Compacted and poorly drained soils contain little oxygen needed for plant roots to survive and grow. Though some plants tolerate soils with low oxygen, most grow poorly or die. Although any type of soil can become compacted, clayey soils cause plants the most difficult challenge.

To check for compaction and drainage of the planting site, dig a hole at least 12 inches deep and 12 inches wide in each section of the planting site when the soil is relatively dry. Soil that is difficult to dig may be compacted. If a pickaxe must be used, the soil is probably too compacted for planting without taking corrective measures. If the ground is easy to dig with a shovel, it is likely not compacted. Fill the holes with water and let them sit overnight. Refill the holes the next day. Measure the water level in the holes every hour until all the water soaks into the ground. Ideal drainage occurs when the water level drops approximately two inches an hour. Most plants with average drainage needs can do well in soils with readings between one to 3 inches per hour. Poor drainage is measured at one inch or less of water per hour, while drainage at four inches and over is too fast. Improve compacted and poorly drained soils by tilling and adding composted organic matter. Organic matter added to fast-draining soils helps to retain crucial moisture for plant growth. Do not till or add soil under the dripline (the area from the trunk to the outer reaches of the branches) of trees and shrubs. Doing so can cause severe damage to their root systems.

Create a mound, known as a berm, on poorly drained sites to elevate the root systems of plants that cannot tolerate wet roots (see Figure 3). Although building a berm may be the only option, it is not an ideal solution because of the various potential problems. This landscaping technique requires careful planning and may require help from an experienced landscape professional. The right plant in the right space is the ultimate key to success.

Subsurface Compacted Layers: Deep tilling or subsoiling relieves compacted subsurface soil layers. Avoid spreading soil loosely over a compacted area as this creates unique challenges when roots grow only in the loose soil but fail to penetrate the compacted subsoil. The resulting shallow root system can create unstable and potentially hazardous large trees. Consequently, plant only small and medium-sized trees in areas where less than two feet of loose soil is spread over compacted subsoil.

In landscapes with subsurface compacted layers, the lowest areas are likely to be wet during certain times of the year. Within a day or two after significant rainfall, evaluate the site and decide if choosing plants tolerant of wet sites will be necessary.

Soil Salinity: Some soils in coastal areas naturally have a high salt content. If unfamiliar with the area or salts are suspected as a problem, test the soil. Also, be mindful that irrigation water may be saline. Analyze well water along the coast to determine the sodium content. If clean water is not available, choose salt-tolerant plants or those that have been growing well in the area with the same irrigation water.

Figure 3. For poorly drained soil, plant trees with one-third of root ball above original grade, see examples above. Excerpted from Newly Planted Trees: Strategies for Survival Forestry Leaflet 17, by Don Ham and Larry Nelson, Clemson University.

Figure 3. In poorly drained soils, plant trees with one-third of root ball above original grade, see examples above. Excerpted from Newly Planted Trees: Strategies for Survival Forestry Leaflet 17, by Don Ham and Larry Nelson, Clemson University.

Soil Depth: In the ideal planting site, the soil layer above bedrock would be at least 5 to 6 feet deep. Dig a hole to learn the depth of the soil layer. If bedrock is close to the surface, or if there is little soil depth, plant trees that are small to medium-sized at maturity. Large maturing trees planted in shallow soil are likely to form large surface roots, which can disrupt foundations, driveways, sidewalks, curbing, and gardens. Furthermore, large trees with shallow root systems can topple over in storms.

Distance to the Water Table: The below-ground variations of a planting site and the surrounding terrain affect the distance from the soil surface to the top of the water table. The location of the water table often varies throughout the year. For instance, the water table that was within inches of the surface in one season may drop several feet below it during another season. When selecting plants, consider sites with water within one to two feet of the soil surface during part of the year as poorly draining.

To determine the distance to the water table, use a shovel, four-inch auger, or backhoe to dig several holes two to three feet deep around the planting site. Wait two hours. If water appears in the hole, the water table is high, suggesting a need to select plants that tolerate wet locations. See fact sheet HGIC 1718, Plants for Damp or Wet Areas, for more information.

If the distance from the soil surface to the surface of the water is less than 18 inches, choose only small or medium-sized wet-site trees. Large maturing trees will adapt to wet sites by developing shallow root systems and can then become unstable in storms. The possible exceptions are trees that grow with their root systems submerged in water, such as bald cypress (Taxodium distichum) and black gum (Nyssa sylvatica). If no water appears in the hole, there is no need to consider the water table when choosing plants for the site.

Underground Utilities: Before any digging or planting, determine the location of underground electric, telephone, and television cables, as well as water, sewer, and gas lines. In South Carolina, call 811 (http://call811.com/) for the underground utility marking service. Member companies will mark underground lines within three business days (not including the day of the call). Appropriately marked utilities provide their approximate location for safe digging. Digging holes without regard for underground utilities risks severe personal injury or death, as well as damage to the lines. Those damaging utility lines are responsible for paying for repairs.

Plant trees that are large at maturity at least 12 feet from major underground utility lines; the rule-of-thumb is to plant as far away as possible. Do not plant trees directly on top of a utility line because tree roots may damage the line and require removal if the line needs servicing.

Medium- to large-maturing trees planted near septic tanks and drain fields can cause damage with their roots. Although the roots of small-maturing trees and shrubs can also invade septic tanks, they seldom cause extensive problems. To be safe, plant a tree at least as far from a potential underground trouble spot as the diameter of its canopy at maturity. For example, a tree expected to produce a canopy 40 feet in diameter should be planted 40 feet from a septic tank or drain field. For more information, see HGIC 1726 Landscaping Over Septic Drain Fields.

Site Preparation & Soil Amendments

Preparation for many planting sites is necessary before planting begins. Needed preparations may include:

  • grading
  • tilling compacted soil
  • installation of irrigation or other utilities
  • the addition of gutters to a roof to control runoff during heavy rains
  • terracing to slow runoff and improve drainage
  • amending the soil

The first step to take in preparing the site is grading the soil to achieve the desired landform. It is crucial at this stage to create adequate surface drainage that directs water flow away from structures to areas where the water can slowly drain into the soil.

Increase the rate of root growth and root penetration into the landscape soil by plowing or tilling to loosen compacted soil. Reducing soil compaction speeds up establishment time and reduces plants’ vulnerability to pests, disease, and drought.

Most plants grow well despite average landscape soils not being modified with soil amendments before planting. However, composted organic matter is a good amendment for large planting beds containing groups of shrubs. Adding composted organic matter helps maintain a desirable soil pH, adds nutrient elements, as well as increases soil nutrient and water holding capacity. While the incorporation of organic matter is an excellent practice to improve soil quality, only add 20 percent organic matter per soil volume. This amounts to roughly two inches of organic matter tilled to a depth of ten inches. Too much organic matter can destroy soil pH and nutrient balance, as well as soil composition. When planting trees and shrubs in individual holes, do not add amendments. Instead, backfill the planting hole with the native soil removed from the hole. For more information on planting trees and shrubs, please see HGIC 1001, Planting Trees Correctly, and HGIC 1052, Planting Shrubs Correctly.

If this document didn’t answer your questions, please contact HGIC at hgic@clemson.edu or 1-888-656-9988.

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