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Large tree roots growing up through a sidewalk causing the paving slabs and tarmac to crack and lift up.

The Design is part two of four in the Street Trees: Site Selection, Planting, and Maintenance in the Urban Landscape series.

Roots! Roots! Roots!

One of the biggest concerns for the design process is the provision of enough space for the roots of the tree to grow. The amount of space required depends primarily upon the tree’s size at maturity and its expected lifespan given the amount of stress to be endured by the tree. An old horticulture maxim is, “If you create a root zone environment in which the roots can thrive, the top of the plant will take care of itself.”

Diagram of a successful landscape: design, site, plant material, specifications

You want to create a design that will minimize root stresses. Some of these stresses may include soil compaction by pedestrians or cars, excessive heat reflecting off sidewalks or buildings, or an imbalance of water and air in the root zone. A good quick rule of thumb for deciding the size of the planting area is to estimate the diameter of the tree at maturity. For each inch of diameter, allow 2 feet along the side of a square planting hole. For example, an oak that is 12 inches in diameter will need a protected root area of 24 feet on one side of the planting hole or 576 square feet of rooting space.

For each inch of diameter of a mature tree, allow 2 feet along the side of a square planting hole.

What age should you use to determine the maturity of a tree? The greater the site stress levels, the shorter the life span of a tree. For city streets and downtown parking areas, estimate maturity at 7 years. Moderate-stress sites in residential street plantings or lowstress sites in residential yards or parks have more generous maturity estimates of 15 and 25 years, respectively. If a circular planting area is specified, determine the diameter of the planting area by estimating the tree diameter at maturity and multiply this number by 2.25. (Example: If you estimate the ultimate desired diameter for a tree you have selected to be 12 inches, multiply 12 x 2.25 = 27 feet diameter required for a circular planting area or 572 square feet root zone area.) Porous pavers can be used to give the desired root zone area if space is limited.

Rejuvenation projects present special problems when finding proper space. Physical constraints in the area may limit options. Therefore, species selection is important, and you must take into consideration that the tree may have a shortened expected life span. However, new development projects call for cooperative planning and allocation of the proper amount of space needed by a tree.

A design feature that can provide an optimal growing environment for trees is the creation of continuous planting beds rather than individual planting holes for each tree. Join concrete spaces and provide walkways between the trees. Mulch the area with compost from the city’s  maintenance and compost program. The result is an inexpensive and aesthetically pleasing way to provide some relief for restricted planting sites.

A good design considers possible drainage problems. If you pour water into a planting hole, and it does not drain within 5 to 8 hours, corrective actions are required. Before taking any expensive corrective measures, dig the hole a few feet deeper to see if there is an impermeable pan restricting drainage. You cannot change the soil, but you can alter the site to minimize the impact of the soil’s poor physical characteristics. Drainage tile can be installed, subsoiling can be used to break up pans, or raised planters can be built. A combination of these measures, along with selection of a tree that tolerates a low oxygen root environment, will enhance establishment success.

Tree Selection

Tree selection for the site is very important to the success of the project. Consider each of the following factors:

  • Soil type, pH, drainage, and volume.
  • Available light and canopy area.
  • Environmental extremes of temperature, pollution, and salt.
  • Potential aesthetic impact.

Our best barometer for the success of a tree in a particular area is experience. Rejuvenation plantings are often very restrictive, and tree selection is crucial. Check with a local certified retail nursery dealer, your county Extension office, or state Urban Forestry office to get a list of trees that would be suitable for your area and planting conditions.

A common mistake is to find one tree that has proven successful and to plant only that tree, creating a monoculture within our cities. This lack of diversity can be disastrous. A recent review of several of our cities’ street-tree inventories reveals that sometimes as much as 85 percent of a city’s street-tree canopy is composed of one species. The decline of the American elm is an example of what the cost of a monoculture can be to our cities.

NOTE: If you are planting in the Black Belt region of Alabama, contact your county Extension office for suggestions for trees for this area. Your area presents special problems for urban trees and proper tree selection is impressive.

Continuous planting beds allow more root zone area and are aesthetically pleasing.

Encourage diversity and continuous experimentation so that we can expand the list of available trees for our cities. Experiment not only with new species but with tree cultivars within these species identified as having greater tolerance of particular urban soils. With proper design and space allocation, your menu of trees from which to select increases.

Tree Size

It is better to invest in smaller trees and plant them properly than to spend your limited funds on large trees and not install and maintain them properly. Smaller trees are preferable for more reasons than money. Smaller trees are easier to transplant and establish and often out-grow larger transplanted trees within the first 2 to 3 years. Be careful not to select trees that are small enough to be subject to vandalism or mechanical injury. Trees with a trunk diameter of 11⁄2 to 3 inches are usually large enough to discourage vandalism and large enough to be easily seen by machinery operators. Smaller balled and burlapped

Seedlings in a tree shelter

Seedlings in a tree shelter

trees establish more quickly than larger trees due to less root loss and subsequent stress experienced by larger trees. For mass plantings, research that is promising and ongoing is using seed or small seedlings with the protection of tree shelters for site plantings. Look for new research releases from the Horticulture Department and School of Forestry and Wildlife Sciences at Auburn University. Be sure the trees you purchase are from a reputable nursery and consult the American Association of Nurserymen’s American Standard for Nursery Stock. This publication outlines the minimum guidelines for acceptable root balls or container sizes for various classifications and sizes of trees, shrubs, bulbs, corms, tubers, and Christmas trees. Extremely valuable for any group involved in the purchase and installation of nursery stock, this publication can be purchased through the American Association of Nurserymen, 1250 I St., N.W., Suite 500, Washington DC 20005.

Trees are grown, harvested,and marketed in three forms: balled and burlapped (B&B), in containers, or bare root. There are new variations to these methods, such as bagged plants or plants grown in foam containers, but these three production and marketing methods comprise the majority of plants grown and sold in the United States. Each of these methods requires different handling for proper establishment.

Selecting and Inspecting B&B Plants

B&B trees are grown in a field and dug up during their dormant season (fall and winter). A portion of the roots and soil are packaged in burlap or a synthetic material to hold the ball together and protect the roots until planting. When planting B&B trees, choose trees grown locally. Having the same soil type and climatic conditions provides roots with a familiar environment and lessens transplant shock. They are also more likely to thrive in local climatic conditions. Locally grown trees are not always available and are not crucial to successful establishment, but their use is a consideration. Balled and burlapped trees have a greater chance of establishment if they put back in the hole and allowed to put out new roots and acclimatize. Request root-pruned plants.

Before planting B&B trees, inspect the root ball to be sure that it is firm. A broken ball means broken roots. Move the trunk of the tree to be sure it is stable and securely anchored by the roots. Also inspect the trunk to see if ties have rubbed or girdled the bark. Wet, mushy root balls may indicate compaction and loss of soil structure and air space.

Purchase trees in balled and burlapped, containerized, or bare root forms.

Planting and Inspecting Container Trees

Container plants are trees grown in a container with a coarse, organic soil or medium. Container trees are rapidly gaining popularity because you transplant 100 percent of the roots. B&B plants, when dug from the field, loose 85 to 95 percent of their roots in the digging process. Root loss creates some obvious stress to the tree. Because of their intact root balls, container trees can be planted year-round when provided with proper care.

Plants are grown in a container in pine bark or some other coarse organic medium. This allows for rapid, healthy growth in a container but can be a limiting factor to establishment in the landscape. The coarse texture of the container medium is very different from landscape soils. The surrounding soil’s attraction for the water is often far greater than the bark’s attraction for the water. If the surrounding soil is dry, water is drawn away from the container root ball. Realizing this fact, it is very important that these plants, as well as the surrounding soil, be watered every 2 to 3 days, depending upon local climate and soil conditions, for the first 6 months. Once the roots have begun to grow into the surrounding soil, reduce watering to one or two times per week for the first year.

Inspect container plants by removing them from the container and inspecting the roots. Make sure roots look healthy and extend to the bottom of the container or fully use the available growing space. Be sure that the roots are not root bound or so matted that establishment will be a problem. If plants are root bound, do not score or cut down the side of the roots, as is often recommended. Because roots circle inside the container, scoring the sides of roots disconnects them from the plant. Physically rub the ball to expose the roots to the surrounding soil or pull out the lowest kinked root and try to straighten it or cut a portion of it off. Of course the best idea is to avoid these types of plants. Containers which use air or copper to prune roots have been designed to help eliminate this problem.

Selection and Handling Bare-Root Plants

Bare-root plants are dug from the field during dormant season with the soil removed from the roots. This offers an advantage of reduced cost in shipping and  can be an acceptable planting method if precautions are taken. Roots are exposed and can easily dry out and die. When these plants are received they should be immediately unpacked and watered. Cover the roots with moist saw dust, compost, soil, or some other media (heeled-in) and place the plant in a shaded area or store it in a cooler until planting. These plants should be received and planted while still dormant or shifted to a container to allow for root growth.

Preparing the Site

Fertilizer and Soil Amendments

When evaluating the site for the selection of trees and the design, it is important to take a soil test. Soil pH or essential element content can be a limiting factor to many trees. Since some of the required elements for plant growth such as calcium, phosphorus, and potassium move very slowly in the soil, it is much easier and more efficient to amend these soils prior to planting. Your county Extension agent can help you interpret your soil test results.

Proper tree planting and area preparation. The ball sits on undisturbed soil, planted no deeper than it was grown.

Proper tree planting and area preparation. The ball sits on undisturbed soil, planted no deeper than it was grown.

 

It is also good to provide organic matter to many of our urban soils. Till 4 to 6 inches of pine bark, or some other available organic compost, into the entire planting area to a depth of 6 to 12 inches (not to backfill). There is no benefit to just adding this material to the backfill in the planting hole, and this practice can be detrimental to establishment. A uniform planting area is needed for good root growth.

The Hole

The minimum-sized planting hole should be 2½ times the diameter of the tree root ball and no deeper than the height of the root ball. The old recommendation of digging a deep hole and loosening soil beneath the root ball was made with good intentions. However, heavy root balls settle in the loose soil, resulting in a planting that is too deep. A tree that is planted too deep will have limited oxygen for root respiration and will develop stress. Research shows that most of a tree’s roots are in the upper 12 to 18 inches of soil and extend out two to three times the width of the canopy. So, provide a root environment that is wide and shallow. It is better to plant too high than too deep. If you want to loosen the soil to a greater depth, leave a firm pedestal for the root ball to rest on to prevent future settling.

Dig the edges of the hole at a 45-degree angle. After digging the planting hole, it is a good idea to make grooves in the sides and bottom of the hole. Many Alabama soils are high in clay and have a glazed appearance after digging, especially when using large mechanical tree spades. Glazed sides restrict penetration of roots into the surrounding soil. These roots may never get out of the original hole to establish themselves in the native soil. This limits their nutrient and water resources and could lead to roots that circle the hole and eventually girdle the trunk. When planting trees dug by a mechanical tree spade, it is better to dig the planting hole with a back hoe and dig the desired wide shallow hole rather than “plugging in” the tree to the same-size planting hole.

Street Trees: Site Selection, Planting, and Maintenance in the Urban Landscape series

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