Urban landscaping from the ground up
Although tree planting is most often promoted as a way to beautify the urban landscape, its benefits run far deeper. In addition to improving aesthetics, trees play a role in cooling America’s cities, controlling erosion, and improving air and water quality. They are investments that can pay dividends for generations.
Unfortunately, some cities undertake tree planting without a plan for success. They choose inappropriateplants or sites, ignore proper planting procedures or neglect the trees after they are in the ground. Consequently, their investments fail.
A successful tree-planting program incorporates preparation, planting and maintenance. By following guidelines for each of the components, municipal arborists can minimize the risk of tree failure and ensure that their communities reap all the benefits of their plantings.
A tree’s long-term health depends first on the site on which it is planted. A substantial surface area and loose, well-mixed soil are essential for growth.
To ensure that a site will accommodate a tree’s roots at maturity, arborists should select a site that has: * 4 square feet of surface area for every inch of diameter the tree is expected to attain, or * 2 cubic feet of soil for every square foot of the tree’s future crown.
Even with adequate space, a tree will not flourish in poorly prepared soil. Unfortunately, defective soil is all too common in urban areas, where trees are planted in soil that is compacted, compromised by construction or subject to flooding and poor drainage. Typically, urban soil is 12 percent air, 12 percent water, 1 percent organic matter and 75 percent minerals. However, the ideal mixture is 25 percent air, 25 percent water, 5 percent organic matter and 45 percent minerals. Soil analysis prior to planting can tell an arborist whether the site is unbalanced in pH, salt, nutrients, toxins or other amendments.
Furthermore, if the site contains foreign matter such as subsoil, concrete, metal, building materials or clay, it must be improved prior to planting. * Heavy clay content can be remedied by adding sand, increasing soil volume by as much as 50 percent. * Organic loam should be added to soil that is pure sand. * High saline can be balanced with gypsum. * pH can be lowered with sulfur or aluminum sulfate. In addition to being balanced, the soil must be permeable to accommodate water, air and roots. If the site contains foreign matter, the worst of the material must be removed, and organic matter such as compost, peat moss or peat should be incorporated. The soil should be prepared 30 inches deep for maximum root vigor.
Selection and planting
Tree selection and planting go hand in hand with site selection and soil preparation. Choosing the wrong tree or planting it improperly translates into wasted time and money, as the tree will surely fail.
While aesthetics are an important part of tree selection, arborists must remember that all trees do not thrive in all climates or soil types. To assist arborists in selecting the foliage that will grow best in their communities, the U.S. Department of Agriculture’s National Arboretum has published the Plant Hardiness Zone map and Hardiness Ratings. (See the map on page 70.) Choices should be based on hardiness, including resistance to pests and disease; diversification; tolerance for urban conditions; and maintenance needs, including water.
Once their selections are made, arborists should adhere to the following guidelines for planting: * The planting area should be five times the diameter of the planting ball, and it should allow for drainage. * A rototiller or shovel should be used to loosen and mix the soil in the entire area to a depth of about 12 inches. * In the center of the planting area, the arborist should dig a shallow hole that will allow the root ball to rest on solid ground rather than in loose soil. * Remove all rope, wire and artificial burlap from the root ball, and set the tree in the hole. Natural burlap should be loosened, rolled back from the top and down the sides of the ball. Position the tree so that it is vertical and will not interfere with traffic or harm pedestrians. (Note: Trees that have been balled and burlapped should be planted 1 inch higher than they were in the nursery to compensate for settlement.) * Add soil gradually. For bare-root trees, the soil should be firm at the base of the roots.
However, do not pack soil with your feet, as that often results in excessive compaction; instead, use water to settle the soil. Add more soil and water, and work the soil under and around the lower roots. Raise and lower the tree gently to eliminate air pockets and to bring the roots in close contact with the soil.
For trees that have been containerized or balled and burlapped, the backfill soil should be lightly packed around the root ball. Again, use water instead of feet. * When the roots are just covered with soil, add water and let the soil settle. * Continue adding soil, and, when you reach the halfway mark, fill the hole with water. * Finish filling the hole with loose soil, and water it thoroughly. * Using the top soil, build a 4-inch-high dike to form a saucer around the circumference of the root ball. The saucer will catch water and prevent runoff for a short time after planting. Fill the saucer with water two more times within 24 hours of planting; even if it is raining, the watering will eliminate trapped air pockets and reduce transplant shock.
At sites where space is limited and digging is restricted (e.g., near sidewalks), the arborist should ensure that the hole is wide enough to accommodate the root ball without any bending. He should remove as much existing soil as possible and replace it with loosened or amended soil. If possible, he should loosen or improve the soil under the adjacent sidewalk to accommodate root growth.
Moisture is essential to long-term tree health, so it is little wonder that maintenance focuses mainly on watering and mulching. However, tasks such as staking, fertilizing and pruning cannot be overlooked.
Trees are rarely irrigated properly, except by nature. A tree’s roots grow 8 to 18 inches underneath the ground surface, and they spread out to a radius equal to the height of the tree. When grass surrounds a tree, the grass roots will compete with the tree roots for moisture, oxygen and other vital nutrients.
Therefore, if possible, trees should have their own irrigation systems, whether they are in the form of sprinklers (with low-volume or micro-spray nozzles) or soaker hoses. Either technology will deliver water at a slow rate, allowing the moisture to penetrate deeply into the roots.
During the first season of a tree’s development, arborists should water the site weekly. After that, occasional watering may be required during droughts; or, on extremely hot or windy days, leaf misting might be needed several times a day.
While irrigation may be needed to get water to a tree, mulch will help retain the moisture. The mulch will reduce evaporation, cool the soil in the summer and warm it in the winter. The tops of most trees can withstand cold temperatures, and even roots will grow until the soil temperature falls to 24 degrees Fahrenheit. In most cases, mulch will suffice in keeping the soil at a suitable temperature. Arborists can use any locally available product that admits moisture and air. They should spread the mulch in the plant saucer at a depth up to 4 inches (if the mulch bed is too thick, it will smother the tree roots) and replace it annually.
While some arborists wrap trees to warm the trunks, they may actually be doing more harm than good. In fact, tree trunks do not need to be wrapped unless the climate is extremely harsh or the bark is so thin that the tree is susceptible to sunburn. Wrapping a tree’s trunk slows the plant’s ability to adapt to a new site; it provides a habitat for insects; and the wrapping string can girdle the tree. If a tree comes from the nursery with a wrapped trunk, arborists should remove the wrapping after planting is completed. In addition to watering and mulching, there are periodic tasks that the arborist can undertake to improve a tree’s development. However, like wrapping, they are potentially detrimental when overused.
Trunk movement is necessary for building strength, so a tree should be staked only if wind is a problem or if the tree develops a lean or was planted with bare roots. Only flexible stakes or guy wire should be used, and they should be removed no later than 18 months after the tree’s planting; otherwise, the tree will not develop the trunk strength necessary to deve lop and grow.
Most roots will respond positively to fertilizer, but the amount of nutrient uptake depends upon the age, health and vigor of the root system. Only 5 percent of a tree’s roots survive the transplanting process and are available to absorb fertilizer. Therefore, it is not necessary to fertilize a tree during planting.
Once new leaves have matured, a fertilizer can be applied lightly, and, when the tree is established, a slow-release fertilizer plus 2 to 4 inches of mulch will encourage plant vigor. Note, however, that fertilizer should be used only if there is a nutrient deficiency — indicated by yellowing leaves (rapid nutrient loss) or inhibited root and shoot growth (slow nutrient loss). Too much fertilizer will cause the tree to grow too quickly, which in turn causes the tree to become stressed and susceptible to insect invasion.
Trees should be pruned at ages 1, 3 and 5. Pruning more frequently will cause rapid branching, producing weak branches that are susceptible to storm damage.
A tree’s health and longevity are tied directly to the diligence of the arborist, who must make appropriate foliage selections, follow established preparation and planting guidelines, and stay up to date on maintenance requirements. Cities have many resources for information on all phases of a tree-planting program (see “Getting landscaping off the ground” on page 72). By investing time up front, arborists can improve their chances of success and ensure continuing growth for their communities’ long-term investments.