By Len Phillips, ASLA Emeritus
Sometimes called “tree vaults”, tree planting pits are also referred to as “tree coffins” — and with good reason. Trees in concrete-laden urban settings struggle with many stresses that inhibit growth such as limited rooting space, air pollution, road salt, excessive temperature, vehicle impacts, etc. Although planting in tree pits is not ideal, in many cases pits offer the only opportunity for planting trees in a streetscape setting.
Tree pits should be constructed so that a continuous channel of soil under the pavement connects the individual pits and allows greater volumes of soil for root growth and water storage. Raised tree planting areas can likewise be designed to accommodate multiple, rather than single, trees.
Individual pits can be above, at or below the surface of the pavement. If they are above, extra provisions must be made for supplemental fertilization and irrigation. If they are at surface level, plant a ground cover at the time of tree installation to discourage foot traffic over the tree roots. If the pit soil level will be 2 to 8 inches below the pavement surface, install an adjustable pit cover or grate to accommodate trunk growth.
Unfortunately, planting success in a tree pit often follows what might be called the “Rule of Four:” the roots of a tree with a 4-inch trunk diameter will fill up a 4-foot by 4-foot by 4-foot pit within four years. This usually results in a growth slowdown or stoppage, and ultimately tree death. A planned street or sidewalk reconstruction offers a prime opportunity to build better tree planting sites. The following strategies are the best bets for ensuring tree survival and green streets.
The best method to prepare a pit for planting is to remove all urban rubble and refill the pit with uniform, good-quality loam. Soil ingredients should be thoroughly prepared and mixed before planting. The soil type should be consistent throughout the pit and to the outermost location available for root growth. Soils for tree pit design must carefully fit the location. Aeration and water — too much or too little — must be considered.
The CU-Structural Soil mix, sometimes called an engineered or load-bearing soil, is an alternative for planting in pits and under sidewalks. This formula, developed and patented by Cornell University, contains approximately 80 percent uniform sized stones mixed with about 20 percent loam and a small amount of material that sticks the soil to the stone. The mixture is designed to preserve large air spaces and ensure oxygen supply to the roots while providing a stable base for pavement. Amsterdam Tree Soil is another soil mix that will hold water for the tree while allowing for the modest compaction necessary for sidewalks.
A tree in a pit often suffers from oxygen deprivation due to poor drainage. In addition to filling the pit with a uniform soil profile, drainage can be facilitated by a system that will drain excess water from the roots. The pits can be linked with a network of pipes.
The typical tree pit has a volume of only 64 cubic feet. Research has shown that a tree with a 20-foot diameter canopy requires 300 cubic feet to have enough water for 10 days without rain. Trees in pits should be deeply watered two or three times during backfilling and deep, regular soakings thereafter. The use of pavers can be one way to enable water and oxygen to permeate the soil.
Citizen volunteers concerned with street tree survival are using an innovative irrigation system in Boston. Used in conjunction with a tree grate, the system consists of a 4-inch black perforated pipe inside a filter sleeve to keep out silt; a “T” joint; and a 4-inch round black slotted drain cap. The perforated pipe is laid in a circle just below the soil surface around the root ball, with the “T” joint leading up to the drain cap. Although the cap is protected beneath the grate, it is easily accessible with a hose. Boston’s tree advocate groups now only plant a tree when they have a volunteer committed to watering it. Once a week, the volunteer uses a hose to fill the system to overflowing, lets it drain, and then fills it again. The entire system costs about $10 per tree, and it holds up for about three years, long enough to get the young tree established.
Plastic irrigator bags are commercially available products that drain very slowly, so as to provide an effectively deep watering of young trees. Several communities have had success with these and most report that vandalism has not been a problem despite the visibility of the bags. A scheduled maintenance commitment is necessary to ensure that the bags are filled regularly.
Choose the right species
A critical step toward the creation of any sustainable streetscape is to select the right tree for the right place. Only small trees or flood plain species can survive in small spaces. Planting in self-contained tree pits dictates the use of slow-growing species that have a relatively small mature size and a tolerance for urban conditions. Near structures, trees with a columnar shape can avoid pruning later.
A tree grate serves as a sidewalk-like surface for pedestrian traffic, protecting the soil from compaction and still enabling water to reach the roots. Tree grates have fallen out of favor for street tree plantings because the concentric rings of the grates are seldom cut away as the tree trunk grows. Eventually the grate girdles the trunk.
Tree grates are available in standard sizes in round, square or rectangular shapes. The designs are Americans with Disabilities Act (ADA) compliant with openings no greater than 1/4-inch and are easily modified to accept up lighting. They can be expanded to not restrict the growth of the trunk.
The two most common mistakes when specifying tree grates are choosing inappropriate sizes and improper installation. If a tree grate is placed directly on the soil instead of suspended on a proper frame, it may cause soil compaction and vertical compression on the roots that will destroy a tree completely. Use tree grates with a minimum 12-inch opening for the tree and with removable sections that can be broken or cut out to allow for the growth of the tree. Fill the space between the finish grade of the tree and the tree grated with gravel larger than 1/4-inch to limit the accumulation of debris under the grate while still allowing air and water penetration.
A concept out of New Zealand — the Soak Stone tree grate — consists of a porous grate of small colored stones that prevents litter, assists tree growth and provides an attractive design. It is manufactured in two halves with an anti-skid surface and is virtually maintenance free. It is covered with pebbles that are contained in a galvanized frame that can be powder coated. The grate can be moved to another site once the trees are established.
Tree guards should extend vertically from tree grates and serve to protect trees in highly active areas. Tree guards should be narrow and painted in a similar color and related to other site furnishings. They are designed to protect trees from harsh weather, vandals and animals. They are manufactured in two or more pieces and are bolted together for ease of shipping and installation around trees.
Instead of tree grates, bricks, cobblestones, or pavers can be used to create a low-walled area surrounding the planting pit that is 6 to 12 inches above sidewalk level. This raised bed encourages pedestrians to walk around and helps limit compaction over the roots closest to the tree. An option is to lay unit pavers level with the sidewalk around the tree. Pavers allow air and water to move into the soil through the spaces between the pavers. Pavers are most effective when used in conjunction with structural soils — avoiding the need for a heavily compacted base of stone dust or sand.
Organic mulch is normally recommended around trees for water retention and weed prevention. However, it is impractical for street trees set level with sidewalks. Mulch may be used in a raised planting bed if the sides are high enough to contain it.
The pruning goal for a pit-bound tree is to keep it at a manageable size that will help it maintain a sustainable shoot-to-root ratio. Excessive pruning may help limit root growth below.
Maintenance on newly planted trees should be planned for at least one year after planting and preferably into the second and third years. Volunteers can be great assistants for watering and for training young trees with structural pruning from the ground. Volunteers are enthusiastic, often protective advocates for public trees, and they are cost-effective labor.
In many communities, planting in pits is one of the primary opportunities to grow trees. Even though pits are not perfect, sound planting and maintenance practices can go a long way toward compensating for the limiting site factors that they present.
Above-ground containers that are decorative and sit on top of a paved surface come in a wide range of sizes, colors, and materials. The sizes for trees range from 90 gallons (44-inch diameter to 18 inches high) up to 450 gallons (70-inch diameter by 30 inches high). They can be made of wood, plastic, metal, ceramic, stone or pre-cast concrete. They are used in areas where they have the most visual impact.
Always pot the right type of plant in the container that would fit its root systems. Be sure that the container offers ample soil support and water retention. Plants need to be continually watered in containers and drain holes are necessary to remove any excess water. Container planting means constant watering — especially in hot dry spells.
Self-watering planters have a reservoir with a hose-fill attachment. By keeping the reservoir filled, the plants will not run out of water. The design provides continuous soak, utilizing capillary action through perforations that expose the soil to the water reservoir. If there are heavy rains, the planters should have built-in overflows to eliminate over-watering.
Len Phillips is administrator of Online Seminars for Municipal Arborists, http://on-line-seminars.com
Note: The author referenced the materials listed below when compiling this overview.
Appleton, Bonnie, “Trees for Parking Lots and Paved Areas.”
Doherty, Karen, David V. Bloniarz and H. Dennis P. Ryan, “Positively the Pits! Successful Strategies for Sustainable Streetscapes.”
K Accents: http://www.kaccents.com
Neely, Dan, and Gary Watson, “The Landscape Below Ground II.”
Watson, G. and Dan Neely, “The Landscape Below Ground.”
Small-stature trees for small planting sites
Karpick Red Maple
Robin Hill Shadblow
White flowering Dogwood
Van Eseltine Crabapple
Japanese Tee Lilac
Summer Snow Japanese Tree Lilac