By Andrea Mosher, Urban Forestry VISTA
Wilmington, DE (June 18, 2007)- What kind of a garden would you have if you hacked off about two-thirds of the roots each time you transplanted a clump of your favorite perennial- likely sparse, brown, and pitiful. It is surprising, then, to see plants transplanted in that way, surviving and growing along our streets and in our yards.
Trees can lose up to 96 percent of their roots when dug from the field for transplanting, and certain transplanting methods can be more stressful to the tree than others (Watson). The age old practice of bare root tree planting, used historically by nursery professionals before the advent of mechanized balled and burlapped (B & B) equipment, is regaining popularity for urban tree planting, in part because of its ability to avoid extensive root damage, which is a main cause of transplant shock. The bare root tree planting method is showing promise as a less traumatic, less expensive, and more practical way to green our yards and communities. This is encouraging as the need for more trees in the urban environment is realized.
Could bare root planting help to reach tree canopy goals?
Trees have a place in treating many of the major problems that come with the urban environment-reducing flooding, screening harsh views, cleaning our air and water, lowering energy costs, increasing property values, promoting neighborliness and curbing violence. Currently Trees for Wilmington, a working group of Mayor Baker’s Wilmington Beautification Commission, is working to develop a plan to increase tree canopy in the city. The ability of the urban forest to function as a natural utility (as opposed to manmade systems, like large stormwater holding tanks) in treating the environmental dilemmas of the city is reinforcing a case for more trees.
Urban forest movements are gaining momentum all across the country as the realities of pollution and need for solutions are recognized. Places like Los Angeles and Baltimore have committed to doubling their tree canopy over the next 30 years-both cities have been working on such efforts for at least 10 years. Based on a growing collection of data supporting the claims that trees positively impact the social, environmental, and economic health of cities, these ground-breaking efforts to ‘re-tree’ our cities are looking smart. Preliminary data analysis of Wilmington’s urban forest using the Urban Forest Effects (UFORE) model shows that existing canopy in Wilmington (~18%) is lower than comparable eastern cities like Baltimore or Annapolis, whose base canopy coverages were 25 and 40 percent respectively, at the start of their tree campaigns.
Another local canopy expansion effort, TreeVitalize- a partnership initiative of the Pennsylvania Horticultural Society, the City of Philadelphia, and the State of Pennsylvania-offers bare root trees to community groups as part of their effort to increase forest cover in southeastern Pennsylvania. The result: over 6,000 bare root trees planted since 1998. With proper planting, maintenance and preservation, a similar effort in Wilmington could work to double our canopy in the next 30 years!
So how could bare root trees facilitate our work to expand the tree canopy in Wilmington? It all stems from the roots…
The way the tree makes its way from the nursery and into the ground outside your house can be stressful enough, but whether the tree has a viable root system is the primary determining factor of whether the tree will survive and thrive in its new location. Current research by Nina Bassuk at Cornell University’s Urban Horticulture Institute indicates that many tree species fare better when transplanted bare root as opposed to B&B or containerized (grown in pots). When properly handled, it seems the bare root method is less traumatic for the trees’ roots, and can lessen transplant shock and recovery time.
The bare root method respects the fact that trees have shallow and wide-spreading root systems. The digging machinery disturbs a wider and shallower profile of soil than a tree spade used for digging B & B trees. The soil is shaken from the roots and the result is a transplant with a much greater percentage of roots remaining. The conventional argument for B & B trees over other methods is that the roots are protected by a soil ball when harvested and in transit. However, when harvested, the tree spade used to dig the B & B tree greatly reduces the root system. Root systems of container grown trees aren’t likely to be lost at the time of transplant, but are often put under extreme stress because the artificial media used in the container loses moisture too easily to the surrounding soil at the tree’s newly transplanted location. Containerized trees are also likely to have circling roots found to be highly detrimental, and often fatal, as the tree attempts to develop.
Time sensitivity is the major challenge of bare root tree planting. Trees must be dug and planted when they are dormant, typically between late November through March. Additionally, once the trees have been dug at the nursery, there is a limited shelf-life. Because the roots are exposed to air and not protected by soil to keep them moist, the roots can easily dry out and become desiccated. Therefore, bare root trees should be re-planted soon after harvesting, and when the trees are out of the ground the roots must be kept moist and cool to prevent desiccation.
Another major reason for tree failure is improper planting depth (Bassuk and Buckstrup 3). The fine absorbing roots are unable to access oxygen when a tree is planted too deep. Bare root trees are easier to plant at the correct depth. The root flare, the important interface between soil and air, is obvious on bare root plants and not as obvious on B & B or containerized plants, whose flares are often buried. When planting a tree bare root, it is also easier to prevent loss due to circling roots. The circling roots common in containerized and B & B trees are easily spotted on bare root trees and can be pruned or straightened before planting.
Of the three tree planting methods, bare root is the least expensive. In Bassuk’s studies, bare root trees were between one-third to one-half as expensive as B & B trees or containerized trees. Bare root trees are lighter and cheaper to ship than B & B trees of the same age/caliper, and they are also typically easier and less expensive to plant. One bare root tree can be lifted and moved by a single person, while a B& B tree can require two to three able adults plus powered equipment to maneuver. Consequently, bare root trees make for ideal community and volunteer planting events; on average a contractor-planted B & B tree will cost $500.00, whereas volunteers planting bare root trees will be able to plant 5-10 trees for the same price.
Although bare root planting promises to be a practical method for many planting circumstances, it is yet to be widely used. As trees gain more attention for the environmental benefits they provide in this age of climate change, and the demand for planting trees in the urban environment intensifies, the bare root method should be an effective way to expand the urban forest canopy.
Creating the Urban Forest: The Bare Root Method
Tree Transplanting and Establishment
Delaware Center for Horticulture
Wilmington Beautification Commission