Operation Tree Transplant

Transporting a large tree specimen necessitates careful planning before the move and comprehensive follow-up after the installation. Preparation involves examining moving routes, analyzing soil types and reviewing grading issues, while ultimate success depends on an attention to detail during the installation. J. Wickham Zimmerman, civil engineer operations manager, Valley Crest, Santa Ana, Calif., discusses these and other considerations for large tree removal and transplant.

1.       Preparation of a Large Tree for a Move – Several things must be taken into consideration before moving a large tree. These considerations are the location of overhead obstructions (such as power lines), underground utilities and existing improvements, such as curbs and sidewalks, as well as protection of the public if this work is to take place in a populated area.

The biggest issue that could arise here is the access to the tree with heavy equipment, such as a crane and truck. If the tree is already boxed and is at a nursery, this will normally not be an issue, however, if the tree is being relocated from its native location it can be very much of a factor.

The first thing to look at is the topography of the land where the tree is located. Is it flat or rolling? Handling a large tree on a hillside is challenging because the crane must be level in order to lift the tree onto the truck. If this is not possible, a flat area would have to be created with earthmoving equipment such as a loader.

Other site issues include the proximity of the tree to a steep vertical drop-off such as a cliff or the edge of a stream. Care would have to be given not to locate the crane and truck too close to the edge, particularly if the soil is unstable

Lastly, the rigging (method of attaching the tree to the crane) is critical to both safety and for the protection of the tree from damage. While rigging involves quite a bit of structural engineering, it is more of an art than a science. The tree’s total weight, including soil, the box and the tree itself, must be taken into consideration, as well as the tree’s center of gravity. The center of gravity is essentially defined as the point on which an object balances. Since trees are rarely perfectly symmetrical and centered in their boxes, determining the center of gravity can be a challenge, and is normally accomplished by trial and error with an experienced rigger, rather than by running extensive structural calculations.

Another factor that plays into this is how the tree is to sit on the truck. If the tree is a very large one, it is not uncommon that it would be set on the truck at an angle to minimize the vertical projection. Since care must be used to not damage the tree’s bark or limbs, the rigging is the key to an efficient and safe move.

2.       Issues Related to Soil Types – If the tree that is being moved is one that is being relocated, and not being delivered from a nursery, the type of soil will have a bearing on the size and method of excavation and loading of the tree onto a truck. If the tree is located in a clay soil, the soil will be denser and heavier than if it is in a sandy soil. Additionally, there are horticultural considerations that would need to be addressed in transplanting a tree from one soil type to another; however, these will not be addressed here (see the April issue of Lawn & Landscape for horticultural information on large tree transport and installation).

If the tree is located in an area of sandy soil (technically called cohesionless soil), the size of the excavation will be much larger than in a clay (cohesive) soil because the sides of the excavation will have a tendency to cave in. This can be particularly troublesome where the access to the tree is limited, or if you are in close proximity to a vertical drop-off such as a streambed. For purposes of excavation and bracing of excavations, the Occupational Safety and Health Administration (OSHA) recognizes four types of soil; Stable Rock, Type A, Type B and Type C. While the difference between types is too complex to get into here, OSHA requires that a “Competent Person” be on site during any open excavation. The “Competent Person” is responsible for determining the soil type and any protective measures that must be implemented to safely perform the excavation. A “Competent Person” is defined by OSHA in 29 CFR Part 1926, but essentially what it says is that this person must have specific training in, and be knowledgeable about, soil analysis, the use of protective systems, and the requirements of the OSHA Standard.

By OSHA requirements, excavations that are deeper than 5 feet require bracing or sloping, depending on the soil type and conditions (such as the presence of water). For a Type C soil (basically sand), the slope requirements are 1-1/2 to 1, which means that for every foot in depth requires the excavation to be 1-1/2 feet wide. In other words, a six-foot deep excavation would be 18 feet by 18 feet. Not only can this dramatically increase the volume of dirt to be handled, but also will require that the crane reach further to lift the tree, thereby requiring a larger crane.

3.       Subsurface Work and Grading Challenges – Normally, subsurface work is not required at the tree’s native location, other than as described above. However, if there are existing utilities in the area where the tree will be planted, those utilities need to be located. In Southern California, a call to Underground Service Alert (USA) is required and they will mark all public utilities within 48 hours. A list of the telephone numbers in each state can be found at www.contractorbulletin.com/CBIHome/CBIHome.nsf/DigSafe. These agencies will typically only locate public utilities, so if the tree is being planted on private property, private utilities will have to be located from as-builts or a private utility location firm.

Grading challenges could include leveling off the area for the crane pad, as mentioned above. The last item that would fall into this category is the backfilling of the hole where the tree was excavated. Since we are removing a volume of dirt roughly equivalent to the root ball, or box size, this dirt must be brought in (imported) to fill the hole once the tree is removed. The replacement dirt should be compacted to prevent future settlement. The soil should be compacted in lifts (layers) to about 85 percent of its maximum density, which could require the addition of water to achieve. If the site were remote, provisions for water would need to be made.

4.       Transportation Challenges – There are many challenges that arise from the transportation of large specimen trees, including the horticultural challenges of protecting the trees from the wind and weather during transportation. Depending on the size of the tree, its orientation on the truck and the type and size of truck or trailer, overhead clearance problems can be the most difficult and expensive to overcome. Before the tree is moved (and really, this type of thing is reviewed prior to selecting the tree for relocation), the route of travel must be evaluated closely for overhead obstructions such as power and other utility lines, bridges, tree branches, etc. Utility lines can be moved, if necessary, but the cost and coordination time can quickly rule the tree relocation impractical. In addition, concern must be given to the weight capacities of any bridges to be crossed and any tight turns that might be impossible for the truck to negotiate. Obviously, the truck driver must be sure to get any permits required by the size of the tree, which could limit the time of day the truck is on the road.

Lastly, local and regional laws must be understood that could affect the trees' relocation, such as requirements to quarantine or laws which may preclude the removal of a particular species altogether. In certain cases, an Agricultural Department inspection and certification might be required, and depending on the local governments, an inspection might be required on both ends, particularly if the tree is being moved across state or jurisdictional lines.

5.       Installation Challenges – The same challenges discussed above will obviously hold true for the installation site, if applicable. Before the selection of a location to plant the tree, several factors need to be considered such as zoning and setback requirements and sight distance requirements if the tree will be near a public roadway.

In addition, consideration must be given to the size of hole that will be excavated, which is largely dependent on the soil type present at the site. Many times the specifications will dictate the size of the tree pit, but a particular area to watch out for is any requirement to over-excavate the bottom. For large trees, this can be a real problem, especially if the bottom is not compacted properly. While loosening the soil at the bottom of the tree pit might sound like a good idea so that the roots can easily penetrate the soil, do not forget the weight of the tree you will be putting in the hole. If the bottom is not properly compacted, the tree will settle with time. This can be unsightly, also will lead to drainage problems, and possibly could result in the death of the tree if the crown of the root ball is covered in order to “fill the hole.”

Similarly, the site should be graded to within one to two tenths of finish grade, before the tree is planted and finish grade should be known at the tree location. In some cases, it may be necessary or desirable to actually have the tree location marked by a surveyor. While the actual location (horizontally) of a tree is rarely that critical, it is imperative to know the finish grade (vertical location).

In poorly draining soils such as clay or clay silt, adequate drainage of the tree pit is essential. While there are many ways to accomplish this, one of the best ways is to put perforated pipe wrapped in a filter sock in the bottom of the tree pit. This perforated pipe is then connected to a solid pipe that in turn is connected to the storm drain system. Obviously, given the recent changes in the water quality regulations, one should consult the appropriate authority to verify that this type of connection is legal. The perforated pipe at the tree pit bottom must extend beyond the root ball size in plan view so that the weight if the tree does not collapse the pipe.

It is also a good idea to place at least one vertical pipe that extends from the tree pit bottom to the surface that can be used as an inspection tube. At any time the maintenance staff can use this pipe to determine if there is standing water in the bottom of the tree pit, after the tree is planted. This tube is usually capped at the surface with a removable cap and does not have to be tied to the drainage piping previously mentioned. In fact, this pipe can be used to pump out the tree pit if excess water accumulates there and there is no way to hard pipe the tree pit to a drain. – J. Wickham Zimmerman