Burke-Gilman Trail Vegetation Management Guidelines

From Seattle Department of Parks and Recreation

Approaches to vegetation management and restoration, including native character, views, tree planting, invasive species, soil erosion control, and hazard tree management.


The purpose of this document is to assist the Seattle Department of Parks and Recreation in the management of existing vegetation, and to facilitate restoration of key areas to functional landscapes, emphasizing the conservation of native habitat. These Vegetation Management Guidelines are based on an inventory of the vegetation found along the trail. Each section of the trail’s landscape has been categorized into one of five different vegetation palettes, according to its current structure and function.

The Guidelines identify areas of critical need, based on the goals and objectives developed through the study of the landscape’s functions. These areas are classified in one of two ways: issues affecting the trail as a whole, and site-specific issues. In general, proposed projects have been developed to strengthen and diversify the overstory and understory of the trail will enrich wildlife habitats, reduce homogeneity of the landscape, and reduce non native exotics that invade the trail. At the same time, focusing new plant selections on a well-chosen palette creates thematic continuity along the length of the entire trail.

The most pressing problem along the trail is the dominance of exotic invasive species. Removal of these invasive species, or at a minimum controlling them, is the main management action required to restore health to much of the landscape found along the trail. Proposed projects identify sections of the trail that can easily be improved or have limited occurrence of invasive species, as well as areas that are in serious decline due to the dominance of invasive species. However, as approximately 72% of the trail is affected by the presence of invasive species, not all occurrences of invasive species were addressed within the projects. The intent of these projects is to provide a basis for managing the highest priority sections of the trail. Those sections not addressed by a project are considered to be lower priority, and should be re-assessed in the future.

The Burke Gilman Trail in Seattle; photo by Stuart Macdonald

The Burke Gilman Trail in Seattle; photo by Stuart Macdonald

Vegetation management goals and objectives

The following five goals define the focus of these Management Guidelines. Objectives for each goal are also provided to help direct and evaluate proposed projects and management activities.

A. Insure the health and longevity of valuable existing vegetation.
A.1 Conserve and properly maintain valuable existing vegetation.
A.2 Diversify vegetation with plant additions and continuous replenishment.
A.3 Select future plants for hardiness and disease-resistance, as well as for native character.

B. Provide landscape continuity along the trail both spatially and over time.
B.1 Provide plant palettes that will achieve consistency throughout the length
of the trail.
B.2 Retain the essential landscape pattern by balancing view opportunities with canopy enclosure.
B.3 Maintain and restore native landscape elements wherever possible.
B.4 Protect the trail's vegetation from detrimental external influences.
B.5 Allow for variations in landscape character where dictated by existing conditions.

C. Maintain trail vegetation while conserving both labor and resources.
C.1 Establish management practices which maximize the efficient use of both labor and resources in the maintenance of trail vegetation.
C.2 Insure that maintenance activities provide the best possible care for vegetation, eliminating impacts on plant health and longevity such as mower damage, weed competition and others.
C.3 Insure that appropriate vegetation is selected and located to accommodate existing infrastructure (utilities, paving, etc.) and current management practices.

D. Foster community appreciation and stewardship for trail vegetation.
D.1 Encourage the involvement of citizens in all aspects of trail management.
D.2 Facilitate public understanding of the management and protection of this trail ecosystem.

E. Protect and enhance vegetation for wildlife and, integrating human and wildlife interests.
E.1 Provide vegetation along the trail that will foster desirable native wildlife.
E.2 Enhance connectivity between habitat elements through landscape continuity.
E.3 Reinforce the vegetated edges of habitat areas to prevent or minimize disturbance.

Vegetation management approaches

The following actions are needed for successful management and restoration of the vegetation along the Burke-Gilman Trail.

A. Native Character

1. Preserve native plant components.

Because the Burke-Gilman Trail is located on the site of an abandoned railroad right-of-way, the landscape is almost entirely disturbed from its natural state. This has allowed the establishment
of many invasive species, limiting the amount of native regeneration. The native plant species
that have survived or re-established are limited in their diversity.

All sections of the trail that contain an established native component should have invasive species removed to preserve valuable plant material. These guidelines give priority to the management of areas that still maintain a native component, this priority being reflected in the project section. Habitat nodes are designated areas where native habitat management is highest priority.

2. Plant native species to restore greater diversity.

The most pervasive canopy tree along the trail is bigleaf maple, with very few other native trees as dominant components. The lack of native species is even more noticeable in the understory, where invasive species dominate along 72% of the trail's length. Planting additional native species will reduce the dominance of invasive species, creating an environment less likely to invite future establishment of invasive species.

3. Utilize plant species that promote native character.

Where a native habitat does not exist or is not planned, use plant species that still maintain a native character. The mixed planting palettes promote this theme. (Appendix D)

B. Views

1. Select trees that do not interfere with existing public view corridors.

Planting palettes provided in Appendix D list trees in different height categories. Where views are to be preserved, any new trees planted should be from the small or medium categories.Taller trees may frame these view corridors. In most cases, small trees should be used in view areas, but if the elevation of the land allows for it, medium-sized trees can also be used.

2. Collaborate with adjacent property owners when planting.

Future plantings along the trail may accommodate private views by selecting and locating plant material so as to maintain view windows that coincide with recent pruning permits. This accommodation will be made on a case-by-case basis, as long as the result does not compromise the character of the trail landscape as outlined in these guidelines. However, the issuance of a pruning permit does not afford the permit holder any rights or privileges beyond the permission to perform pruning as specified in the permit. All pruning will be done in accordance with the Purpose and Policy of the DRAFT Department of Parks and Recreation Tree Policy.

3. Protect tree health and longevity during all pruning.

Due to the lack of healthy, mature trees along the trail, existing trees should be preserved.Topping of trees reduces photosynthetic capacity for food production (leaves and needles) and increases susceptibility to disease through large open wounds. Limit pruning to windowing and top thinning, allowing the removal of no more than 20-30% of the live crown. Neither of these practices should be excessive, as plants could be forced into decline. All pruning will be done in accordance with the Purpose and Policy of the Department of Parks and Recreation Tree Policy. Refer to ANSI A300 Pruning Standards for correct pruning practices.

C. Plantings

1. Thoroughly assess the site as part of project design

The Burke-Gilman trail contains numerous utility services, including electrical (both above and
below ground) cable, telephone, sewer and drainage. Before beginning any project, all utilities must be located. Locations can be approximated from existing City GIS data. However, some private or Parks and Recreation services may not be included. Prior to any digging, call Dial-a- Dig 48 hours before starting any new plantings. Trees and large shrubs should not be planted within four feet of an existing underground utility. Shrubs and herbaceous plant material can be planted over existing utility lines with the understanding that future maintenance activities may require the removal of these plants.

2. Design projects to minimize long-term maintenance

New projects cannot add to the existing workload of district field staff. Therefore, all projects must be designed to naturalize successfully after the initial three year establishment period. New plantings of large trees near the trail should incorporate root barrier to protect the pavement. Select and place all vegetation to minimize encroachment on established clearing limits.

3. Provide establishment care as an integral part of any planting project

All projects should be fully planned for intensive establishment care for the first three years following planting. This would include: weeding, watering, fertilization, plant replacement, etc. If necessary, projects may be required to include irrigation systems, weed mats or other infrastructure to insure successful establishment.

4. Limit the damaging effects of root growth.

To help prevent roots lifting trail pavement and entering utility pipes, root barriers should be used when planting trees near the trail. Nylon mesh (Jason Mills Q889 with extra firm finish) or commercial solid barriers work well in limiting the extension of large roots. These should be specified as part of any new planting project within 20’ of the paved trail surface.

5. Establish and maintain a grass-free zone around the base of existing and newly planted formal row trees.

Create grass-free zones at the base of existing row trees, and establish this zone on all new row plantings. The zone should be mulched to a depth of 2-3", and should have a diameter of at least 3'. Grasses over the surface root zone of a tree compete directly with those roots for moisture and nutrients, leading to reduced tree vigor and health. Also, a grass-free zone provides protection to the trunk from mower and trimmer damage that can seriously affect the health of a tree.

6. Stake newly-planted trees only as necessary and remove stakes after initial growing season.

Stake only those trees that are susceptible to windthrow or potential lean problems. Stakes should be placed outside the root-ball, and ties should loosely secure the tree so as to eliminate the occurrence of girdling. Removal of stakes and ties after the initial growing season is important. Failure to do so promptly can lead to trunk girdling, windthrow when stakes and ties are finally removed, and poor development of taper.

D. Invasive Species

1. Aggressively remove invasive species from canopy, tree trunks and understory.

The removal of invasive species from much of the trail will assist in the restoration of native habitats, the diversifying of plant species present along the trail, and the improvement of the health, vigor, and longevity of existing vegetation. Projects laid out in this plan call for the elimination of invasive species from areas including:

  • Native species that are dominant but have minimal invasive component that could spread.
  • Existing trees and understory plants that are being seriously affected (health, vigor) by the presence of invasive species.
  • Establishment of invasive species on trunk or in canopy of otherwise healthy trees.

There are many other areas along the trail where invasive species are established and could cause future problems for existing vegetation. These areas should be monitored and dealt with on a prioritized basis. The highest priority areas are identified within the guidelines as projects requiring immediate action.

E. Soil Erosion Control

1. Introduce plant species that reduce soil erosion.

The use of plant species contained in the soil erosion control palettes will help to reduce soil erosion on steep slopes. Many of these plants have deep rooting systems which bind the soil on slopes. Existing vegetation should not be cleared from steep slopes as it is likely assisting the binding of the soil. Rather, soil erosion control species should be introduced to improve soil stability.

Note: slope instability differs from soil erosion in that erosion occurs due to the movement of water across a surface, whereas instability generally results from sub-surface conditions. When soil becomes saturated, a lubricated seam can occur between the saturated soil and an adjacent impermeable layer. This event can lead to the sliding of the heavy, saturated soil from a slope.

2. Consult with a Geotechnical Engineer in areas of slide potential.

As noted, two major areas of the trail are susceptible to slide activity. Therefore, in areas of high
slide potential, a geotechnical engineer should be consulted before any activities take place on
any slope.

F. Hazard Tree Management

1. Annually inspect and evaluate potential hazard trees.

Red alder and bigleaf maple make up much of the native canopy along the trail. In particular, as these two species decline they are prone to break-up and overall failure. Weak attachments of branches often lead to the dropping of large branches, causing potentially hazardous conditions along the trail. Hazard tree evaluation is based on the likeliness of tree failure, the size of the failure, and the proximity to potential targets. This evaluation should take place annually, using the International Society of Arboriculture's (ISA) "Tree Hazard Evaluation Form" and Matheny and Clark's "A Photographic Guide to the Evaluation Of Hazard Trees in Urban Areas." (ISA, 1994), with a report of "risk" trees being made to the urban forester for further evaluation.

2. Remove all black cottonwoods from trail vicinity.

Black cottonwood (Populus trichocarpa) has a tendency to drop large branches during the summer, even if the tree appears to be healthy. The condition is called "summer branch drop" and often occurs during or soon after hot, calm summer afternoons. Due to the rapid growth, large size, and proximity to the trail of many cottonwoods, this condition could cause a potentially dangerous situation. Cottonwood roots also heave pavement where they grow close to the trail. Due to these factors, and to avoid continual monitoring and maintenance, it would be advisable to remove all black cottonwood from within 50 feet of the trail.

Attached document published May 1999


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