Safeguarding Seattle’s Green Legacy: An In-Depth Guide to the Regulations for Protecting Exceptional Trees

Seattle forest

Safeguarding Seattle’s Green Legacy: An In-Depth Guide to the Regulations for Protecting Exceptional Trees

Seattle forest

Mount Rainier, Seattle, USA

Seattle, a city known for its lush landscapes and dedication to sustainability, has specific regulations enshrined in law to protect the trees that add value to the community. According to the Seattle Municipal Code Chapter 25.11, or the Tree Protection chapter, exceptional trees are not only acknowledged but are given special attention. Let’s explore what constitutes an exceptional tree and what protection measures are in place.

Defining Exceptional Trees

An exceptional tree is considered an important community asset due to its historical, ecological, or aesthetic value. This determination is made by the Director according to standards and procedures laid down by the Department of Planning and Development.

An exceptional tree may be:

  1. Designated as a heritage tree by the City of Seattle, or
  2. Considered rare or exceptional due to its size, species, condition, cultural or historic importance, age, and/or contribution as part of a grove.

Size Thresholds

The size of a tree, specifically the diameter at breast height (dbh), is a key criterion in determining its exceptional status. If the dbh is equal to or greater than the threshold diameters listed in Table 1, the tree may be considered exceptional, except if it fails to meet specific risk criteria.

For species not listed in Table 1, the threshold diameter is either 30” or 75% of the largest documented diameter for that species in Seattle. If no specific measurements are listed, the threshold diameter is 30” or 65% of the largest documented diameter for that species in Washington.

Tree Grove Protection

A grove, which consists of eight or more trees with a diameter of 12” or more forming a continuous canopy, is also treated as exceptional. Trees part of a grove should not be removed if their removal may affect the health of the grove. Street trees are not included in determining a grove.

Measurement Protocols

The measurement of the tree’s diameter is taken at 4.5 feet above average grade. If branches or swelling interfere with measurement at this height, the diameter is measured at the most narrow point below 4.5 feet. Various methods are used for slopes, multiple trunks, or other specific situations.

Risk Assessment

Exceptional status does not mean the tree is immune to removal. A qualified professional may determine if a tree or grove should be removed based on risk assessment. The criteria include considerations of crown size, disease, potential damage, risk mitigation options, and survivability post-construction.

Certain species such as red alders, black cottonwoods, and bitter cherries may not be considered exceptional except as part of a grove.

Qualified Professionals

A qualified professional should have specific certifications and experience in tree evaluation, risk assessment, and protection during construction.

Environmental Considerations

Seattle’s regulations also take into account the environmental implications of tree protection, especially concerning rare or unique plant or wildlife habitat, wildlife travelways, and habitat diversity of significant aesthetic, educational, ecological, or economic value.

A Comprehensive List of Tree Species

The policy provides a comprehensive list (Table 1) of both native and non-native species and their threshold diameters for exceptional status, ranging from species like the Sitka Spruce to non-native species like the Giant Sequoia.


Seattle’s commitment to preserving its green heritage is evident in the detailed regulations aimed at protecting exceptional trees. These measures contribute to the city’s beauty and biodiversity, maintaining a vital link with nature amidst urban growth.

For anyone looking to understand, assess, or work with trees in Seattle, Chapter 25.11 of the Seattle Municipal Code serves as an essential guide, ensuring that the city’s treasured trees continue to flourish and enhance the community’s quality of life.

Table 1

Species Threshold Diameter
Native Species
Red ALDER – Alnus rubra Not Exceptional except in grove
Sitka ALDER – Alnus sinuata 6 in
Oregon ASH – Fraxinus latifolia 2 ft
Quaking ASPEN – Populus tremuloides 1 ft
Paper BIRCH – Betula papyrifera 1 ft 8 in
CASCARA – Rhamnus purshiana 8 in
Western Red CEDAR – Thuja plicata 2 ft 6 in
Bitter CHERRY – Prunus emarginata var. mollis Not Exceptional except in grove
Black COTTONWOOD – Populus balsamifera ssp. trichocarpa Not Exceptional except in grove
Pacific CRABAPPLE – Malus fusca 1 ft
Pacific DOGWOOD – Cornus nuttallii 6 in
Douglas FIR – Pseudotsuga menziesii 2’6 in
Grand FIR – Abies grandis 2 ft
Black HAWTHORN – Crataegus douglasii 6 in
Western HEMLOCK – Tsuga heterophylla 2 ft
MADRONA – Arbutus menziesii 6 in
Bigleaf MAPLE – Acer macrophyllum 2 ft 6 in
Dwarf or Rocky Mountain MAPLE – Acer glabrum var. Douglasii 6 in
Vine MAPLE – Acer circinatum 8 in
Oregon White or Garry OAK – Quercus garryana 6 in
Lodgepole PINE – Pinus contorta 6 in
Shore PINE – Pinus contorta ‘contorta’ 1 ft
Western White PINE – Pinus monticola 2 ft
Western SERVICEBERRY – Amelanchier alnifolia 6 in
Sitka SPRUCE – Picea sitchensis 6 in
WILLOW (All native species) – Salix sp. 8 in
Pacific YEW – Taxus brevifolia 6 in
Non-native Species
Orchard (Common) APPLE – Malus sp. 1 ft 8 in
European ASH – Fraxinus excelsior 1 ft 10 in
Green ASH – Fraxinus pennsylvanica 2 ft 6 in
Raywood ASH – Fraxinus oxycarpa 2 ft
European BEECH – Fagus sylvatica 2 ft 6 in
European White BIRCH – Betula pendula 2 ft
Atlas CEDAR – Cedrus atlantica 2 ft 6 in
Deodor CEDAR – Cedrus deodara 2 ft 6 in
Incense CEDAR – Calocedrus decurrens 2 ft 6 in
Flowering CHERRY – Prunus sp. 1 ft 11 in
Lawson CYPRESS – Chamaecyparis lawsoniana 2 ft 6 in
Kousa DOGWOOD – Cornus kousa 1 ft
Eastern DOGWOOD – Cornus florida 1 ft
American ELM – Ulmus americana 2 ft 6 in
English ELM – Ulmus procera 2 ft 6 in
GINGKO – Ginkgo biloba 2 ft
Common HAWTHORN – Crataegus laevigata 1 ft 4 in
Washington HAWTHORN – Crataegus phaenopyrum 9 in
European HORNBEAM – Carpinus betulus 1 ft 4 in
Common HORSE CHESTNUT – Aesculus hippocastanum 2 ft 6 in
Red HORSE CHESTNUT – Aesculus x carnea 2 ft 6 in
KATSURA – Cercidiphyllum japonicum 2 ft 6 in
Littleleaf LINDEN –Tilia cordata 2 ft 6 in
Honey LOCUST – Gleditsia triancanthos 1 ft 8 in
Southern MAGNOLIA – Magnolia grandiflora 1 ft 4 in
Paperbark MAPLE – Acer griseum 1 ft
Japanese MAPLE – Acer palmatum 1 ft
Norway MAPLE – Acer platanoides 2 ft 6 in
Red MAPLE – Acer rubrum 2 ft 1 in
Sugar MAPLE – Acer saccharum 2 ft 6 in
Sycamore MAPLE – Acer pseudoplatanus 2 ft
MONKEY PUZZLE TREE – Araucaria araucana 1 ft 10 in
MOUNTAIN-ASH – Sorbus aucuparia 2 ft 5 in
Pin OAK – Quercus palustris 2 ft 6 in
Red OAK – Quercus rubra 2 ft 6 in
Callery PEAR – Pyrus calleryana 1 ft 1 in
Austrian Black PINE – Pinus nigra 2 ft
Ponderosa PINE – Pinus ponderosa 2 ft 6 in
Scot’s PINE – Pinus sylvestris 2 ft
London PLANE – Platanus acerifolia 2 ft 6 in
Flowering PLUM – Prunus cerasifera 1 ft 9 in
Coastal REDWOOD – Sequoia sempervirens 2 ft 6 in
Giant SEQUOIA – Sequoiadendron giganteum 2 ft 6 in
Japanese SNOWBELL – Styrax japonica 1 ft
American SWEETGUM – Liquidambar styraciflua 2 ft 3 in
TULIP TREE – Liriodendron tulipifera 2 ft 6 in
WILLOW (All non-native species) 2 ft
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