Trees You’re Not Allowed to Plant in Portland

Plants are often chosen for the smell, beauty or fruit produced without further consideration, causing an epidemic of invasive and nuisance species nearly every city is up against. Specialized companies are becoming increasingly important in cities’ fighting against damaging species as a tree company can help identify, remove and replace nuisance landscaping.

The Portland Plant List is a thorough resource covering native and nuisance plant species including trees, shrubs, flowers and weeds. It identifies common plants that are considered beneficial to Portland’s people and ecosystems, as well as those the city hopes to eradicate. Portland’s trees are classified into four categories.

•      Native species are trees that took root in Portland prior to European settlement. These are the best choice when planting new trees as birds and other native animals rely on these trees for a secure habitat and food source.
•      Ornamental trees are non-natives that are commercially sold, typically for use in landscapes. Though native animal species may adapt to find a home in ornamental trees, they are not historically part of the diet of Portland’s native invertebrates.
•      Nuisance species are non-native trees that threaten health and safety. Nuisance species may degrade natural habitat or be toxic to humans or soil. They’re bad, but not as bad as invasive species.
•      Invasive species are detrimental to native ecosystems. They spread at an inflated rate, overtaking native ecosystems and posing threat to human health, the environment and the economy. Invasive species displace native vegetation and destroy biodiversity.

Know Your Zone

Though the city’s natural resource protection program relies on both native and nuisance plants, the trees listed on Portland’s Nuisance Tree List are legally prohibited from being planted within the Environmental Overlay Zones, Greenway Overlay Zones and the Pleasant Valley Natural Resource Overlay Zone. Not sure if you live in one of these zones? Find out here where the City of Portland identifies 15 overlay zones in total.

•      Environmental Overlay Zone: protects resources that the City of Portland identifies as providing benefits to the public. The environmental regulations encourage development that protects the site’s public areas.
•      Greenway Overlay Zone: protects, enhances and maintains the natural, scenic, historic, economic and recreational qualities of land along the city’s rivers. Water quality is a huge factor in protecting the Greenway Overlay Zone. Any plant that threatens the health of the vegetation corridor protecting the water’s health is strictly forbidden.
•      Pleasant Valley Natural Resources Overlay Zone: protects and conserves natural resources in the Pleasant Valley neighborhood. The zone’s codes restore and enhance stream corridors, wetlands and forests, and protect existing floodplains. Streams and riparian areas are recognized as amenities for the community and codes aim to enhance connections between upland and riparian habitats within the neighborhood and those of Powell and Clatsop Buttes, as well as Butler Ridge.

Portland’s Forbidden Trees

In addition to the three designated overlay zones, trees named on the Nuisance Plant List cannot be planted along city streets or city landscaping areas. The trees included in the list are invasive, threatening the health of native habitats and humans, putting financial strain on public and private landowners. Nuisance plant species are ranked based on distribution and level of invasiveness. A Tree company that offers skilled removal services can help identify nuisance and invasive species and plan the best method for removal. Specialists can help you determine which trees pose the biggest threat and therefore take priority in the removal process. A Tree company can also suggest which native species would best replace the removed tree.

Norway Maple (Acer platanoides)
Originally from Europe and Western Asia, the Norway Maple is a broad deciduous tree that chokes out native vegetation. Once established, its canopy casts a heavy shade that prevents regeneration of native seedlings. It is also thought to have allelopathic properties, meaning it releases toxins that prevent the growth of other plants.

Sycamore Maple (Acer pseudoplatanus)
Also native to Europe and Western Asia, the Sycamore Maple grows well in disturbed sites including abandoned fields, roadsides and vacant lots. It does well in coastal areas due to its high tolerance of salt, extreme soil and pollution. Because a single tree produces a large number of spawn, Sycamore Maples can quickly choke out native vegetation.

Horse Chestnut (Aesculus hippocastanum)
Native to Asia, Horse Chestnuts are deciduous trees that can reach 80-feet tall. It’s dangerously tolerant of even the harshest alkaline soils. Once established, its thick canopy drowns out sunlight and fights native species for moisture and nutrients.

Tree-of-Heaven (Ailanthus altissima)
Sometimes called Chinese Sumac, the Tree-of-Heaven over-produces seeds, reproduces through its roots, and is proven to have allelopathic properties. Originally from China, the U.S. started using this tree abundantly in city landscaping as early as the 1700s.

Cutleaf Birch (Betula pendula)
Sometimes called Silver Birch, this species is invasive because of how quickly it colonizes open space. Grasslands are important to ecosystems and Cutleaf Birch trees (that thrive in wet areas) threaten these ecosystems.

English Hawthorn (Crataegus monogyna)
The English Hawthorn is a deciduous tree whose leaves do not change color in the fall. It quickly dominates wetlands and prairies and interbreeds with native hawthorn, spawning hybrids that are difficult to identify.

English Holly (Ilex aquifolium)
English Holly is common in Christmas decorations, but doesn’t always carry so much cheer. It creates a deep shade that prevents native species from germinating. It’s recognized as one of the most common invasive species in the Pacific Northwest and is extremely poisonous when ingested by humans.

Golden Chain Tree (Laburnum watereri)
The Golden Chain Tree has beautiful yellow flowers that makes it an appealing ornamental tree, however, it is deeply poisonous to humans and animals.

Empress/Princess Tree (Paulownia tomentosa)
The Princess (or Empress) Tree is native to China and produces pods of beautiful purple flowers that rapidly produce seeds, displacing native species especially in disturbed areas.

White Poplar (Populus alba)
Native to Europe, Siberia and Central Asia, White Poplar (also known as Silverleaf Poplar) can grow to be as tall as 70-feet and have a diameter as thick as two feet. It spreads through root suckers and can thrive in a variety of soils. White Poplar outcompetes native vegetation by over-producing seeds and easily healing itself from damage. It chokes out native species especially along forest edges and in fields where sunlight is most prevalent.

Sweet Cherry (Prunus avium)
A deciduous tree in the rose family, Sweet Cherry trees create enough shade to stave off native species during germination. Because native animal species eat the berries, Sweet Cherry trees can easily take over an area.

English Laurel (Prunus laurocerasus)
The English Laurel is an evergreen tree common in heavily-wooded areas. Native to southwestern Asia and southeastern Europe, it’s considered a noxious weed and ingesting its seeds can be fatal to humans. Like the Sweet Cherry, English Laurel seeds are spread by birds. English Laurel is extremely tolerant of a variety of soils resilient to disturbance, making it easy for this tree to overtake native vegetation.

Black Locust (Robinia pseudoacacia)
Black Locusts are fast-growing trees that sprout rapidly through a connected system of underground root suckers. It invades disturbed habitats and dormant fields by crowding out native vegetation in prairies, oak savannas, and upland forests.

European Mountain Ash (Sorbus aucuparia)
European Mountain Ash thrive in the coastal rainforests of Oregon, using highly-desirable fruits to tempt birds into spreading its seeds that love wet soil. Its ability to rapidly reproduce means it easily replaces native rainforest.

Siberian Elm (Ulmus pumila)
Siberian Elm trees can overtake a disturbed prairie in just a couple of years. It thrives in poor soils and its seed germination rate is high. Seedlings establish quickly in sparsely-vegetated areas and it was originally chosen for landscapes because of its hardiness. Some companies still sell Siberian Elm as a windbreak tree despite its invasive nature.