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Most City offices closed Wednesday, June 19, to observe Juneteenth

The City of Portland recognizes Juneteenth as a formal day of remembrance to honor Black American history and the end of slavery in the United States. Learn about Juneteenth.

Invasive plants and the damage they can cause to our environment and economy

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Invasive plants can reproduce rapidly and spread quickly, taking space, nutrients, water, and light from other plants. If uncontrolled, they can damage parks, streams, and infrastructure. Learn more about the damage caused by invasive plants.

What is an invasive plant?

Many plants have done well in the Willamette Valley because of the soil, rainfall, and light available here. For centuries, people and animals of this region have come to depend on the food and shelter provided by the many different kinds of native plants

Some plants were introduced to the Willamette Valley by people often for gardening or agricultural reasons. Most of them don’t cause long-lasting problems. We often call them “exotic” or “non-native” plants. Some plants may be annoying or create problems when they grow where we don’t want them. We often call those “weeds.”

But, only a few plants are considered “invasive.” 

Invasive plants cause long-lasting problems to the environment. Invasive plants can reproduce rapidly and spread quickly. Animals, people, water, or wind can move them from yards and roadsides into parks and forests.

Once started, they can begin to crowd out other plants. They can form dense patches, and use up the light, water, soil nutrients, or space that other plants depend on. As a result, other plants can start to die off and with them the animals which depend on them.

Why are invasive plants a problem?

Invasive plants cause long-lasting damage to the environment. They can lead to: 

  • Erosion and poor water quality. Invasive plants often have shallow roots, which can let soil wash into creeks when it rains. When there is a lot of soil in the water, it is hard for fish, insects, and other animals to live and eat there.
  • Reduced biodiversity of plants and animals. A healthy plant community usually has a lot of different kinds of plant. A variety of herbs, shrubs, and trees means a variety of food and shelter (habitat) for a lot of different animals.  These plants and animals balance each other in a way that lets them all exist.  When invasive plant species push out other kinds of plants, the animals that depend on the wide range of plants also can disappear.
  • Reduced habitat. Invasive plants can reduce the available habitat. For example, when invasive plants like Japanese knotweed or Himalayan blackberry grow dense patches next to streams, they provide little shade and keep trees from growing that normally provide shade. Without shade, streams get warmer and fish have a harder time breathing. In some streams, fish and other animals can’t live.  
  • Reduced tree canopy. Invasive plants may not only prevent trees from growing. They can also cause them to fall down. Tree-climbing invasive plants like English and Irish ivy can make the tops of trees heavier or shade the tree leaves. Both can make the tree more likely to break or fall over in wind. Plants like garlic mustard may kill off soil fungus that young trees depend on to get water and nutrients.  
  • Increased fire risk. Climbing vines may also make it easy for wildfires to travel from tree to tree. Dense patches and monocultures of invasive plants create fuel for wildfires making the fires more difficult to control and more likely to threaten nearby structures. For example, dense patches of invasive blackberry have led to fires along rail lines in North Portland.
  • Increased costs. When invasive plants damage city infrastructure such as by interfering with a natural area’s capacity to control rain and runoff or by killing tree canopy in a forested park, the City must spend time and use funds to remove them. When farmers have to control damaging pasture weeds (some of which can make their cattle sick) that cost can be transferred to consumers through the cost of beef. 

Stop invasives before they get started 

Catching and controlling invasive plants early is the most effective and least expensive way to stop them. And here’s where you can help. Learn more about how to identify and stop invasive plants and what you can do to help prevent their spread into our parks and natural areas. 

Find more information

The following websites have additional information on how invasive plants threaten our environment and economy.