Invasive Plant Removal - Questions and Answers

What are the invasive non-native plants?

The most common non-native invasive plants are:

  • English Ivy
  • Himalayan Blackberry
  • English Holly
  • English Laurel

Other invasive plants are Japanese knotweed, giant hogweed, clematis, field bindweed, creeping buttercup, reed canary grass, policeman's helmet and herb Robert. Almost all of the invasive plants were originally introduced as landscaping plants that escaped into the natural environment.


What are native plants?
Native plants are the plants that existed in this area of Washington before 1860, the year of the arrival of the settlers in Washington. WSU Extension Native Plant Guide

Why is native habitat important?
The birds, butterflies, mammals, reptiles and amphibians that live here are adapted to nesting, feeding and living in the plants, shrubs and trees that were here before we, the settlers, arrived. If you remove their habitat, they too will disappear.

Native Important

Why is ivy bad on trees?
The added weight of the ivy can cause mature trees to come down in a storm. For example, on a large tree in Olympic National Park, 2,100 pounds of ivy was removed from a single tree.

  • Once ivy is in the tree canopy it shades out the trees leaves or needles.
  • English ivy can strangle a tree, especially at the base of the trunk.
  • Dense ivy cover deprives the tree's bark of normal contact with air and microorganisms.
  • Thick ivy mats accelerate rot.
  • Fast-growing ivy competes for nutrients and water.
Ivy enveloped trees

Why not just let nature take its course?
That would work except for the fact that we have introduced non-native invasive plants that overtake our native plants and alter the natural re-growth of our native plants. Our City will not be the same place in the future if we do not get the invasive plants under control. Native ground cover and habitat will be overrun by ivy and we will have ivy deserts.

Our mature trees will gradually disappear with no new young trees to replace them because ivy suppresses the sprouting of seedlings!

So what if the trees disappear?
There is no upside to the loss of urban forest. Every community that has made the decision to lose urban forest has been on the decline. There are measurable economic savings to a healthy urban forest:

  • Trees improve property values; homes with mature trees sell much better.
  • Trees reduce air pollution by removing carbon dioxide from the air, reducing the greenhouse effect. Our forests are our lungs. Diminished air quality can be traced back to a lack of urban forest.
  • Trees reduce storm water runoff costs by naturally controlling water runoff. Engineering solutions to surface water runoff are much more expensive. Functioning forests work better for storm water and ground water cleanliness than any engineered solution.
  • Trees reduce energy costs by shading houses.
  • Trees reduce the air temperature by giving off droplets of water.
  • Trees help muffle sounds such as cars.

Why is ivy bad when growing on the ground?

  • Ivy provides nesting places for rats and other vermin.
  • English ivy's shallow root structure makes it a poor choice for erosion control and ivy limits understory regeneration of native trees, shrubs and plants by blocking sunlight and shading out native plants. (i.e. No re-growth of native plants.)
  • Fast-growing ivy competes with native plants for nutrients and water.
  • Ivy changes the natural succession of the forest. If you look around, you will see few young trees to replace the mature trees when they come down. Ivy also creates "ivy deserts," where other plants, native to our environment, do not grow.
Blackberry and ivy on trees

What can I, a private landowner, do?
Home owners are strongly encouraged to clear ivy from their trees and remove ivy and other invasive plants from the "natural areas" on their property. For more information on ivy removal see the Ivy Out website.

Why should I remove invasive plants and do restoration now? Why not wait?

  • The invasive plants do not stop growing. The longer you wait, the more invasive plant material will need to be removed.
  • The longer you wait, the fewer viable native seeds will be available under the ivy for re-growth, increasing the costs of replanting and the loss of local genetic diversity.
  • The invasive plants keep spreading and destroying more native plants as they grow. More native plants will have to be purchased and replanted to fill the space left by the invasive plants.
  • The cost of removing invasive plants, buying and replanting native plants will only increase with time. It takes a generation to grow a mature tree.
Trillium in bloom

Who do I call if I want to have ivy or other invasive plants removed from my property?
If you have landscaped property, you can get a gardening service to remove ivy and other invasive plants. Get multiple bids. It will probably be cheaper to have it done in the winter when gardening services are less busy and looking for work. Also the roots are removed more easily in the winter. In addition to commercial landscaping/gardening businesses, you might try EarthCorps, a non-profit organization. If you have property that is currently undeveloped, is steeply sloped, contains a ravine, stream, creek or wetland, or other sensitive elements, contact City Hall at 425-744-6279 for more information and what process may apply before proceeding with any work.

What is to prevent the ivy and other invasive plants from just growing back?
Your property needs to be monitored for re-growth and removal of the invasive re-growth annually. This is nowhere as difficult as the initial removal. If you spent 100 hours initially removing ivy, the second year will take 10 hours and future years will only take an hour.

What do I use to cover the bare soil after the ivy or other invasive plants have been removed?
Bare soil needs to be protected from heavy rains. The rain can cause erosion and close the pores in the soil. Be sure to protect soil from where ivy has been pulled by planting trees, shrubs and ground cover and putting down a mulch. Initially, it can be covered with 4-6" of wood chips. (Usually available for free from tree services.) Beauty bark is not a good choice. For a good article on the reasons for using arborist wood chips and not beauty bark see the article by Linda Chalker-Scott, formerly of University of Washington at Horticultural Myths.