When invasive plants like ivy or clematis dominate the groundcover, there is very little root structure to bind the soils. That’s why large areas dominated by invasive plants are more likely to erode during flood events than areas with a diverse understory of trees and shrubs, which provide more root structure diversity.

Native plant roots extend deep into the soil, and many species have wide, branching fibrous root structures that bind the soils and reduce erosion. Erosion releases sediment to streams, increases stream turbidity, and impairs water quality.

Invasive plants provide less streamside cover and shade, which increases stream temperatures. Invasive plants, such as Japanese knotweed or Himalayan blackberry, form monocultures (areas entirely dominated by one species) next to streams, which prevent tree establishment.

Invasive plants can reduce the amount of tree cover by preventing trees from becoming established, causing them to fall down prematurely, or reducing their growth rate. A Harvard University study showed that garlic mustard reduces soil fungi and inhibits the establishment of tree seedlings.

Dense cover by Himalayan blackberry can prevent sunlight from reaching seedlings or saplings. Dense ivy or clematis in the tree canopy can weight down trees making them more susceptible to blow downs and decreasing their growth rates by shading the leaves.

Monocultures of invasive plants create fuel for wildlfires. Ivy or clematis vines climb trees and can become a conduit for fire to reach the tree canopy, where it is more difficult to control and more likely to threaten nearby structures.

Invasive species monocultures can increase the frequency of wildfires. Cheatgrass, for example, is an annual grass that grows in early spring. By summer, cheatgrass is dry and ecosystems dominated by cheatgrass are more likely to catch fire.

Invasive plants aren't just a Portland problem. There are regional, state and federal efforts to combat invasive vegetation. The Oregon Invasive Species Council estimates that invasive plants cost the U.S. economy $120 billion dollars annually in lost crop and livestock production, control efforts, property value damage, and reduced export potential.

These costs are passed on to consumers through higher prices for agricultural products. For example, money a farmer spends for star thistle control in pastures is reflected in the price of your steak. The Oregon Department of Agriculture estimates that 21 invasive plant species in Oregon reduce personal income by $83 million per year.

Increasing efforts to prevent and control invasions is the most cost effective and ecologically successful approach. The U.S. Congress Office of Technology Assessment reports that a dollar spent on early weed control prevents $17 spent in future control efforts. If early intervention is not implemented and a species becomes widespread, eradication may not be feasible so the damages are permanent and money is still spent to control the future spread and contain the population.