Vashon and Maury Island are special places. They feature uncommon habitat types and niches for native plants and animals that don’t occur much on the mainland. Beaches, bluffs, bays, creeks and forests all host their own special kinds of nature. Added to this are the many gardens, farms and fields that make the island such a welcoming place to live and visit. Unfortunately, the Island’s natural areas and farms are also being threatened increasingly by the impacts of invasive plants.

Invasive plants are those introduced species that thrive in their new environment at the expense of that new environment. Invasive plants spread beyond where they are put, and once they get into a new area they can overwhelm the other plants. 

Nationwide, invasive species are the second biggest threat to endangered and threatened plants and animals, following only habitat loss in importance. On islands, the problem is even more intense due to the limited size of the natural areas and the large amount of new species introduced relative to the resident native species. Invasive plants are capable of creating ecological deserts where few native flora and fauna can survive. 

Often invasive plants cause other types of damage as well. Gorse and Scotch broom create fire hazards, posing a risk to both homes and forests. Tansy ragwort overwhelms pastures and poisons horses. English ivy covers trees with so much weight and dense growth that the trees are much more likely to be toppled in a wind storm. Knotweed roots are coarse and brittle, increasing erosion into creeks and Puget Sound. 

Purple loosestrife can grow so thickly in a wetland that waterfowl can’t make their homes or find food and shelter. Reed canarygrass and yellow flag iris can accumulate so much sediment that they fill in creeks and drainage ditches. Giant hogweed’s juices cause the skin to be so hyper-sensitive to light that it can cause painful, watery blisters and dark scars that last for months or even years.  

All of these invasive plants are present on Vashon; some are quite abundant, but most are still relatively limited. In fact, Islanders and the county have had so much success in fighting back against some of them, such as purple loosestrife, hogweed and gorse, that they’re much less abundant now than in the past.

There are a few other invasive plants that are also worrisome on Vashon and Maury Island.  In wooded areas, the pretty garden escapee policeman’s helmet is a concern. Although it is popular as a garden plant, when it escapes into the woods or along a stream, it can become so densely packed that it overwhelms native plants. 

Perhaps even more damaging in forests and harder to control is yellow archangel. It grows from where it is planted or dumped and can overwhelm native forest plants. Once it gets going in the woods or along a trail, it moves into and over the forest understory vegetation, eventually creating a dense carpet. And giant hogweed, once planted in Island gardens as an ornamental, continues to pop up in new locations on the Island. Old homesteads or abandoned gardens are sometimes the source of new infestations.  

Spring is a great time to find out what invaders are lurking in your forests and fields and to make a plan to stop them. The next time you’re out wandering around, look for unusual plants that seem altogether too happy and see if you can figure out what they are. If you already know you have a problem with invasives, now is the perfect time to find a solution and get started. Left alone, invasive plants will only be harder to stop in the future.

— Sasha Shaw is the education specialist with the King County Noxious Weed Program.