Management Strategies for Invasive Plants

Urban Forests In Danger

Invasive plants are one of the biggests threats facing urban forests in the Puget Sound region.  After decades of invasion by English ivy, Himalayan blackberry, English holly, clematis, and other non-native species, more than half of Seattle's forests are completely overrun by aggressive non-native weeds.  Vines climb up into the canopy, covering leaves and blocking photosynthsis.  Their weight alone is enough to break branches and bring whole trees down in strong storms.  Understory invasives choke out native plants on the ground and prevent new seedlings from regenerating the canopy.  Invasive plants grow quickly and aggressively, leaving behind what biologists call an ecological "dead zone," unable to provide habitat for native wildlife or perform many of the green functions our cities rely on.

Basic guidelines for invasive plant removal

 For more information, or to partner with the GSP, read the Forest Steward Field Guide. [Word 1.33 MB]

Weed Pulling and Cutting with Hand Tools

Hand Pulling – Most appropriate for small herbaceous plants and some vines.
Hand Tools – Most appropriate for shrubs and some vines.

• Maintain safe distance of at least 10-feet between those using tools
• Work carefully to minimize soil disturbance
• Avoid pulling non-target plants, especially native plants
• Use tools of appropriate size for the job to avoid stress on both hands and tools

Cutting tools: Hand pruners for stems < 1", Lopers for stems 1-2", Handsaws for stems > 2".
Root-removal tools:  Hand-picks for herbaceous plants, large picks, shovels and weed wrenches for shrub or tree roots and rhizomes.

Weed Waste Disposal

• Weed wastes are generally not appropriate to dispose of in Seattle’s green waste recycling system as this encourages the spread of weed seed.
• Stems of many plants may be gathered into central composting piles out of trail-view corridors and composted on-site, this includes blackberry canes and ivy vines.
• Composting piles should have at least 2 layers of cardboard underneath (1 box folded flat = 2 layers) to prevent plants from re-rooting, especially blackberry canes. An optional addition for control is to wrap the pile in burlap, tucking burlap in deeply between the weedy plants and cardboard.  Overlap layers of burlap by at least 6” and seam together at points with cut wire.
• Reproductive parts such as seed-heads and roots should be collected separately and placed on top of composting piles so they do not sprout or root in the soil.
• Bohemian knotweed and other noxious weeds are capable of re-sprouting from plant fragments.  All plant parts of Bohemian knotweed, purple loosestrife, garlic mustard, and giant hogweed should be removed from the site and disposed of in a land-fill or hot composted in landscape fabric on site to reduce size, then later disposed of in a landfill.  Flowering parts of Purple loosestrife and tansy ragwort should be clipped into plastic bags and disposed of in a landfill immediately.

More Information:

Weed-Specific Methods

Best Management Practices for our most common invasives.

Washington Native Plant Society Website

Lots of great resources, including current research, native plant lists, and information on local chapters