Lakeside Living
Aquatic weed management in Lake Washington and Lake Sammamish
Some aquatic weeds are controlled because they interfere with human needs such as boating and swimming in the lakes. Others pose a threat to the environment. For property owners along Lake Washington and Lake Sammamish, the most bothersome aquatic weeds in the lakes are Brazilian Elodea and Eurasian Milfoil. For years milfoil had been the most problematic, but recently, Elodea has proven so aggressive that it is actually displacing Eurasian Milfoil and posing an even greater threat to the lakes' ecology. While it seems unlikely that we will be able to eradicate these two problem weeds from our state's second- and third-largest lakes, we can at least make efforts to control them.
How do we define aquatic weeds? Noxious vs. nuisance weeds
Noxious weeds
- Are legally defined as non-native plants that are highly destructive, competetive, and difficult to control.
- Are classified (in descending priority) by current plant abundance, geographical distribution, perceived threat, and eradication potential as Class A, B or C.
- Washington State Weed Law (RCW 17.10) requires that landowners control listed species, depending on class designation.
Nuisance weeds
- Nuisance weeds are native plants that are creating problems in certain situations.
- Eradication is not a goal for native plants, but they may need to be controlled when they get out of balance.
- When controlling, it is important to designate a "conservancy" area to preserve natural habitat that provides refuge and and a food source for wildlife that share the lake.
Invasive aquatic weeds
- Displace native plants with rapid dense growth.
- Reduce both open water and access to it.
- Prevent light and oxygen penetration into growth areas.
- Use dissolved oxygen when dead plants decompose, which threatens fish survival.
- Raise water temperature through plants absorbing sunlight at the water surface.
- Can block or clog intakes and water control structures.
- Reduce biodiversity and affect the aquatic food web.
For more information, visit these pages:
King County iMAP Main Page, then scroll down for a link to the Noxious Weed locator map
King County Aquatic Weed Identification.
What weeds pose the greatest problems
in Lake Washington and Lake Sammamish?
- Eurasian Milfoil;
- Brazilian Elodea;
- Water Lilies;
- Pond Weed;
- Purple Loosestrife;
- Garden Loosestrife; and
- Yellow Flag Iris.
Eurasian Milfoil: Class B weed of concern, control strongly encouraged
- Native to Eurasia, first introduced through aquarium industry.
- First identified as a problem in Lake Washington in the 1970's.
- Forms dense mats of vegetation just below the surface.
- Propogates by fragments that can float around for an extended period and establish new colonies.
Brazilian Elodea: Class B, where
control and containment required by law
- Native to areas in South America.
- Submersed, freshwater perennial plant can root in water up to 20 feet deep.
- Found in lakes, ponds, pools, ditches, and quiet streams.
- Forms dense monospecific stands that can cover hundreds of acres.
- Bush plant with dense whorls of bright green leaves .8 inches long and arranged usually in four leaves per whorl.
- Brazilian Elodea has very showy little flowers, whereas our native elodea (Elodea canadensis) has tiny, inconspicuous flowers.
- Our native elodea is smaller than Brazilian Elodea and generally has three leaves per whorl (leaves are .4 inches long).
Control methods
Mechanical weed control
You must have a copy of the Aquatic Plants and Fish Handbook #APF- 1-98 by the Washington State Dept. of Fish and Wildlife. This booklet serves as your permit.
- Weeds are found in lakes, ponds, pools, ditches and quiet streams.
- Hand pulling works well in easily accessible areas.
- Hand digging is effective, particularly for emergent weeds along shorelines.
- Weed cutters and rakes are great tools for lakeshore owners to use for small scale projects to keep weeds in check. It's no more maintenance than mowing the lawn each week (see photo above).
- The ArrowTooth is a particularly effective rake made in western Washington.
- Remember to bag all weed fragments or you'll just be spreading the problem.
Bottom barriers
- Opaque cloth that blocks sunlight, while allowing gases to move. This is key for the survival of tiny benthic organisms that live in the lake bottom.
- Used in small, discrete areas like boat launches or swimming beaches to suppress growth of aquatic weeds.
- Not practical for large scale infestations.
- Need regular cleaning because plants will root in sediment that accumulates on top of the barrier.
- Material and installation techniques are key to success (see photo at right).
- Don't use in shallow water below four feet.
- Barriers must be secured to bottom.
- Use durable material that does not billow.
- Must be durable.
Mechanical harvesters
- Employ battery or motorized boat-mounted weed cutters.
- Often used where mechnical means prove impractical, especially for larger areas, although they are usually less suitable for residential lakeshore properties where there are many docks.
Chemical treatment
Aquatic herbicides can ony be applied by a Washington State Dept. of Agriculture-licensed applicator. You cannot apply herbicides to the water as a do-it-yourself homeowner.
- Herbicide must be Environmental Protection Agency- and Washington Dept. of Ecology (DOE)-approved aquatic formulas.
- All chemicals are not the same. Some pose fewer risks than others. Some last longer in the water than others – and longer than described on the product's label.
- National Pollution Discharge permit is still required from DOE, and it requires notificiation of adjacent properties. Permit revisions were under way for 2006.
- Hydraulic Permit Approval from Washington Dept. of Fish and Wildlife has different levels of permitting, depending on the scope of work.
- Local jurisdictions may require further permits; the King County Dept. of Development and Environmental Services may require a clearing and grading permit for bottom barriers.
- Integrated vegetation management plans are required after two years of chemical treatment.
- Community cooperation and consensus is necessary for long-term management.
Chemical treatment should be used as a last resort. Remember that chemicals do not respect property boundaries.
Aquatic vegetation management plans
- Mix of various control methods.
- Most environmentally sensitive.
- Offer a long-term solution.
- Better for larger scale projects, as opposed to individual homeowners, but can make the work of individual homeowners more effective.
- Best cost-benefit solution.
- May involve several years of work for incremental treatment.
There are many useful web sites on Aquatic Weed Control:
The Washington
Dept. of Fish and Wildlife
Washington
Dept. of Ecology
King
County Noxious Weed Control Program