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WSU/Pend Oreille Extension introduced the Sense of Place series in 1999, with a focus on place-based stewardship education. Since 2001, a partnership with the Kalispel Tribe of Indians Natural Resources Department (KNRD) has supported this newsletter and allowed us to expand class offerings through EPA funding. Further staff support comes through Renewable Resources Extension Act (RREA). Many thanks to our partners and to you, our readers, for your continued enthusiasm for "digging" into the natural history and culture of this part of the world.

The Stages of Weed Incursion


by Sharon Sorby • Pend Oreille County Weed Board

Foreign plants arrive in our area through a number of vectors – accidentally as hitchhikers or contaminants, and intentionally as introduced crops or ornamentals. Most introductions do not create problems. The ones that become invasive noxious weeds go through a succession of population increases: Introduction, Pioneering, Colonization, Lag Phase, and Exponentiation.

Aquatic noxious weed introductions are usually the result of escaped intentional ornamental plantings or as a contaminant with an aquaria dump. Flowering rush and yellow flag iris are escaped ornamentals. The problem with these two plants is that they displace native shoreline vegetation that is better suited for bank stabilization and for providing habitat – food, resting sites and nesting material – for the animals and waterfowl that use the water’s edge.

Flowering rush is also detrimental to recreational uses including impediment of boat passage due to prop fouling, blockage for swimming, and loss of open water for near shore fishing. The flowering rush infestations provide ideal habitat for great pond snails (Lymnaea stagnalis), which are an intermediate host for the trematode parasite (Trichobilharzia ocellata) that causes swimmer’s itch.

The introduction of flowering rush into Flathead Lake was first noted in 1964. It has now reached the exponentiation level, spreading to over 1,000 acres, and is affecting the shallow bays and altering the lake’s

morphology. It has also started to migrate down the Clark Fork River and was discovered two years ago in Lake Pend Oreille. Due to Idaho’s new aggressive aquatic invasive species program, steps to eradication were started immediately. Last year, a quick survey of likely habitat sites in the Washington portion of the Pend Oreille River did not discover any invading plants.

In the garden catalogues, flowering rush has an attractive open pink flower head. However, the genotype that is invading the Upper Columbia Basin rarely flowers. Spread is primarily through water movement assisted rhizome transport. There are two leaf types, one is triangular in cross-section, erect and stands above the water surface. The other is strappy and floats in the water column. It can grow to a depth of 13 feet.

Yellow flag iris was intentionally introduced into the West Branch of the Little Spokane River as an ornamental planting in the 1940’s. It has since steadily spread upstream and completely infests Fan Lake in Pend Oreille County. It is continuing its spread upstream and is colonizing Davis, Diamond and Sacheen Lakes.

It also occurs on the Pend Oreille River where I have been watching it over the last 15 years. It spent this time in its lag phase of infestation, present but not spreading significantly. In the last 3 years, it has started to enter the exponentiation level of infestation as it is spreading to new segments of shoreline at a rapid rate. It now infests both shorelines as scattered clumps from the border with Idaho to Boundary Dam. With cooperation from PUD and Seattle City Light, we are planning an aggressive control project to stop yellow flag iris from spreading along the entire river shoreline. We are currently awaiting funding.

Yellow flag iris has an attractive yellow iris flower. The rhizome root appears the same as the garden iris variety; however it is chambered with air pockets allowing it to grow in water where garden iris would drown. It has long, flat and sharply pointed leaf blades much like garden iris, and it produces large swollen seed pods designed to float on the water. It spreads primarily by seed, although roots that break off can float to new locations and establish a new colony.

When faced with managing infestations, catching noxious weeds at the first stage of introduction is critical to successful prevention. Catching them before the final stage of exponentiation is the key to successful eradication. Do your part—never plant noxious weeds into natural waters, or into garden ponds where they could escape into natural water ways. And, if you see either of these weeds along the Pend Oreille River, please note the location and report to ssorby@pendoreille.org or cmack@wsu.edu.

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