Drainage-scale herbicide treatment of Japanese, giant, and bohemian knotweed (Fallopia japonica, F. sachalinense and F X bohemicum) control within the Sandy River Gorge, Oregon, USA

  • Published source details Soll J., Kreuzer D., Strauss K., Dumont J., Jeidy L., Krass M., Aldassy C. & Nemens D. (2006) Sandy River Riparian Habitat Protection Project Report 2006. The Nature Conservancy report.


The Sandy River Watershed, a sub-basin of the Columbia River Basin, drains an area of 508 square miles (131,600 ha) within Multnomah and Clackamas Counties in Oregon. Despite its location near Portland, Oregon's largest population center, it supports rare and characteristic wildlife including twenty-two species of state or federal concern, such as Chinook Onchorhnychus tshawytscha and Coho salmon O.kisutch, and steelhead O.mykiss. Two major sections of the Sandy River Watershed have federal 'Wild and Scenic River' and/or an 'Oregon State Scenic Waterway' designation.

The Sandy River Watershed's tendency towards catastrophic flooding and its proximity to developed landscapes (Portland, Gresham, Sandy, the Hoodland Corridor and the growing urban/suburban fringe), active nurseries and farms make it particularly vulnerable to water quality issues and invasions of well known noxious weeds such as Japanese knotweed Fallopia japonica (syn. Polygonum cuspidatum), giant knotweed F.sachalinense and bohemian knotweed F. X bohemicum, English ivy Hedera helix, Himalayan blackberry Rubus discolor, Scots broom Cytisus scoparius and new and previously unknown species of horticultural origin.

Knotweed (Fallopia japonica, F. sachalinense and F. X bohemicum) is able to rapidly colonize fresh sediment deposits and other low nutrient, disturbed sites, reproduce vegetatively, grow rapidly, form dense monocultures, and tolerate poor soils and long periods of submersion. These characteristics adapt it perfectly for life in the dynamic riparian and floodplain systems of the Pacific Northwest. They also enable knotweed to fundamentally alter the function of riparian systems by contributing to stream bank erosion, which can result in more sediment in the water and broader, shallower, and warmer waterways. Knotweed can also out-compete native vegetation including trees (through suppression of seedlings and saplings), which can result in an overall decrease in stream shading, decrease in woody debris, loss of appropriate riparian wildlife habitat, and ultimately a loss of aquatic invertebrate biodiversity.

Study area: A total of 196 microsites (groups of knotweed patches in a defined area) in the Sandy River Gorge between river miles 6 and 19 were identified. (Note these 196 sites were also part of a larger-scale study of the entire Sandy River Watershed, including tributaries (see: Case 675, Watershed-scale herbicide treatment of Japanese, giant, and bohemian knotweed (Fallopia japonica, F.sachalinense and F. X bohemicum) control in the greater Sandy River Watershed, Oregon, USA).

Treatment: In 2001, a total of 287 knotweed microsites were established, and visited or treated annually. Some sites were “lost” each year, due to lost flagging, changing river conditions and patch disappearance but 196 microsites were successfully tracked and treated through 2006, and received a total of six to eight treatments each. Eleven treatment combinations were used:

• 2001 Foliar (triclopyr 5%)

• 2002 Spring cut and fall foliar spray (3% glyphosate)

• 2003 3 ml inject (% Product name) + foliar (5% glyphosate)

• 2003 5 ml inject + foliar (5% glyphosate)

• 2003 5 ml inject only

• 2004 5 ml inject + foliar (8% glyphosate)

• 2005 5 ml inject + foliar (glyphosate 8%)

• 2005 5 ml inject + foliar (glyphosate 4% + triclopyr 1%)

• 2005 foliar (glyphosate 4% + triclopyr 1%)

• 2005 5 ml inject

• 2006 5 ml inject + foliar (glyphosate 4% + imazapyr1%) and digging

Directly comparing treatment types is difficult due to changing environmental conditions and treatment practices over time. However, all treatment types greatly decreased stem counts in knotweed patches. Total stem count for the 196 sites before treatment in 2001 was 31,113, which was reduced to 2,439 stems in 2006, an 85% reduction (Figure 1). In 2006, 156 of the original 196 microsites (80%) had no knotweed regrowth (Figure 2).
Some other important results include:

• No patch over 300 stems has been successfully eradicated through the use of glyphosate, even after six to eight treatments.

• When multiple herbicide treatments do not eradicate a patch, regrowth is typically too small to be injected.

• The number of microsites with epinastic growth is increasing.

• Glyphosate treatments seem to cause epinastic and stunted growth.

• While many smaller patches have been eradicated completely, others have remained, with very low stem counts or with significant epinasty, for years without dying.

• Some patches that appear dead and send no growth above ground for one of more years can have significant underground living tissue. Some such patches eventually produce above-ground growth two or more years later.

• Treating epinastic growth with herbicides does not kill large knotweed rhizomes.

• Initial treatments usually produce the largest decrease in a knotweed infestation. Subsequent treatments provide less control.

For detailed results and analysis see Soll et al. 2006, which can be downloaded from:

Output references

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