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

  • 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 growth 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: The incorporated 800 microsites (groups of knotweed patches in a defined area) on over 120 river and creek miles in the greater Sandy River watershed, including its tributaries. Note this study includes data from a smaller-scale study of 196 sites within the Sandy River Gorge only (see Case 674, 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).

Treatment: Treatment applied to a microsite varied depending on the field season, property owner, and condition of the plant at the time of treatment. Treatments include any combination of cutting, foliar spraying, stem injection, and digging up root biomass. 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 only

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

Foliar spray in large areas was done using plastic 1.5 gallon hand carried chemical spray units and backpack sprayers, or plastic hand-mister spray bottles for smaller areas. In 2005 a treatment of 4% v/v glyphosate, 1% triclopyr, and 1% v/v R-11 surfactant was used. In 2006 a treatment of 4% v/v glyphosate, 1% v/v imazapyr, and 1% v/v R-11 surfactant was used.

Direct stem injection involved poking a 0.1 inch (0.2cm) hole through both sides of a knotweed stem just below the first or second node. A small amount (1 to 5 ml) of undiluted glyphosate was injected downwards into the hollow chamber of the stem with a 14 gauge needle and a 60 ml syringe. 2001-2005, stems of 0.75 inches (c. 2cm) diameter were injected with 1.5 ml, those of 1.00 inch with 3 ml, and stems of 1.25 inches with 5 ml. In 2006, all stems of 0.75 inches or greater were injected with 3 ml, smaller stems received lesser amounts depending on the capacity of the hollow chamber, and stems too small to accept injections were sprayed.

Digging involved hand-digging knotweed stems and rhizomes, extracting as much plant matter as possible, and removing the debris from the site. This was only performed in inland areas with no risk of flood displacement.

Despite the discovery of new knotweed patches each year, the total number of known stems in the watershed has decreased in response to treatment. For the 750 sites discovered prior to 2006, the known stem count has declined from 176,705 to 25,963 stems, an 85% reduction. In addition, average stem count per site has decreased since 2002 (Figure 1). Directly comparing treatment types is difficult due to changing environmental conditions and treatment practices over time. However, all treatment types tested greatly decreased stem counts in treated knotweed patches.

Of the 750 previously treated microsites, 375 (50%) had no new shoots (NNS) in 2006, while many others exhibited epinastic growth (stunted, discolored shoots). On average, 33 percent of NNS patches produced aboveground growth two or more years later, probably due to belowground biomass. In 2006, digging at 28 sites revealed living rhizome tissue in about half of the excavated patches.

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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