Stem injection control of invasive Japanese, giant, and bohemian knotweed (Fallopia japonica, F.sachalinense and F. X bohemicum) control along the Clackamas River, 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 Clackamas River Watershed, a sub-basin of the Columbia River Basin, drains an area of nearly 1,000 square miles (259,000 ha) within Clackamas County, Oregon. Despite its location near Portland, Oregon's largest population centre, it supports rare and characteristic wildlife including Chinook Onchorhnychus tshawytscha and Coho salmon O.kisutch, steelhead O.mykiss, cutthroat trout O.clarki and native lamprey.

The Clackamas River Watershed's tendency towards catastrophic flooding and its proximity to developed landscapes (Portland, 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 (F.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: Sites invaded by knotweed spp. were identified along the Clackamas River, Oregon. Staff from The Nature Conservancy and the Metro Parks and Greenspaces Program conducted the treatments in July and September of 2003, with comprehensive follow up treatment and data collection through August 2005 and spot follow-up in 2006.

Treatments: Two main issues were addressed in the herbicide application experiment: application dosage and timing. Two sites, each with 30 patches of knotweed, were selected for treatment. Stem numbers varied from 23 to 185 stems at the first site, and from 21 to 114 stems at the second site. Neither site had been previously treated.

Direct stem injection involves poking a 0.1 inch (0.2cm) hole through both sides of a knotweed stem just below the first or second node. The undiluted herbicide (glyphosate) was injected downwards into the hollow chamber of the stem with a 14 gauge needle and a 60 ml syringe. Stems of .75” diameter were injected with 1.5 ml, those of 1” with 3 ml, and stems of 1.25” with 5 ml. In the 5ml + spray group, stems < 1” in diamter were foliar sprayed to “just wet” with 5% v/v solution of Rodeo herbicide with 1% Li-700 surfactant.

The first site was treated in July 2003, and the second was treated in September 2003. Initial post-treatment data (see below) was collected in August 2003 (for sites treated in July) and October 2003 (for sites treated in September). One year post-treatment data were collected in July 2004. All treatment sites were re-visited in September 2004, and all remaining regrowth stems in the test plot were foliar sprayed using a tank mix of 8% v/v Rodeotm herbicide with 1% LI-700. Comprehensive follow-up treatment and data collection was performed through August 2005, and spot follow-up, including excavation of two sites, was performed in 2006.

Data collection: Data collected at each site included: stem number, typical stem diameters, typical height, patch sizes, shading, general soil type, and photographs. Patches were stratified by stem number then randomly assigned to one of four treatment groups (1.5 mL stem injection, 3 mL stem injection, 5 mL stem injection, or 5 mL stem injection plus foliar spray) or a control group.

All stem injection and foliar spray treatments dramatically reduced the stem number, diameter, height, and spread of knotweed patches between 2003 and 2005.

Effect of treatment date: Treatment date (July vs. September) had no significant effect on stem reduction at any level of treatment (P = 0.80). However, stem counts in 2006 were slightly higher than the preceding two years, suggesting that patches that were not eradicated might be slowly recovering (Figure 1). Excavation of two sites unearthed dead knotweed rhizomes at one site, and living knotweed rhizomes that had survived treatment at depths of two inches (5 cm) to one foot (30 cm) at the second site. This indicates that monitoring of aboveground biomass alone may not be a sufficient measure of the effectiveness of knotweed treatments.

Effect of application method: Two years after treatment, stem injection with glyphosate effectively reduced stem number in all treatment groups (ANOVA, P<0.0001) (Figure 2). Treatment group mean stem reductions ranged between 88.6% (± 28.2%) in the 5 ml group and 99.0% (± 1.6%) in the 5 ml + foliar spray group, with an average of 94.5% (± 15.0%). Although the 5 ml + foliar spray had the highest mean reduction in stem number in both years, the four treatment groups were not significantly different from each other (Tukey's HSD, P = 0.61).

In addition, new stems growing from treated patches were greatly reduced in diameter and height compared to control patches (Figures 3 and 4). Treated stem height reduction = 99.0% (± 3.0%), whereas control stem height reduction = -133.0% (± 103.1%). Dose per stem had no clear effect on the height or diameter of the next summer's stems. However, stems in the 5 ml + foliar spray treatment were the most consistently small, with no stems greater than 0.5 m in height or 0.5 cm in diameter.

Treated stems also showed irregular growth, with excessive branching, narrow, twisted, or discolored leaves. In 2005, new stems grew less than a metre beyond the 2003 patch boundaries in 23.4% of treated patches, while control patches had grown so that original boundaries were impossible to determine. In some cases, untreated patches adjacent to the target patches displayed reduced and/or abnormal growth following treatment.

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

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