By North Santiam Watershed Council
The Labor Day fires of 2020 impacted approximately 44% of the North Santiam Watershed, greatly affecting the native plant communities we value in the Santiam Canyon. As we are all working hard to recover and rebuild from the devastation, there is still one battle we will need to wage, now and for many years to come: invasive weeds. As ash and debris are cleared and hazard trees are salvaged, we are left with disturbed and exposed landscapes and soils. Unfortunately, these newly exposed soils are more susceptible to weed infestation.
The seeds of invasive weeds are spread easily by wind, birds, animals, vehicles, and equipment. Once introduced, many noxious and invasive plant species have the potential to outcompete our native plant communities that are not yet recovered from the fires. The first line of defense against invading weeds is prevention. Given our shared landscape, it is important for all of us to help prevent the spread and control invasive weeds. It will take all of us to win this battle.
One way to help stop the spread is through Early Detection and Rapid Response, or EDRR. This strategy is used to identify and then eradicate weeds as quickly as possible. Weed treatments are most time- and cost-effective when populations are small. In the North Santiam Watershed, experts have identified the following weeds as having high priority for treatment: False Brome, Italian Thistle, Spotted, Meadow and Diffuse Knapweed, Garlic Mustard, Yellow Archangel, and Knotweeds. These invasive plants have been determined to be the greatest threat within burned areas.
Weeds are always a problem, so what’s the urgency?
Post-fire soils are much more readily taken over by invasive plant species. The risks of new infestation are high, especially in areas where invasive species were absent before the fire. In addition, with so much clean-up, logging, and rebuilding taking place, there are now newly disturbed areas for weeds to grow. So much new soil disturbance also gives dormant weed seeds ideal conditions to grow.
All the weeds burnt up in the fire, so they can’t come back, right?
Many weeds did not die in the fire. Existing invasive plants may have only been top-killed in the fire. Roots and seeds may have survived. Soil is a great insulator, and in most areas where the soils were only moderately affected, large invasive-plant seed banks may still exist. For example, Scotch broom seeds can remain dormant in the soil for up to 80 years. Ground-disturbing activities now bring these seeds to the surface.
Species of Concern in Fire-Impacted Areas & Common Control Methods
False Brome (Brachypodium sylvaticum)
False brome is a perennial bunchgrass native to Eurasia and North Africa that generally stays green throughout the year. It has spread extensively in areas of western Oregon. It is highly invasive in shaded woodland, open prairie, and roadsides and can outcompete native forest understory and grassland vegetation.
Prevention: Seeds of false brome can be carried on shoes and vehicles, so special care should be taken to clean these off after entering areas infested with this plant. Watch for new patches of this plant especially after other grasses have started to turn brown (August to November or later).
Treatments: Small infestations can be dug up.Herbicides can be used from midsummer through fall or after the rainy season begins. Follow the product label and all laws and regulations regarding herbicide use on the site. To reduce the amount of herbicide used, a multi-year mowing regime can be used to exhaust the seed bank before starting herbicide treatment. A combination of mowing in early July followed with a fall treatment of herbicide is also effective.
Garlic Mustard (Allaria petiolata)
Introduced from Europe originally as a food plant, this species is now a serious concern in forests across North America. Garlic mustard is an invasive, non-native biennial herb that spreads by seed. Although edible for people, it is not eaten by local wildlife or insects.
We currently do not know of any locations of this plant in the North Santiam Watershed, but it is a problem in Clackamas County and in the Portland metro area. It is especially important to eradicate any garlic mustard plants that might be found within the fire area.
Treatments: Hand-pulling individual plants is effective if the entire root is removed. Flowering or seeding plants must be put in a bag and discarded in garbage destined for landfill. Carefully and thoroughly clean off boots, clothes, and tools before leaving the area to avoid carrying the tiny seeds to new sites. Herbicide may be needed for large, dense infestations and should be applied in the spring or fall on seedlings and rosettes, taking care to avoid native and other desirable plants. Follow the product label and all laws and regulations regarding herbicide use on the site.
Italian Thistle (Carduus pycnocephalus)
Native to the Mediterranean, southern Europe, and North Africa to Pakistan, Italian thistle is now widespread in temperate zones and a major pest in Australia, New Zealand, South America, and South Africa. It was accidentally introduced into the United States in the 1930s. Italian thistle dominates sites and excludes native species, crowding out forage plants in meadows and pastures. The blanketing effect of overwintering rosettes can severely reduce the establishment of other plants. Most animals avoid grazing on it because of its spines.
Treatments: Controlling Italian Thistle can involve a variety of methods including digging, tilling, grazing by sheep, pulling, and herbicide use before the plant flowers (May–June). When pulling or digging, cut the plant at least 4” below the soil surface to prevent regrowth. Follow the product label and all laws and regulations regarding herbicide use on the site.
Spotted Knapweed (Centaurea stoebe)
Spotted knapweed is a native of Europe and Asia. It was introduced to North America in the 1890s as a contaminant in agricultural seed and through soil discarded from ship ballasts. Spotted knapweed has many negative impacts on the landscapes it invades. For example, it releases a chemical that hinders native plants’ root growth and displaces vegetation. Also, infestations can decrease food quantities for wildlife and livestock. As well, large infestations can increase erosion and runoff. Spotted knapweed is not very common in the North Santiam Watershed.
Treatments: You can hand pull or dig up individual plants, making sure to remove as much root as possible. Plants in sandy soil pull easily, but those in hard packed soil will require a shovel or stout trowel. Sites where plants have been pulled need to be watched closely for new knapweed plants, as disturbed soil favors germination of any seeds present.
Knapweed that is periodically mowed will generally continue to flower and produce seeds, so mowing alone is not recommended. Herbicides can be effective at time of stem elongation (usually late April to early May), before flowers open. Remember to follow the product label and all laws and regulations regarding herbicide use on the site. Double-check the label for any site-specific restrictions.
Meadow Knapweed (Centaurea x moncktonii)
Meadow knapweed, from Europe, is a hybrid of black and brown knapweed. It invades pastures, parks, lawns, industrial sites, tree farms, vacant lands, railroads and roadsides. Its foliage is coarse and tough and not generally palatable to livestock. Meadow knapweed outcompetes grasses and other pasture species and is difficult to control. It threatens wildlife habitat and causes problems for Christmas tree growers. Meadow knapweed is the most common knapweed found in the North Santiam Watershed.
Treatments: Rototilling or plowing will eliminate knapweed. Cultivating with a disk will control young plants and seedlings, but established plants can survive if the root or root fragments remain. Mowing will not control knapweed effectively. If using herbicides, the timing of application is critical to success. Meadow knapweed should be sprayed with selective herbicides from the time when the rosettes of lower leaves are actively growing until the plant reaches the bud stage (usually April –May). Remember to follow the product label and all laws and regulations regarding herbicide use on the site.
Diffuse Knapweed (Centaurea diffusa)
Diffuse knapweed is a European plant introduced to North America by accident. It threatens wildlife habitat and rangeland and impacts Christmas tree growers. Knapweed outbreaks in pasture and range areas cause significant losses by reducing the available forage for grazing. This plant is only known in a few isolated locations in the North Santiam Watershed.
Treatments: Individual plants can be pulled or dug up. There is usually a large seed bank, so continued monitoring and removal of new plants is essential. Herbicide control is most effective in the rosette stage (April-May). Mowing may cause increased plant growth.
Yellow Archangel (Lamium galeobdolon)
Yellow archangel is a fast-growing perennial ground cover that may be either trailing or upright depending on conditions. Yellow archangel, also known as yellow Lamium, is very competitive and fast-growing in forest habitats. When it is dumped with yard waste or escapes from intentional plantings, it spreads quickly into forested areas and outcompetes native understory plants. Before this plant was designated as a noxious weed in Oregon, it was commonly sold at nurseries and used in hanging baskets because of its trailing growth habit.
Prevention: If you already have this plant and would like to minimize its invasive nature, contain it in flower beds by regular trimming, or dig it up and replant into pots. Because yellow archangel spreads readily by stem cuttings, it is very important to discard plant material in such a way as to prevent spreading.
Treatments: Roots are not deep, so plants can be hand-pulled or dug up. This method can be labor-intensive. Herbicides can be effective on yellow archangel, especially if combined with manual control and monitoring for surviving plants. Take care to avoid harming native vegetation by selectively spot-spraying. Remember to follow the product label and all laws and regulations regarding herbicide use on the site.
Knotweeds (Polygonum & Fallopia spp.)
Knotweed forms dense thickets, shading out native plants and excluding native animals. It outcompetes nearby vegetation for soil nutrients and light. Additionally, it decreases property values because of potential damage to asphalt, concrete, or foundations by the rhizome and the long-term investment in the management of the plants. Last, it can induce bank erosion and lower water quality.
Treatments: Knotweeds are generally difficult to control and require several years of repeated cutting and herbicide treatments. If cutting small infestations, be sure to bag the cut material and put it in the trash; do not compost or leave it on the ground. Digging is not effective. Treat foliage with herbicide in late summer or early fall after flowering.
Contact Information: For more information on the identification and treatment of the above species please contact Jenny Meisel, with the Marion Soil & Water Conservation District (503) 949.9212 [email protected] or reach out to the North Santiam Watershed Council staff at (503) 930-8202 [email protected].