Lewis County Weeds of Concern

As a Lewis County landowner, you might have one (or many) noxious weeds that are causing you concern. We've compiled a list of some common weeds that occur in Lewis County. To this list, we've also added some weeds that we consider "high priority" noxious weeds, as they are not yet widespread or they pose a particular threat to our economy, our ecosystem, or our health. Clicking on the link below each image will take you to the Washington State Noxious Weed Control Board webpage where there is in depth information specific to identification and/or control methods for each of these noxious weed species.

For the complete 2021 Lewis County Weed List click here. The 2021 APPENDIX is helpful in understanding terms, designations, and weeds classified as "select", to view the appendix click here.

Garlic Mustard (Alliaria petiolata) is not known to occur in Lewis County but has been documented elsewhere in the state. Due to it's early growth in spring, it easily outcompetes native plant species that are slower to emerge from winter dormancy. Garlic mustard changes the plant community by exuding chemicals into the soil that disrupt the growth of neighboring plants and plant-mycorrhizal fungi connections which are important for tree seedling health. True to its name, new leaves will give off a strong garlic odor. Also, garlic mustard is the only mustard that occurs in Washington with a white flower. If you see this weed growing anywhere in Lewis County, please contact our office! Weed classification: A

Butterfly Bush (Buddleja davidii) is a mainstay in many Lewis County gardens. They start to cause problems when they escape the garden and start growing uncontrolled along riverbanks, railways, and roadways. Due to its popularity, it is pretty easy to identify. If you see the butterfly bush growing in natural spaces or if you'd like information on how to control an unruly one in your landscape, please contact us. We are able to make site specific recommendations for control and even offer some plant alternatives. Weed classification: B Select

Knotweeds (fallopia species) are very aggressive escaped ornamentals that many Lewis County landowners struggle with. Capable of forming dense stands quickly, knotweeds crowd out all other vegetation, increase streambank erosion, and significantly degrade wildlife habitat. A rather attractive plant, knotweed have bamboo like stems that grow 4 – 8 feet tall, heart shaped leaves 4 – 6 inches long, and clustered tiny white flowers appearing in August – September. Knotweed is currently known to occur along many creeks and rivers in Lewis County, including the Cowlitz River, Tilton River and Chehalis River. Control methods, including chemical recommendations, are available by clicking here. Weed classification: B Select

Meadow knapweed (Centaurea × gerstlaueri) is an attractive weed that can easily pass as a wildflower. Meadow knapweed is known to be alleopathic, which means it exudes chemical signals into the soil, inhibiting the growth of other plants. This allows for accelerated development of dense knapweed populations that outcompetes, or replaces, critical native forage that wildlife and livestock replay on for food. Currently known to be present in Winlock, Vader, and the Packwood area, this noxious weed also establishes itself easily into the pasture environment, outcompeting the desirable pasture grasses and small grains. Upright stems, 20 – 40 inches tall, leaves up to 6 inches long and 1 ¼ inch wide, rose-purple flowers the size of a nickel appear in July/August. This noxious weed can be a difficult one to control once it is established. For more complete control and management information click here. Weed Classification: B designate

Poison hemlock (Conium maculatum) is known to occur at many locations in Lewis County. Easily spread by seed, it seems like poison hemlock 'pops up' someplace new every year. Relatively easy to identify when mature, poison hemlock foliage looks very similar to wild carrot early in the spring. The leaves and stem of the wild carrot are covered in a fine hair, whereas poison hemlock lacks hair on both the leaves and the stem. When mature, poison hemlock can stand 4 – 8 feet tall. These stems have distinct mottled purple spots along most of the stem. Lacey white flowers may appear late May to August. Please note that every part of this plant is acutely poisonous to humans and animals. If you believe that you have poison hemlock growing on your property, please contact us so that we may work with you to get this under control. Weed classification: B Select

Scotch broom (Cytisus scoparius) is a well known weed adversary in here in Lewis County, and across much of the state. Admittedly, this noxious weed will likely never be eradicated, but we still try to control its spread into natural areas and our forests. If left untreated, scotch broom will easily take over fallow pastures, meadows, and grasslands. During the dry months, scotch broom also increases the danger of wildfires. Lewis County Noxious Weed Control staff treats many acres of scotch broom each year. We also work with private landowners to make positive progress toward successful long term scotch broom control. If you are dealing with scotch broom on your property and think you would benefit from a site specific recommendation, please contact our office! Weed classification: B Select

Spotted knapweed (Centaurea stoebe) is a very aggressive knapweed species that can quickly infest large areas. Like other knapweeds, infestations increase production costs for ranchers, degrade wildlife habitat, decrease plant diversity, increase soil erosion rate and pose wildfire hazards. Spotted knapweed is known to occur in Winlock, Randle, and Packwood areas. Growing as either a biennial or a perennial, spotted knapweed will often grow to be 2-3 feet tall, but can be taller. Plants start as a rosette (cluster of radiating leaves) of deeply lobed leaves the first year and then produce flowering stems. The flowers are pink to purple, with brown tipped bracts. Weed classification: B designate

Tansy Ragwort (Jacobaea vulgaris) might be Lewis County's 'banner' noxious weed. Tansy ragwort grows in disturbed sites, including pastures and roadsides. All parts of the tansy ragwort plant contain alkaloids that are toxic to most livestock. These alkaloids can buildup in the liver resulting in eventual liver failure. Tansy ragwort is often mistaken for common tansy or common St. Johnswort. Tansy can be identified by it's upright stems, 1 – 4 feet high, leaves alternate, and clusters of yellow daisy like 13-petaled flowers. Control of tansy ragwort is most successful when action is taken before the plant matures and goes to seed. In the spring of the year, when the soil is still moist, tansy can be pulled by hand fairly easily. Broadleaf selective herbicides can also be used very successfully when applied on young rosettes in the fall of the year, or in the following spring. Mowing tansy ragwort is almost never ideal as flowering heads can still produce mature seed even after it has been cut. When dealing with tansy ragwort in a pasture setting, re-seeding and fertilization will go a long way toward giving desirable grasses a competitive edge over the tansy seedlings. Weed classification: B Select

Yellow archangel (Lamiastrum galeobdolon) is a ornamental escapee that is now being spotted in natural areas and National forests. The aggressive nature of this creeping groundcover makes it very good at dominating forested understories. The yellow archangel offers insufficient cover and forage for wildlife. Care must be taken to keep this noxious weed in its intended location and not let it get out! Weed classification: B

Himalayan blackberry (Rubus bifrons) is a notorious noxious weed common across all of Lewis County. The control of this invasive weed can cost millions of dollars for both control and it's estimated economic impacts. This species spreads aggressively and has severe negative impacts to native plants, wildlife and livestock. Weed classification: C

Bull thistle (Cirsium vulgare) may outcompete native plants and desirable wildlife and livestock forage plants. It can invade most any disturbed habitat and grow in dense thickets. Weed classification: C

Canada thistle (Cirsium arvense) Once established, it spreads quickly replacing native plants. It grows in circular patches, spreading vegetatively through roots which can spread 10 -12' in one season. It poses an economic threat to the agriculture industry by reducing crop yields. Weed classification: C

Common Catsear (Hypochaeris radicata) is a serious weed in lawns, pastures and waste areas. It is extremely aggressive in lowland pastures and lawns. It is also thought to be poisonous and is believed to be the cause of Australian Stringhalt in horses. Weed classification: C

English Ivy (Hedera helix 'Baltica', 'Pittsburgh', and 'Star'; Hedera hibernica 'Hibernica') can outcompete native plants, reducing animal foraging habitat. It inhibits regeneration of understory plants and kills understory and overstory trees by shading them out. Ivy may cause storm damage trees by the added weight in the canopy that also may act as a sail. The sap of the stems can cause skin irritations and rashes to sensitive individuals. Consuming large amounts of leaves and fruits can be toxic to people and cattle. Weed classification: C

Field Bindweed (Convolvulus arvensis). Once established, field bindweed is nearly impossible to fully eradicate. It outcompetes native plants species and can reduce crop yields. It forms an extensive root system, often climbing or forming dense tangled mats. Weed classification: C

Oxeye daisy (Leucanthemum vulgare) aggressively invades fields where it forms dense populations and decreases plant species diversity. Oxeye daisy decreases crop yields and is a weed of 13 crops of 40 countries. It is a particular problem in pastures. Weed classification: C

Other Weeds

Creeping buttercup (Ranunculus repens) is not classified as a noxious weed by Washington State, however this weed is very common in Lewis County and poses some threat to livestock due to the fact that it is mildly toxic. It is also very aggressive in compact, nutrient poor soils. Often growing in wet to moist soils, control of this weed requires patience, persistence, and repeat efforts. Cultural control efforts should include promotion of healthy competitive forage and good soil drainage. Applying lime can increase the soil pH, which is better for grass populations but less suitable for creeping buttercup, which prefers acidic soil. Small patches may be dug out so long as all roots and stolons are removed. Mowing is not effective due to its low growth habit; mowers may also spread plants. Incomplete hoeing efforts may actually create more new plants due to root fragmentation. Chemical: Use a selective, systemic, postemergent herbicide (e.g., containing MCPA) to not harm grasses or spot spray with a non-selective, systemic, postemergent herbicide (e.g., containing glyphosate). Apply to actively growing vegetation. If treating an area with herbicide, fence off the area and exclude it from grazing temporarily.

Horsetail (Equisetum species) Toxic to: Equines, Chickens in particular; rarely for Cattle, Sheep, Goats, possibly Alpacas/Llamas. Plant part toxic or injurious: All. Toxin: Thiaminase, complicated by several other compounds; much more toxic to equines than to ruminants. Anecdotal reports claim that palustrine (an alkaloid) can cause lameness in cattle. Most toxic when: Often in spring before emergence of good forage, or rarely in fall in heavily grazed pastures. Most toxic events occur from feeding of contaminated hay. Toxic in hay? Yes; may be more toxic when dried than fresh. Most sources list toxic amount as 20% of total forage/hay intake over 1-5 weeks for a horse. Symptomology: Unlike brackenfern thiaminase poisoning, appetites may remain normal until toxicity is severe but weight loss may be observed. Other symptoms include weakness, difficulty moving, rigid muscles, trembling; rapid, weak pulse; constipation; seizure and coma. Action to take if plant is suspected: Call the vet immediately. Remove animals from pasture or stop feeding suspected hay. Less advanced cases may recover when Equisetum is removed from the diet. More serious cases require massive thiamine doses and possibly supportive treatments. 39 Horsetail Family Equisetum Toxic to Classification 40 Photo Credits: upper right, Rich Old; lower left, David Cappaert, Bugwood.org; lower center, Ben Legler; lower right, Rich Old. Scouring rush, Equisetum hyemale Vegetative growth Common horsetail, Equisetum arvense Giant Horsetail, Equisetum telmateia Smooth horsetail, Equisetum laevigatum Control measures: Very difficult to control due to extensive rhizomes and high spore production, silica content, and waxy cuticle. Cultural: Drainage of excess water, raising the pH if needed and increasing fertility of soil aids the competitive ability of forage grasses and makes environment less ideal for Equisetum species. Creating shade by planting trees and shrubs may help long-term. Mechanical: Removing stems before cones appear may help reduce spores, but must be repeated after each period of regrowth and also for many years due to rhizomes. Mechanical methods alone will generally not control Equisetum species. Tilling or hoeing are not recommended as root fragments and spores spread plants. Chemical: The PNW Weed Management Handbook recommends MCPA, applied when horsetail fully emerges and before grain or grass is in boot stage. Check resource for further information and updates.