What’s Up With Weeds: August

County Agent Ron Davis looks at a field of tansy, Clackamas County, ca. 1946

August is the month when a yellow flower has folks across Clackamas County seeing red. The yellow flower at the heart of the problem is Tansy ragwort (Senecio jacobaea). This pernicious weed is poisonous to livestock and often impacts neighborly relations.

Tansy ragwort has long been the scourge of rural landowners with grazing livestock. Horses and cows are especially susceptible to poisoning. Most grazing animals will avoid eating tansy ragwort unless the available forage in infested pastures drops below sustainable levels.

Contaminated hay is another common culprit, where it becomes impossible for feeding animals to avoid consumption.

Tansy ragwort has a long history in Clackamas County. It was one of the few plants regulated under the former county noxious weed control district that was formed in 1949. At that time, landowners in Clackamas county could be cited for having flowering plants on their property. Back then, neighbors would come together for community tansy pulls to keep their horses and livestock safe and to avoid the dreaded visit from the weed inspector.

Cinnabar Moth caterpillar on Tansy Ragwort

In the 1960s, several insects were introduced as biological controls to reduce the abundance of Tansy ragwort. This insects feed on the plants and weaken or kill the Tansy. The most recognizable of these is the crimson red Cinnabar moth; the caterpillar for the moth feeds on the flowering plant during the summer months.

With the introduction of the flea beetle in 1971, we had found the one-two punch needed to reduce the Tansy ragwort problem to low levels. With the reduction of Tansy ragwort, Clackamas County dissolved the Clackamas Weed Control District on August 3rd, 1989 citing the effectiveness of the biological controls as well as budget constraints as the chief reasons for its dissolution.

In the 23 years since the Weed Control District was dissolved, Tansy ragwort has continued to persist in Clackamas County, but fortunately at much lower levels than those encountered in previous generations. The biological controls introduced in the 1960s and 1970s are still working on our behalf!

In years with especially wet springs we see a strong resurgence of Tansy ragwort seedlings. Over the past three wet springs we’ve seen more Tansy ragwort than usual. This year alone included the wettest March in Northwest Oregon on record, which coupled with May thunderstorms led to the the wettest spring on record, and was followed up by the second wettest June on record. As a result the Tansy ragwort is soaring to the top of Clackamas County’s least wanted list.

Folks throughout Clackamas County are once again taking note of the yellow flowers blooming around them, and calls come pouring in asking for assistance in controlling this weed. But by the time Tansy’s yellow flowers appear in pastures and hay fields, the best management of this weed is a good pair of leather gloves, a strong back, and a healthy dose of perspiration. Mowing and cutting plants only spreads the poisonous vegetation around, making it more difficult for livestock to avoid. Tansy ragwort is normally a biennial plant, but mowing can cause it to behave like a perennial, meaning it will tend to come back year after year.

Tansy Ragwort is manageable

Some of our customers remember the neighborhood tansy ragwort pulls that were common many years ago…but now they seem to be a thing of the past. Tansy still poisons livestock but also affects the relationships of once cordial neighbors. Each year the Clackamas County SWCD receives a large number of calls from neighbors reporting and complaining about their neighbors tansy plants.

The weed inspectors of previous generations are not likely to return to Clackamas County anytime soon. The best advice we can give folks during this time of year is to talk with their neighbors and let them now about the issue, and have them [contactweedwise] if they need help developing a plan for their property.

All is not lost! There is still hope. Remember that Tansy is manageable. If you have a lot of Tansy plants and are feeling overwhelmed, focus first on areas where grazing animals are present or along fence lines to help with your neighborly relations. Pull flowering Tansy ragwort plants and dispose of them as trash or pile them up away from grazing animals and then burn them when allowable.

If plants are already going to seed, cut off the seed heads and dispose of as trash. Try to avoid spreading seed further. Follow up the next spring by pulling emergent rosettes when the ground is still wet or use an approved herbicide recommended by the Pacific Northwest Weed Management Handbook that is appropriate for your property. Be sure to always read and follow the label directions. Work to keep good vegetated cover on your ground. Avoid overgrazing, reseed good pasture grasses as needed, and implement rotational grazing practices when possible to rest vegetation over time.

Don’t let Tansy Ragwort poison your livestock, or your relationship with your neighbors: do the neighborly thing and control your Tansy ragwort.

So what’s up with weeds in August?

Grass-like plants

  • Jubata grass (Cortaderia jubata) is an annual shrub that follows the same timeline as Pampas grass, flowering in July and August. Dig up the entire plant.
  • Common reed grass (Phragmites australis) is a grassy perennial. Plants that were emerging in April, May, and June are now flowering.
  • False brome (Brachypodium sylvaticum) is a perennial grass. Plants that emerged this winter are beginning to flower. Please [contactweedwise] if you see false brome!

Herbaceous flowering plants

  • Russian knapweed (Acroptilon repens) bolts in May with flowers appearing in June, July, and August. Control can be through handpulling, tilling, mowing, or digging. Root fragments resprout so it’s important to get all of the plant out of the ground.
  • Garlic mustard (Alliaria petiolata)was forming rosettes through the winter and flowers in April and May. Seeds may be present and viable from June through September! You can hand pull plants, although root fragments can resprout. Please [contactweedwise] if you see garlic mustard!
  • Spotted knapweed (Centaurea biebersteinii) follows the same timeline as Diffuse knapweed. Dig up the entire plant (easier when soil is moist in the spring).
  • Purple starthistle (Centaurea calcitrapa) sprout from seed in winter and develop into rosettes in spring. Starthistle plants flower from July to Spetember. Please [contactweedwise] if you see purple starthistle in your area.
  • Meadow knapweed (Centaurea debeauzii) follows the same timeline as Diffuse knapweed. Dig up the entire plant (easier when soil is moist in the spring).
  • Diffuse knapweed (Centaurea diffusa) is a herbaceous perennial. In May, plants bolt. Flowers appear in May, June, and July. Dig up the entire plant (easier when soil is moist in the spring).
  • Yellow starthistle (Centaurea solstittialis) sprout from seed in winter and develop into rosettes in spring. Starthistle plants flower from May to July. Please [contactweedwise] if you see yellow starthistle in your area.
  • Rush skeletonweed (Chondrilla juncea) is bolts in April, May, and June. When it flowers in July, it sets seed quickly!
  • Paterson’s curse (Echium plantagineum) emerges in March and April, flowering all summer.
  • Orange hawkweed (Hieracium aurantiacum) can be dug up in March and April. Be sure to get the roots and runners. Flowering occurs in May and June, and can extend later, with flowers and seeds occurring from July through September. More information is available on the Oregon Department of Agriculture’s page about Orange hawkweed. Please [contactweedwise] if you see Orange hawkweed!
  • Yellow-flag iris (Iris pseudacorus) is a herbaceous perennial that emerges in April and flowers in May and June. Seeds are set from July through September
  • Yellow archangel (Lamiastrum galeobdolon) is a herbaceous perennial the grows throughout the winter and flowers from April to June. Seeds are set in July and August.
  • Kudzu (Pueraria lobata) is a vining perennial. Leaves emerge in April, May, and June. Flowers emerge in July. Please [contactweedwise] immediately if you think you have spotted Kudzu in Clackamas County!
  • Lesser celandine (Ranunculus ficaria) is a herbaceous perennial. Rosettes form in January and February, and the plants flower in March. In April, seeds are set. Remove all of the bulblets and tubers.
  • Tansy ragwort (Senecio jacobaea) is a herbaceous biennial. Rosettes form in March and April, and flowers in May, June, and July. If soil is moist, dig up the rosettes. If you remove the plant during flowering, bag the plant so seeds can’t spread.

Shrubby plants

  • Blessed milk thistle (Silybum marianum) overwinter as rosettes that bolt in March, April, and May. The plants flower in June and July. Mow or hand pull before flowers fully develop.
  • Butterfly bush (Buddleia davidii), a tall deciduous shrub, displays new leaves in April, May, and June, but doesn’t generally flower until July and August. Control by digging up the entire plant. Our Weed Wrench might work!
  • Giant hogweed (Heracleum mantegazzianum) is a tall herbaceous perennial. Plants emerge in March and April from winter rosettes, and accelerate in growth in April and May. When the plants flower in June and July, remove the heads and cut off the plant at the base…but be very careful because the sap of this plant is caustic. ODA says this about Giant hogweed: “This plant is a public health hazard. Do not expose bare human skin to the plant or breathe the smoke from fires if it is being burned. The plant exudes a clear watery sap which sensitizes the skin to ultraviolet radiation. Humans often develop severe burns to the affected areas resulting in blistering and painful dermatitis. Blisters can later develop into purplish or blackened scars.” Please [contactweedwise] immediately if you spot this plant!
  • Gorse (Ulex europaeus) is a single to multi-stemmed spiny shrub that buds in February and flowers in March, April, and May. It goes to seed in June and July. Dig up small plants. Please [contactweedwise] if you see gorse!
  • Policeman’s helmet (Impatiens glandulifera) is a herbaceous annual the begins to emerge in April and grows through May. Flowers appear in June and July.
  • Purple loosestrife (Lythrum salicaria) is a herbaceous perennial that emerges in May and June. Flowers begin to appear in July.
  • Scotch thistle (Onopordum acanthium) is a herbaceous biennial that overwinters as rosettes. Flower stalks emerge in June and flowering is in July and August.
  • Spurge laurel (Daphne laureola) is an annual shrub that emerges in March and flowers in April, May, and June. Berries appear in June and July. Dig up as much of the plant and root system as possible. Please [contactweedwise] if you see spurge laurel!
  • Knotweeds include Japanese knotweed (Polygonum cuspidatum), Giant knotweed (Polygonum sachalinense), Himalayan knotweed (Polygonum polystachyum), and hydrid knotweeds began emerging in April. With warmer temperatures, growth accelerates throughout May and June. Flowers emerge in July and August. Mechanical control requires frequent, persistent effort: cut twice each month from April through August. In the fall, additional steps are needed, including cutting and covering the plants. Knotweeds are so persistent that you need to do this repeatedly for five years to achieve control.

Aquatic plants

  • Parrot’s feather (Myriophyllum aquaticum) is a perennial with underground rhizomes that overwinter. Stems emerge in April and May. Flower stalks appear in June and July. Removing the entire plant, including root fragments, is necessary.

Report weeds!

Visit our page on reporting weeds to file a report.


The featured images on this page are public domain or licensed under the Creative Commons Attribution-NonCommercial 3.0 United States (CC BY-NC 3.0) license.


Flickr Commons: OSU Special Collection and Commons (https://www.flickr.com/photos/osucommons/5687462905/)

Photo courtesy of Samuel Leininger, Clackamas County Soil and Water Conservation District

Photo courtesy of Eric Coombs, Oregon Department of Agriculture, Bugwood.org

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Clackamas SWCD