A Call to Action: Post-Fire Prevention & Control of Invasive Weeds

The Labor Day fires of 2020 impact­ed approx­i­mate­ly 44% of the North San­ti­am Water­shed, great­ly impact­ing 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 infestations. 

The seeds from inva­sive weeds spread eas­i­ly by way of wind, birds, ani­mals, vehi­cles, and equip­ment. Once intro­duced, many of the nox­ious and inva­sive plant species have the poten­tial to out­com­pete our native plant com­mu­ni­ties that are still yet to recov­er 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 high pri­or­i­ty for treat­ment:  False Brome, Ital­ian This­tle, Spot­ted, Mead­ow and Dif­fuse Knap­weed, Gar­lic Mus­tard, Yel­low Archangel, and Knotweeds. These inva­sive plants are deter­mined to be the great­est threat with­in burned areas.

Com­mon Misconceptions

Weeds are always a prob­lem, so what’s the urgency?

Post-fire soils are much more sus­cep­ti­ble to being tak­en over by inva­sive plant species. The risks of new infes­ta­tions 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 new areas for weeds to grow. Also, with so much new soil dis­tur­bance, dor­mant weed seeds may now be in 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 impact­ed large inva­sive plant seed banks may still exist. For exam­ple, Scotch broom seeds can live dor­mant in the soil for up to 80 years. Ground dis­turb­ing activ­i­ties most like­ly have brought these seeds to the surface.

Species of Con­cern in Fire Impact­ed Areas & Com­mon Con­trol Methods

False Brome

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 wood­lands, open prairies, and road­sides and can out-com­pete 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 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 mid-sum­mer 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

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 the garbage. 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, with care tak­en to avoid native and oth­er desir­able plants.  Fol­low the prod­uct label and all laws and reg­u­la­tions regard­ing her­bi­cide use on the site.

Ital­ian Thistle

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

Spot­ted Knap­weed (Cen­tau­rea stoebe)

Spot­ted knap­weed is a native of Europe and Asia. It was intro­duced to North Amer­i­ca in the 1890’s as a con­t­a­m­i­nant in agri­cul­tur­al seed and through soil dis­card­ed from ship bal­lasts. Spot­ted knap­weed has many neg­a­tive impacts to 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 aids in 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 the 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

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 out-com­petes 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: Roto-tilling 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 between 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

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

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 the for­est habi­tats.  When it is dumped with yard waste or escapes from inten­tion­al plant­i­ngs, it spreads quick­ly into forest­ed areas and out-com­petes 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.

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 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 out-com­petes with near­by veg­e­ta­tion for soil nutri­ents and light. Addi­tion­al­ly, it decreas­es prop­er­ty val­ues from the poten­tial of asphalt, con­crete, or foun­da­tion dam­age from the rhi­zome and the long-term invest­ment in the man­age­ment of the plants. Last­ly, 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 cut mate­r­i­al and put in the trash, do not com­post or leave 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) 391‑9927 or reach out to the North San­ti­am Water­shed Coun­cil staff at (503) 930‑8202