A Pacific Northwest Extension Publication • Idaho • Oregon • Washington

Fig 1. Meadow hawkweed infestation. Fig 2. Orange hawkweed.

AIntroduction

ncient Greeks thoughtthat hawks ate the sap

of hawkweeds tosharpen their eyesight. Today,however, hawkweeds are aneyesore to those whose landthey have invaded (Figure 1).These rapidly spreading,tenacious weeds, introducedfrom central and northernEurope, are recent arrivals inthe west but are quickly mak-ing their presence known.

Hawkweeds belong to thechicory tribe of the sunflowerfamily and are closely relatedto dandelion, sowthistle,prickly lettuce, and chicory.Plants in this tribe have flowerheads with only ligulate flow-ers (like dandelion, not daisy),and contain a milky sap.

Five species of weedy hawk-weeds are in the United Statesand Canada. Two species areeasily recognized. Orangehawkweed (Hieraciumaurantiacum) (Figure 2) hasbright orange flowers, andmouse-ear hawkweed (H.pilosella) has a single, yellowflower. Mouse-ear hawkweedis found only in coastal Wash-ington and Oregon (For more

Hawkweedinfestation in thePacific Northwest.

information on mouse-earhawkweed, see PNW409).

Other hawkweeds havemultiple, yellow flowers andare difficult to distinguish fromone another. They are knownas meadow hawkweed (H.pratense) (Figure 3), yellow-devil hawkweed (H.floribundum), and king-devilhawkweed (H. piloselloides).

Hawkweeds are notorious fortheir complex and confusingclassification. Species aredifficult to distinguish becausethey interbreed freely, andmany of our hawkweeds looklike hybrids. For the purposeof this bulletin, these yellow-flowered hawkweeds willcollectively be called meadowhawkweed.

The authors—Robert H. Callihan, LindaM. Wilson, Joseph P. McCaffrey, andTimothy W. Miller

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PNW 499

HawkweedsHieracium aurantiacum, H. pilosella,

H. pratense, H. floribundum, H. piloselloides

Once established, hawkweedquickly develops into a patchthat continues to expand untilit covers the site with a solidmat of rosettes. Forage speciesin pastures and abandonedfarmland are choked out by theadvancing front of hawkweed.Hawkweeds can pose a seriousthreat to native plant diversity.Hawkweeds threaten lawnsand gardens, too. Many timesorange hawkweed will escapefrom the gardens they areplanted in, spreading intolawns, adjacent fields, androadsides.

DistributionUntil recently, hawkweeds

were found only in the easternUnited States and Canadawhere they are troublesomeweeds in pastures, abandonedfarmland, and mountainmeadows. Orange hawkweedwas first recorded in Washing-ton in 1945 and is now widelydistributed in the Northwest.It owes much of its spread toits cultivation in gardens whereit escapes into neighboringfields, roadsides, and farmland.Orange hawkweed seems tospread more slowly thanmeadow hawkweed but itnonetheless has formed largeinfestations in certain areas likeIdaho’s Bonner County, andMontana’s Flathead County.

Meadow hawkweed wasfirst recorded in SpokaneCounty, Washington, in 1969.It has reached its greatestabundance in northern Idaho,with the largest infestations inBenewah, Kootenai, andBonner counties. In Washing-ton, meadow hawkweed israpidly spreading in the north-eastern counties of Okanogan,Ferry, Stevens, Pend Orielle,

and Spokane. Infestations ofmeadow hawkweed wererecently identified in westernWashington in Skagit,Snohomish, and Whatcomcounties.

In Montana, meadow andorange hawkweeds are spread-ing in several northwesterncounties with the largestinfestations in Flathead, Lin-coln, and Sanders counties.King-devil hawkweed wasrecently found in GlacierNational Park. Oregon doesnot report a major problemwith hawkweeds, but the statehas areas susceptible to hawk-weed invasion.

Across the United States andCanada, introduced hawk-weeds are associated withoxeye daisy, Canada golden-rod, dandelion, goatweed,cinquefoil, Canada bluegrass,and Kentucky bluegrass. In theInland Northwest and in thewetter, coastal region of theNorthwest, hawkweeds livepredominantly in permanentpastures and hayfields, moun-tain meadows, clearings inforest zones, roadsides, andabandoned farmland at eleva-tions of 2,100 to 5,400 feet.Introduced hawkweeds are notexpected to become a problemin any dry habitat usuallyassociated with IntermountainWest rangelands. Hawkweedsprefer soils that are well-drained, coarse-textured, andmoderately low in organicmatter. Although they cangrow in open woodlands, theydo not tolerate shade very well.

IdentificationA basal rosette of hairy

leaves with several cord-likerunners (stolons), and slenderflower stems with a terminal

cluster of small, dandelion-likeflowers identify the invasivehawkweeds. The narrow,spatula-shaped leaves are 4 to 6inches long, dark green above,and light green beneath. Plantsvary considerably in the typeand amount of hairs on theleaves. Each rosette producesfrom two to eight flower stemsthat are 10 to 36 inches tall.Stems have short, stiff hairs,contain a milky sap, and mayhave one to three small, clasp-ing leaves below the midpointof the stem. Each inflorescenceconsists of five to thirty brightyellow (or orange in the case oforange hawkweed) flowerheads, from 1/2 to 3/4 inch indiameter. Heads contain allray flowers arranged in a flat-topped cluster (Figure 4). Thetwelve to thirty tiny blackseeds are ribbed and have atawny tuft of bristles on theflattened end (Figure 5).

Stolons (Figure 6) begin togrow from buds in the axils ofrosette leaves only when plantsbegin to flower. Depending onthe growing conditions, plantsproduce from four to eightleafy stolons that can reachlengths of 4 to 12 inches. Moststolon tips develop into small,daughter rosettes that becomethe next generation of plants.

Many native hawkweedspecies grow in the UnitedStates and, except for whitehawkweed, all have yellowflowers. Native hawkweedsdiffer from the introducedhawkweeds because they lackstolons and have numerous,upper stem leaves and abranched, open, flower ar-rangement. In addition, nativehawkweeds are not weedy.

Biology & ecologyHawkweeds are perennial

herbs with fibrous (not tap)roots, that reproduce by seeds,stolons, rhizomes, and, in somecases, buds on the roots. Seedscan be produced either withpollen (sexually) or withoutpollen (asexually). Althoughmost new hawkweed infesta-tions are probably started byseeds, most established popula-tions expand vegetatively.Studies in Ontario, Canada,showed that once meadowhawkweed gains a toehold at anew site, less than 2 percent ofthe plants in the patch comefrom seedlings. Once estab-lished, vigorous stolon growthquickly expands the colony,forming dense patches that canhave as many as 3,200 plantsper square yard. The slender,leafy stolons elongate throughthe summer and form daughterrosettes at their tips. As rootsanchor these young rosettes,the stolons die and the youngplants become independent ofthe mother plant. Hawkweedsregrow each year from short,below-ground rhizomes, whichactually look like small rootcrowns.

Plants require a certainnumber of daylight hours perday in order to flower. Atlower elevations this occursaround mid-June. Seeds ripenby early August. Fall-germi-nated seeds generally perishduring the first winter, so mostseedlings establish in thespring. Studies in easternCanada showed that seeds arenot carried far by the wind.Minute barbs along ribs on theseeds enable them to stick tohair, fur, feathers, clothing, andvehicles and be carried longdistances.

ControlEarly detection and eradica-

tion are important to preventnew infestations of orange andmeadow hawkweeds. Eradi-cate small infestations bycarefully digging out theshallow-rooted rosettes. Whendigging or cutting rosettes,avoid scattering the stolons,rhizomes, and roots. Plantsquickly regrow from stolon andrhizome fragments, and frombuds on scattered roots. Inlawns, mowing does not killthe weeds because the low-lying leaves are missed by themower blades. Althoughmowing prevents seed produc-tion by removing flower stems,repeated mowing encouragesfaster vegetative spread.Proper care of lawns withirrigation and fertilization willusually control the hawkweed.Hawkweeds do not persist incultivation because crops canoutcompete hawkweeds,especially where herbicides areused in the cropping system.

Where perennial grasses,legumes, and other beneficialbroad-leaved plants are mixedin with the hawkweed, fertiliz-ers can help control hawkweedby increasing the competitiveability of more desirable spe-cies. This is particularly trueon range and pastures becausethese lands are generally notpriority areas for supplementalfertilization and soil nitrogenlevels may be inadequate foroptimal grass health. Depend-ing on soil productivity andgrass condition, a single nitro-gen application may be suffi-cient for grasses to competi-tively suppress hawkweedgrowth for 3 to 5 years. Goodgrazing management willextend this period.

Spring treatments with bothherbicide and nitrogen fertil-izer appear to be the besttreatment for hawkweeds. Fallfertilizer applications are notrecommended because exces-sive nitrogen loss throughleaching will occur during thewinter.

Selective herbicides areeffective in controlling hawk-weed if used properly. Forchemical control recommenda-tions, refer to the Pacific North-west Weed Control Handbook, anannually revised publicationavailable from the extensionoffices at University of Idaho,Washington State University,and Oregon State University.

Biological control uses theplant’s natural enemies such asinsects, mites, nematodes, andpathogens to control the weed.The feeding action of insectsweakens the plant and hampersits competitive ability to invadenew sites. There are manyexamples where biologicalcontrol of weeds has beeneffective, for example, knap-weed, yellow starthistle,goatweed (St. Johnswort), andleafy spurge. Biological controlfor hawkweeds is a new pro-gram still in development, sobiocontrol agents are notcurrently available. However,hawkweed studies now beingconducted in Europe willdetermine which insects and/or pathogens will specificallyattack only the invasive hawk-weeds and not native speciesand other valuable plants.These biocontrol agents will beintroduced only after proven tobe safe.

Long-term hawkweedmanagement in the westernUnited States requires carefulimplementation and integra-tion of a host of management

Fig 4. Meadow hawkweed flowers.

Fig 6. Meadow hawkweed stolons enable rapid growth of large colonies.

Fig 3. Meadow hawkweed.

Fig 5. Meadow hawkweed seeds withtawny bristles.

Pacific Northwest Extension publications are jointly produced by the three PacificNorthwest states—Idaho, Oregon, and Washington. Similar crops, climate, and topogra-phy create a natural geographic unit that crosses state lines. Since 1949, the PNWprogram has published more than 400 titles. Joint writing, editing, and production haveprevented duplication of effort, broadened the availability of faculty specialists, andsubstantially reduced costs for the participating states.

Pacific Northwest Extension publications contain material written and produced forpublic distribution. You may reprint written material, provided you do not use it toendorse a commercial product. Please reference by title and credit Pacific NorthwestExtension Publications.

Published and distributed in furtherance of the Acts of Congress of May 8 and June 30,1914, by the University of Idaho Cooperative Extension System, LeRoy D. Luft, director;the Oregon State University Extension Service, Lyla Houglum, interim director; andWashington State University Cooperative Extension, Harry B. Burcalow, interim director;and the U.S. Department of Agriculture cooperating.

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and control strategies. Theseinclude not only herbicides,fertilizer, and biocontrolagents, but also managinggrazing, limiting disturbance,early detection, and coopera-tion between private andpublic landowners.

About the authors

Robert H. Callihan is extension professoremeritus of weed scienceLinda M. Wilson is research supportscientist of entomologyJoseph P. McCaffrey is professor ofentomologyTimothy Miller is Extension supportscientist of weed science in the Universityof Idaho’s Department of Plant, Soil andEntomological Sciences in Moscow.