Narrowleaf hawksbeard: Identification, Biology and Integrated Management

Narrowleaf hawksbeard is a highly adaptable and difficult-to-control weed present in northcentral and northeastern Montana, northwestern North Dakota and in parts of Canada. As a facultative (fall and spring germinating) annual it can be a problem in reduced and no-till fields, rangeland, land in the Conservation Reserve Program (CRP), along roadsides and in other disturbed areas. Narrowleaf hawksbeard can reduce crop yields and forage quality due to competition and low palatability.

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Figure 1. Narrowleaf hawksbeard in a field.

Origin and Distribution

Narrowleaf hawksbeard (Crepis tectorum) is native to Eurasia and occurs from Siberia through Mongolia to France. It was introduced to eastern North America as early as 1890 and was recorded in Alaska in the early 1970s. In Montana, narrowleaf hawksbeard has been recorded in 16 counties, and the largest and problematic populations occur in the northeastern part of the state (Figure 1 and Figure 2), where it was introduced as a seed contaminant of alfalfa seed and via natural dispersal from infestations in southern Saskatchewan and Alberta. Narrowleaf hawksbeard is a state-listed noxious weed in Alaska and Minnesota and in the Canadian provinces of Alberta and Manitoba.

Biology, Identification, and Invasion Potential

Narrowleaf hawksbeard germinates predominantly in the fall, but it can also germinate in the spring. After fall emergence, plants overwinter as basal rosettes, before bolting and flowering in the spring. Spring seed germination begins when soil temperatures reach 41°F (5°C). Flowering plants range in height from 4–30 inches. Narrowleaf hawksbeard has a shallow taproot. Basal leaves can be toothed or slightly lobed and can resemble dandelions and prickly lettuce, two closely related species (Figure 3). Leaves on bolting stems are narrow and less than one inch wide. All parts of the plant exude a milky sap when broken. To distinguish hawksbeard from similar species, look for soft, fuzzy hairs and two rows of bracts on the flower buds (Figure 4), lobed basal leaves that may be absent at flowering, and alternately arranged narrow leaves on the flowering stem. Hawksbeard is easy to pull from the ground due to its annual habit, while perennial and biennial look-alikes are difficult to pull.

Two main flowering periods occur annually for narrowleaf hawksbeard. The first is late June through July and the second is September through October; though some flowering can occur throughout summer when conditions are favorable. Plants have 5–20 flowering heads. In each head there are 30–70 yellow ray-and-disk flowers (Figure 5). Each plant can produce hundreds to thousands of seeds that can remain viable in the soil for two to three years.

The seeds of narrowleaf hawksbeard are readily wind dispersed because the seed is attached to a fluffy pappus, and the rough seed coat attaches to animal fur and clothing. Seeds exhibit very little dormancy with over 90% germinating within the first year.

Narrowleaf hawksbeard can grow in alkaline as well as acidic soils and is adapted to a wide range of soil types from a sandy loam to heavy clay.

Figure 2. Counties colored green indicate where narrowleaf hawksbeard has been recorded in Montana.

Figure 3. Narrowleaf hawksbeard rosette.

 Figure 4. Flower bud of narrowleaf hawksbeard showing the distinguishing soft, fuzzy hairs and two rows of involucral bracts.

Management Strategies

To manage narrowleaf hawksbeard with herbicides in cropland, two applications are needed, one in fall, followed by one in spring. Fall applications are critical for optimum control of fall-emerging plants, which will be more robust, bolt more quickly and produce more flowers and seeds than spring-emerging plants. A fall application of glyphosate at 21 ounces per acre (oz/ac) with a 5.5 lb active ingredient per gallon product followed by another spring pre-plant glyphosate application of 32 oz/ac results in effective control. Other herbicides are also effective, but herbicide choice and tank mixes depend on the crop, subsequent crop, timing of application, and plant back intervals of the herbicide (Table 1). If planting wheat after a pre-plant glyphosate application, tank- mixing other broadleaf herbicides that will have some residual control of narrowleaf hawksbeard is an option. Examples of such herbicides include those with the active ingredients florasulam, fluroxypyr, sulfonylureas, 2,4-D, bromoxynil, and dicamba.

Post-emergence herbicide applications in wheat that provide greater than 80% control include Everest (flucarbazone-sodium), Affinity BroadSpec (thifensulfuron-methyl + tribenuron-methyl), Huskie (bromoxynil + pyrasulfotole), GoldSky (florasulam + fluroxypyr + pyroxsulam), Quelex (halauxifen-methyl + florasulam), Starane Flex (florasulam + fluroxypyr), and Talinor (bicyclopyrone + bromoxynil) (Table 1). Always read and follow label directions when using herbicides and be cautious regarding plant back restrictions if planning on planting broad-leaved crops such as peas, lentils, chickpeas, mustard, canola, or flax the following year. Narrowleaf hawksbeard resistance to Group 2 (ALS inhibitor) herbicides was documented in Alberta, Canada in 2011.

Figure 5. Narrowleaf hawksbeard (Crepis tectorum) is in the Asteraceae family and has showy yellow flowers.

In pulse crops, herbicide options are limited for both pre-plant and in-crop herbicides, so it is important to control narrowleaf hawksbeard during the cereal grain rotation. A fall glyphosate treatment to peas at 21 oz/ac followed by a spring pre-plant application of saflufenacil (Sharpen (2 oz/ac)) mixed with glyphosate (21 oz/ac) controls narrowleaf hawksbeard before planting because earlier seeding of this cool season pulse crop ensures treatment when rosettes are smaller. Glyphosate (21 oz/ac) mixed with flumioxazin (Valor 2 to 3 oz/ac) in the fall followed by a pre-plant glyphosate (21 oz/ac) spring application is also effective in managing narrowleaf hawksbeard prior to planting to pulse crops. In alfalfa, imazethapyr (Pursuit (2 to 3 oz/ac)) has proven effective when the alfalfa is less than 6 inches tall.

Tillage controls weeds by uprooting plants and subjecting them to desiccation, or by burying plants deep enough that they cannot re-emerge from the soil. Tillage has been effective on narrowleaf hawksbeard.

In non-cropland settings, including CRP, rangeland and pasture, the current recommendation is to apply one pint/ac 2,4-D in fall after other broadleaf species have gone dormant or in spring before they break dormancy. Rates lower than one pint per acre (pt/ac) 2,4-D are not effective in managing narrowleaf hawksbeard. Results from a trial in Valley County indicated that early spring applications were likely to control both the fall and spring emerging plants, signifying spring would be a more efficacious time to apply in CRP, range and pasture lands.

Table 1. Herbicides and tank mixes to control narrowleaf hawksbeard in fall, spring, and in cereal crops.

¹ Methylated seed oil, ² 2,4-D LV6