With the widespread adoption of minimum and no-till practices in field crops, Narrow Leaf Hawksbeard (Crepis tectorum has become one of the more difficult and potentially yield-robbing weeds in agronomic crops. It is technically classified as a winter annual, but this is only half of the story--it also has a strong flush in the spring and summer from both seed and overwintering rosettes. Because of this, it can quickly become troublesome when not kept in check.
Some call hawksbeard ‘dandelion on steroids’ because its flower might resemble dandelion from a distance but can grow to 12-39” (30-100 cm) in height. While maturing, it exhibits one main stem, branched, hairless, and leafy. The term ‘narrow-leaf’ depicts its 0.5” (13mm) wide leaves, unlike dandelion. Composites like hawksbeard will characteristically contain a milky sap in the center of the stem when cross-sectioned. Other weeds that can be confused for hawksbeard include sowthistle and prickly lettuce, due to the shape of the leaves. With hawksbeard, look for leaves arranged in an alternate pattern along the stem.
Normal rates of 2,4-D and related phenoxy herbicides are not sufficient to control Narrow-Leaved Hawksbeard. This is particularly challenging in no-till and minimum-till fields where the soil is not inverted by plowing or rotated fairly often. Control must occur early in vegetative stages to suppress this weed. It is very tolerant to leaf-absorbed and translocated herbicides once the plants bolt before flowering.
The invasiveness associated with Hawksbeard has two important factors that lead to its success as a yield reducer. First, a single plant can produce as many as 50,000 seeds. The winter annual phase sets its seeds in mid summer; while seeds from the spring germinating seeds mature later in the summer and early fall. They germinate readily, and have very little if any dormancy. Hence, over the years of conventional tillage systems, plowing and finishing the seedbed often reduced the population of this weed; but as tillage is reduced, they have greater windows of opportunity to take root and reseed. Seed can remain viable for up to five years in certain climates and conditions. The other characteristic trait is that hawksbeard has a deep taproot, robbing moisture and nutrients from the crop as it tries to compete.
The good news, if any, is that an actively growing and healthy crop suppresses the weed’s ability to take hold as long as it canopies over before the hawksbeard emerges.
All that being said, it becomes clear that an Integrated Crop Management approach is the best way to address Hawksbeard. Though tillage greatly assists in suppressing Hawksbeard because it reduces new seed introduction, it should be followed up with growing a competitive, healthy crop which canopies well. Having a ‘leg up’ on this absence-presence weed can go a long way to suppressing future seed production from Hawksbeard. Use an integrated weed management approach that focuses on the prevention of seed production and the establishment of a competitive crop stand.
Herbicides, when included in control programs, are typically applied in the fall or spring, according to its predominant flush’s life cycle. In the fall, labelled rates of 2,4-D or glyphosate can be effective; but due to the higher rate recommendations for 2,4-D, carryover to sensitive crops planned for the following year should be understood (ex: peas, canola, lentils, others).
Spring applied burndown herbicides in minimum and no-till systems can also be effective. This can often be best achieved with tank mixes of glyphosate, 2-4-D, bromoxynil, MCPA, and other related compounds. The key with these burndown treatments it to kill off the rosettes when they are small, (15 cm across) and always before they have bolted. Translocation is greatly slowed and roots are well developed by this point.
Scouting the field prior to spring treatment is important so as to optimize rate and timing of any herbicide control program; according to rosette development, weather conditions, and other factors. Further, it is important to check the field after the crop begins development to catch any early post emergent weeds germinating in the crop and assessing the need for treatment or cultivation.
Consult your local Certified Crop Advisor or Specialist for more information on best materials to use and cultural practices which are recommended for your geographic area.