Agronomy Library > Soil Conservation

Control of Narrow-leaved Hawk's-beard in Reduced Tillage Systems
Author: Mirza N. Baig & Peter Gamache
Date Created: August 23, 2006
Last Reviewed: August 23, 2006

1. Introduction
Narrow-leaved hawk's-beard (Crepis tectorum L.) is a commonly occurring weed species in northern Alberta and northwestern Saskatchewan. It is usually a problem weed in forage crops. Recently, it has become an increasing problem in reduced tillage systems because of lack of tillage to disrupt its life cycle and its unusual growth habit in behaving as both a summer and winter annual.

2. Description 
Narrow-leaved hawk's-beard is a summer or winter annual; it has a deep taproot and reproduces by seed. The stem is erect, up to 1 m tall, branched and leafy, with a milky white sap. Basal and lower leaves are 10 to 15 cm long and 4 cm wide, toothed or deeply lobed and backward pointing.

Flower heads are bright yellow, containing only ligulate florets, and are surrounded by hairy involucral bracts. The seeds are 2.5 to 4 mm long, ribbed, dark purple to brown with a pappus of hair almost as long as the seed attached to the top (Najda et al. 1982). At the seedling stage, the weed can be easily confused with dandelion; however, dandelion leaves are a darker green than those of narrow-leaved hawk's-beard.

3. Growth and Reproductive Characteristics
Seedlings can emerge throughout the growing season; however, the main flush emerges from mid-May to mid-June, and the second flush is from early August to mid-September. The first flush develops as summer annuals, but the second flush develops as winter annuals, forming low rosettes in the late summer and early fall, which over-winter and resume their growth the following spring.

The over-wintering rosettes of narrow-leaved hawk's-beard are oval to somewhat linear, have several light green leaves, which are 2.5 to 5 cm long and 0.5 to 1.3 cm wide (see Alberta Agriculture's Weed Seedling Guide, Agdex 640-9). Rosettes resume growth very early the following spring. The rapid development of the flowering shoot (called bolting) gives narrow-leaved hawk's-beard a competitive advantage over the crop.

Narrow-leaved hawk's-beard has many bright yellow, dandelion-like flower heads, up to 2 cm wide when fully expanded. Flowers appear in mid-June from the winter annuals and throughout the rest of summer and fall from the spring-germinated annuals.

Narrow-leaved hawk's-beard is a prolific seed producer; each plant is capable of producing 3360 – 49,420 seeds that can easily be spread by the wind. Seeds of winter annuals are set from mid-July to mid-August, and seeds from summer annuals mature from early August to mid-September. Seeds germinate rapidly and have little or no dormancy. They can remain viable for up to five years.

4. Management Strategy
Use an integrated weed management approach that focuses on the prevention of seed production and the establishment of a competitive crop stand.

4.1 Tillage – Narrow-leaved-hawk's-beard starts growing early in the spring; therefore, it is difficult to control in high or low disturbance seeding systems, and a herbicide must replace tillage to achieve any control of this weed.

4.2 Chemical control--Depending on the life cycle (whether summer annual or winter annual), narrow-leaved hawk's-beard can be controlled by fall or spring-applied herbicides.

4.2.1 Fall applications--Under good weather conditions, fall germinating rosettes can be best controlled with a late fall (mid Oct. – until soil freeze-up) application of 2,4-D or glyphosate. However, herbicide residue from 2,4-D could affect germination of peas, lentils or canola the following spring (Control of Winter Annuals in Reduced Tillage Systems).

4.2.2 Prior to seeding (Weed Burn-down) applications – Over wintered rosettes of narrow-leaved hawk's-beard can be controlled prior to seeding with glyphosate alone or glyphosate containing herbicides (glyphosate + 2,4-D, glyphosate + Buctril M, glyphosate + Express Toss-N-G0, glyphosate + MCPA and PrePass), provided the rosettes are less than 15 cm across and have not bolted. The best results with this technique can be obtained by following these guidelines:

· As soon as you can get on the land, scout the fields for narrow-leaved hawk's-beard infestations and determine the stage of weed growth. This step ensures the correct glyphosate application rate for weed size. If the rosettes are less than 8 cm tall or across, use 0.45 kg a.i./ha. Use 0.67 kg a.i/ha rate if rosettes are 8-to 15 cm across.

· Seed a competitive crop into a warm, moist seedbed as soon as possible.

· After seeding the crop, scout the fields for newly germinated seedlings of narrow-leaved hawk's-beard, and, if necessary, apply an in-crop herbicide (Table 1) to control it and other broad-leaved weeds.

4.2.3 Spring in-crop applications
Over-wintered rosettes--In-crop herbicides have limited efficacy on fall emerged narrow-leaved hawk's-beard since rosettes of this weed are greater than 15 cm in diameter and have already bolted. The in-crop application time of early to mid-June in western Canada is too late; the rosettes cannot be controlled at this date.

Summer annuals—A number of herbicides such as 2,4-D, 2,4-DB, Ally plus 2,4-D and Refine Extra are registered for the control of spring-germinated seedlings in cereal and forages (Table 1). For best results, application should be made to small, actively growing seedlings.

Table 1. Registered herbicides for control of narrow-leaved hawk's-beard in cereals and forages 
 

Product

Leaf stage

Registered crops

Wheat

Barley

Oats

Fall rye

Forage grasses

Forage legumes

2,4-D

Seedlings

x

x

x

x

 

2,4-DB*

2 - 4 LS

x

x

x

 

 

Fall appl.

Ally + 2,4-D

Seedlings

x

x

 

 

x

 

Champion Extra

1- 6 LS

x

 

 

 

 

 

Express Pack**

< 10 cm across

x

x

 

 

 

 

Harmony Total

Young and actively growing

x

 

 

 

 

 

K2 + 2,4-D

x

 

 

 

 

 

Refine Extra

x

x

x

 

x

 

 
* 2,4-DB containing products are Cobutox 600, Embutox 625 and Caliber400, ** Fall rosettes and spring seedlings

Note: For up-to-date control information, consult the latest edition of Crop Protection (Agdex 606-1) available from Alberta Agriculture, Food and Rural Development.

Herbicide resistant crops--certain glyphosate-based products such as Roundup Transorb, applied early in Roundup Ready canola and corn, are also an option for controlling annual narrow-leaved hawk's-beard, provided the weed has not started to bolt. When selecting a glyphosate product, consult the product label about weeds controlled and rates.

5. Summary
Narrow-leaved hawk's-beard can be a problem in direct seeded systems because of its unusual ability to behave as both a summer and winter annual. This weed can be most effectively controlled in the late fall (before soil freeze-up) by using herbicides such as 2,4-D, 2,4-DB or glyphosate. An early pre-seed weed burn-down treatment of glyphosate or glyphosate containing products such as PrePass is also an option for the control of this weed. Summer annuals can be effectively controlled by in-crop applications of 2,4-D, 2,4-DB and Refine Extra.

6. References
Hartman, M, Gamache, P., and M. N. Baig. 2004. Controlling Narrow-Leaved Hawk's-Beard in Direct Seeding Systems. Agdex 519-11. Alberta Reduced Tillage Linkages.

Najda, H.G.; A. L. Darwent and G. Hamilton. 1982. The Biology of Canadian Weeds: 54. Can. J. of Plant Sci. 62: 473-481.