The first step in developing a successful weed management program is being able to properly identify the species that are infesting a field. But why? Why is weed identification so important?
Simply stated, because not all weeds are created equal. Species differ with respect to their emergence timing, life history traits, competitive interactions with the crop, potential to harm livestock, and sensitivity to available herbicides, among many other characteristics.
Consequently, the type and timing of weed control events need to be designed to target problematic species so that control is maximized, weed seed return to the soil seedbank is minimized, and crop yields are protected.
Unfortunately, weed identification is not always a simple task, especially when it comes to members of the Asteraceae (sunflower family), one of the largest plant families in the world.
Several species of Asteraceae can be found in California’s alfalfa fields, including: hairy fleabane (Erigeron (Conyza)bonariensis), annual sowthistle (Sonchus oleraceus), prickly lettuce (Lactuca serriola), and common groundsel (Senecio vulgaris). This blog post has been developed to help you distinguish among these similar looking weeds.
Erigeron bonariensis – Hairy fleabane:
Distribution: Hairy fleabane (also called asthma weed and flax-leaf fleabane) is an annual (sometimes biennial) herb that is not native to California. According to the USDA’s PLANTS database (https://plants.usda.gov), the species can be found in California, Arizona, Nevada, Oregon, and Washington, and across the southern US as far north as Virginia.
Leaves: Leaves are greenish-grey in color, hairy, and are arranged alternate to each other up the stem. While the first leaves are roughly oval- to spatula-shaped (spoon-shaped), later leaves tend to be more linear with entire to weakly-toothed margins. Leaves of hairy fleabane are also twisted, crinkled or wavy in appearance.
Flower stalks, flowers, and seedheads: At bolting, hairy fleabane produces a many branched stem (0.5 to 3 feet [0.15 to 0.9 m] in height) upon which numerous cream- to yellow-colored, flowers are held. Lateral branches can be longer in length than the main stem; most branches tend to arise from near the base of the flowering stalk.
Flowering occurs, predominantly, mid-summer through fall. The resultant seedheads resemble those of dandelions, although they are much smaller in size (0.2 to 0.3 inches [5 to 7 cm] in diameter). Individual seeds possess a pappus (0.12 to 0.16 inches [3 to 4 mm] in length) on one end that facilitates wind-dispersal.
Roots: Hairy fleabane plants produce taproots along with fibrous lateral roots.
Notes: Hairy fleabane can be confused with horseweed (Erigeron canadensis) at the seedling and young rosette stage. Hairy fleabane and horseweed populations with resistance to glyphosate and with multiple resistance to both glyphosate and paraquat have been confirmed in California.
Sonchus oleraceus – Annual sowthistle:
Distribution: Annual sowthistle is not native to North America but has become naturalized throughout the US and Canada.
Leaves: The first true leaf is round to egg-shaped; following leaves are spoon-shaped with prickly margins. Older leaves are hairless and deeply lobed with the terminal lobe being large and triangular in shape.
Flower stalks, flowers, and seedheads: Stems are erect and hollow between nodes and can reach heights of 4 feet [1.2 m]. Leaves on the stems are clasping at the base. Broken or cut leaves and stems will exude a white sap. Because annual sowthistle can be both a summer and a winter annual, flowers can be produced year-round when conditions are favorable.
Flowers (0.5 to 1 inch [1.2 to 2.5 cm] in diameter) resemble those of dandelions (although a lighter shade of yellow in color). Like other species in the Asteraceae family, pappuses (0.2 to 0.3 inches [5 to 8 mm] in length) are attached to individual seeds, giving the seedhead it’s cotton ball-like appearance. Seeds are wind-dispersed.
Roots: Annual sowthistle produces a short and thick taproot with many lateral roots.
Lactuca serriola – Prickly lettuce:
Distribution: Prickly lettuce is a winter or summer annual (sometimes biennial) that is not native to North America but has become naturalized throughout the US and Canada.
Leaves: Early leaves are elongated and club-shaped with weakly-toothed margins. Later leaves can be deeply- and pinnately-lobed with rounded indentations. The midrib vein is pronounced and covered with a row of spines on the underside.
Flowers stalks, flowers, and seedheads: Plants exist as a basal rosette before sending up a single flower stalk that can reach heights of 6.5 feet (2 m). Leaves alternate along the stem. The stems are covered with stiff hairs that give them a spiny appearance and are branched at the terminal apex. Stems and leaves exude a milky sap when cut or broken.
Flowers (0.3 to 0.4 inches [8 to 10 mm] in diameter) are pale yellow; flowering occurs, typically, from April to October. Like other members of the Asteraceae, prickly lettuce seeds possess pappuses (0.16 to 0.2 (4 to 5 mm) in length) that facilitate wind dispersal.
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Roots: Plants produce deep tap roots with fibrous lateral roots.
Senecio vulgaris – Common groundsel:
Distribution: Common groundsel is not native to North America but has become naturalized throughout the US and Canada. The species is, predominantly, a winter annual and sometimes a summer annual.
Leaves: The youngest leaves are egg-shaped with shallowly-toothed edges. The base of the leaves may be purple on the underside. Later leaves are arranged in a rosette, are more deeply (but irregularly) lobed, and may be hairless to hairy.
Flower stalks, flowers, and seedheads: At maturity, groundsel plants can send up a single stem or else a series of branches from the base; stem heights can reach 2 feet (0.6 m). This species has often been described as having a ‘ragged’ or ‘scraggly’ appearance.
Common groundsel produces small, yellow flowers in clusters that may be drooping. Individual flowers are surrounded by green bracts with black tips. Flowering can occur throughout the year. Like other members of the Asteraceae, a pappus, a tuft of hair that aids with wind dispersal, is attached to each seed.
Roots: Common groundsel plants produce small taproots with fibrous lateral roots.
Notes: The presence of common groundsel can be problematic for alfalfa and forage producers; the species produces pyrrolizidine alkaloids, that can lead to liver damage in livestock following consumption.
Even hay with a small amount of contamination can be problematic as pyrrolizidine alkaloid poisoning can accumulate in the livestock over months of feeding.
While similar in general appearance (i.e. alternate leaves arranged as a basal rosette, flowers produced on upright stems, and seeds with pappuses that allow for wind dispersal), hairy fleabane, annual sowthistle, prickly lettuce, and common groundsel all possess unique morphological and phenological characteristics that differentiate them from one another.
Properly identifying these and other species that interfere with alfalfa production is the first step in developing a safe and effective weed management program. For example, the presence of glyphosate-resistant hairy fleabane may reduce the value of glyphosate as a stand alone active ingredient for use in Roundup Ready alfalfa.
For more information about UC recommendations for pest control, including weeds, in alfalfa visit: here.
Weed identification is also critical for detecting the presence of poisonous plant species in hay and forage. Repeated consumption of common groundsel can result in irreversible liver poisoning of livestock; a quick and proper ID of this species is an important tool for limiting animal exposure to toxic alkaloids.
Many tools are available to assist growers, PCAs, and other crop consultants with weed identification, including the reference material listed below. Your local county farm advisor can also be an extremely useful resource for determining the identity of suspect species.