ONA REPORT

published in

THE FLORIDA CATTLEMAN AND LIVESTOCK JOURNAL


February 2012

Common Winter Weeds and Their Control

by Dr. Brent Sellers


For questions or comments regarding this publication contact: Brent Sellers

Winter is here, literally, as nighttime temperatures the past couple nights have been near or below freezing in south Florida.  For weeds like TSA, pigweed, crotalaria and other “summer” weeds freezing temperatures bring on their demise, which is what I like to think of as Mother Nature’s herbicide program.  Unfortunately, winter annual weeds have adaptations to survive these extremely cold temperatures and will continue to flourish.  Additionally, these plants are often the only green tissue in pastures and hayfields after a frost.  In this article, we will provide some information on several of the most common winter annual weeds and their control.



Fireweed. Fireweed (Urtica chamaedryoides) is native to Florida, but has only recently become problematic in pastures (Figure 1). This winter annual species is commonly observed in bare ground areas (near feeding pens and under fences) as well as near and under trees where forage grasses tend to be less dense.  Fireweed is particularly troublesome because it possesses stinging hairs that easily embed in skin. Once exposed to the toxin, severe irritation can occur for several hours. Though generally avoided by cattle, horses are more likely to browse fireweed and develop stress symptoms.

Fireweed leaves are opposite in arrangement and resemble that of a strawberry plant, but the plant as a whole has little resemblance to strawberry. Leaves are triangular to heart-shaped in outline, but are bluntly and coarsely toothed. The plant has square stems that are generally 4 to 20 inches tall. Stems are relatively weak and are often supported by surrounding plants.  The plant flowers in spherical clusters and individual flowers are small and pale green in color.  Small stinging hairs are found on the stems, petioles and leaves. These hairs contain irritants which have been shown to cause respiratory stress and local allergic reactions when ingested or inhaled.

Our research with this seasonal species showed that 2,4-D and Telar were ineffective on fireweed. GrazonNext, Remedy (triclopyr), and Pasturegard were found to be highly effective. Within 2 weeks of application, over 90% of the fireweed plants were dead and the remaining individuals were yellow and dying. By 6 weeks after treatment, no fireweed could be found.



Wild Radish. Wild radish (Raphanus raphanistrum; Figure 2) is a member of the Brassicaceae plant family with cabbage, turnip, and mustard, and is one of the most common and problematic pasture weeds in the Florida Panhandle. Generally, wild radish germinates during the fall months when soil temperatures drop below 65° F. Studies indicate a chilling requirement is necessary to break dormancy. In addition, wild radish has a thick fruit pod from which the seed does not shatter free easily. Therefore, the pod must decay before the seed can be released to germinate. 

After emergence, wild radish forms a rosette of leaves throughout the winter and early spring. Seedling wild radish plants possess heart-shaped cotyledons and the first true leaves will be slightly serrated and indented about two to three times as long as wide. As the leaves mature, the serrations will be jagged and more deeply indented. In addition, the leaves are covered with stiff hairs, giving a bristly feel to the touch. The wild radish plant remains in rosette form through most of the winter, reaching approximately 10 to 14 inches across at the base. In the late winter to early spring, as the temperature and day length increase, the plant bolts. Bolting is a process in which the internodes (regions of the stem between leaves) begin to lengthen and a flower stalk forms at the top. In wild radish, multiple flower heads form on several branches arising from a single flower stalk. The flowers are generally yellow but occasionally may be white.

Some of the most effective and inexpensive herbicides for wild radish control are growth regulators such as 2,4-D and dicamba (Banvel, Clarity, etc.). These herbicides provide excellent control of wild radish when properly applied. Tolerance of cool season forages to herbicides will vary according to species. Generally, wheat is the most tolerant and oats are least tolerant to 2,4-D applications. The timing of an herbicide application is critical for effective wild radish control. Research has shown that >90% wild radish control can be consistently achieved when 2,4-D is applied to plants less than 6 inches in height. By delaying the application until the plant reaches 12 inches, control drops to approximately 70%. However, if wild radish begins to flower before 2,4-D is applied, less than 50% control should be expected. Therefore, herbicides should be applied early to achieve the greatest wild radish control while avoiding herbicide injury to winter forage. For bermudagrass hay fields, control of wild radish is usually attempted well after flowering and seed development. Control of fully mature plants with 2,4-D can be very difficult. In these situations, metsulfuron (MSM 60, others) at 0.2 oz product/A is most effective. Depending on temperature at time of application, metsulfuron may require 3 to 5 weeks to control mature wild radish. But this herbicide is highly effective on wild radish and is safe on bermudagrass at any stage from dormant to full green up.



Carolina geranium. Carolina geranium (Geranium carolinianum; Figure 3) is a multi-branched, low-growing winter annual that is problematic in bare areas of pastures as well as roadsides.  However, some research has stated that this species is readily grazed and digestibility ranges from 68 to 78% with 11-19% crude protein. Stems are greenish to pink-red, and are densely hairy.  Leaves are deeply lobed, and generally are no larger than a quarter in diameter. Flowers are pink to purple in color and the seedpod resembles a “stork’s bill” at maturity.  Seed production is relatively high and germination commonly occurs as early as September in some regions of the state.

Control of Carolina geranium is relatively easy as it is sensitive to many of the growth regulating herbicides including 2,4-D and dicamba.  We have also found that it is extremely sensitive to Milestone and GrazonNext herbicides.  For bermudagrass hayfields, metsulfuron (MSM 60, others) at 0.3 oz/A is highly effective and will control many other winter annual weeds as well.



Butterweed. Butterweed, or cressleaf groundsel (Packera glabella; Figure 4) is a winter annual weed that appears to be increasing in density over the past several years.  This is problematic as this species is toxic to all livestock.  Butterweed initially forms a large rosette of leaves in the late fall and winter months prior to bolting in early spring, a growth habit similar to wild radish.  Rosettes are highly variable in shape and leaves are deeply lobed.  Bolting stems are hollow, succulent, and light green in color with many red veins running the length of the stem. Many bright yellow flowers are produced on the end of the stems.

While we do not have a tremendous amount of data on this species, we have observed that applications of 2,4-D + dicamba during the rosette stage provides satisfactory control of this species, while products like  Milestone, GrazonNext, and Pasturegard are more effective once plants have begun bolting. In bermudagrass hayfields, metsulfuron at 0.3 oz/A has provided satisfactory results. 




Heartwing sorrel. Heartwing sorrel (Rumex acetosella; Figure 5), also known as sheep sorrel, sourgrass, Indian cane, and many others, is another common winter annual (sometimes classified as a creeping perennial) that most do not recognize until the reddish flowers appear in late spring.  The stem is somewhat woody at the base of the plant and plant height ranges from 1 to 2 feet, with little or no branching.  Lower leaf blades are somewhat arrow-shaped with one to two basal lobes.  Upper leaves on the flowering stalk tend to be more slender and usually without the basal lobes. Presence of sorrel species in a pasture may be an indicator of low pH as this species tends to thrive under acidic conditions, however, it has been observed growing in pastures where the pH is optimum for forage growth.

Control of sorrel species can be achieved by applying 2,4-D or 2,4-D + dicamba prior to flower stalk emergence for optimum control.  After bolting, Pasturegard and triclopyr products have provided more satisfactory results than 2,4-D-containing products. In bermudagrass hayfields, 0.3 oz/A of metsulfuron is extremely effective.



Cut-leaf evening primrose. Cut-leaf evening primrose (Oenothera laciniata; Figure 6) is a winter annual, or sometimes biennial, that is found throughout Florida.  While this weed is more often found in citrus groves, it has been observed in some pastures and many hayfields, especially those that have been converted from an abandoned citrus grove. Germination typically occurs in late fall and a basal rosette is formed with untoothed leaf margins.  As the plant matures, leaf margins are deeply toothed, and hairs cover the top sides of the leaves.  Most often cut-leaf evening primrose has a prostrate growth habit and stems can reach 3 feet in length.  Stems are usually reddish in color, hairy, and can be either simple or branched from the base of the plant.  Flowers are bright yellow and typically open in the evening and petal fall from the plant within 24 hrs of opening.

Control of this species is relatively easy as size of the plant at application is not as restrictive as with other species.  Herbicides such as 2,4-D, 2,4-D + dicamba, triclopyr, Pasturegard, Milestone and GrazonNext all provide excellent control of cut-leaf evening primrose.  Metsulfuron can be applied at 0.3 oz/A in bermudagrass hayfields. 


Cudweeds. There are around three different cudweed (Gnaphalium sp.; Figure 7) species that are common in Florida.  In general, the plants have basal rosettes and the leaves and seedheads are covered with white hairs, giving the plants a wooly appearance.  Some species only have hairs on the underside of the leaves, whereas others have hairs on all surfaces.  Generally, cudweeds begin to emerge as early as October and begin to grow an upright stem as early as January in some parts of the state.  Typically, cudweeds are problematic only in hayfields, but they are also commonly found in bahiagrass pastures.

Herbicides that are effective on cudweed include 2,4-D, dicamba, 2,4-D + dicamba.  GrazonNext, although it contains 2,4-D has not performed consistently on these species.  Metsulfuron at 0.3 oz/A has resulted in approximately 85% control in our research plots in bermudagrass hayfields.



Henbit.
Henbit (Lamium amplexicaule; Figure 8), a member of the mint family, is a winter annual with square stems that is typically found in hayfields.  Stems are usually purplish in when growing in full sunlight.  Leaves are opposite, egg-shaped with bluntly toothed margins.  Flowers are reddish purple.  Henbit is typically found in north Florida, but some infestations have been observed as far south as Polk County.

Henbit control is relatively tough compared to the other previous species.  Applying 2,4-D alone is often inconsistent and control is often considered fair at best.  Applying dicamba, or 2,4-D + dicamba appears to provide fair to good control.  If henbit is in a bermudagrass pasture, paraquat can be used as long as the bermudagrass is dormant; this treatment is likely the best option for controlling this species.  Winter weed control can be relatively easy and inexpensive in most cases.  Typically, these weeds are less problematic in bahiagrass pastures than in bermudagrass, stargrass or limpograss hayfields.  The first hay cutting of the year is usually accepted as low quality due to winter weeds.  A single, well timed herbicide application can eliminate many of these weeds, resulting in premium quality hay from the first cutting. Additionally, removing these weeds will allow the hayfield to transition from dormancy more quickly.