ONA REPORT

published in

THE FLORIDA CATTLEMAN AND LIVESTOCK JOURNAL


March 2011

Fireweed Biology and Control in Pastures

Dr. Brent Sellers
University of Florida/IFAS


For questions or comments regarding this publication contact: Brent Sellers


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 north and central Florida, particularly 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.  Due to the many frost events this winter, we are expecting fireweed to be more prevalent than in the past.

Fireweed is particularly troublesome because it possesses stinging hairs (Figure 2) 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. These symptoms commonly manifest themselves as weight loss, or difficulty in swallowing and breathing for many days after consumption. In extreme cases, young horses have died after rolling in fireweed and becoming over-exposed to the toxins in the leaf hairs.

Biology.  Fireweed seedlings are extremely small (Figure 3), and many don’t see this plant until it becomes a problem.  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 (Figure 4). 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.

Control.  Our research with this seasonal species showed that 2,4-D and Telar were ineffective on fireweed (Table 1).  Glyphosate, which can be applied when pasture grasses are dormant, provided control ranging from as high as 80% to as low as 30% under research conditions. Because glyphosate inconsistently provides fire weed control and can severely injure pasture grasses that are not fully dormant we do not recommend the use of this product for fireweed control in pastures. Weedmaster at 1.5 qt/A did not provide acceptable levels of control, but GrazonNext, Remedy, and Pasturegard were all 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.

It is our recommendation that GrazonNext, Remedy, or Pasturegard herbicides be used for effective control of fireweed. These herbicides can be applied any time of year to warm-season forage grasses. There are no grazing restrictions for beef cattle with these herbicides, but lactating dairy animals must be removed for 0 and 14 days with GrazonNext and Remedy, respectively, and one season for Pasturegard.   Mowing provides no benefit to control of this species. In fact, mowing has been found to result in smaller plants, but with many more stinging hairs. Additionally, the seed is surrounded by a sticky substance that can be transported by mower blades to areas not infested with this weed.  If not controlled, fireweed generally disappears in May with the onset of summer temperatures.

Table 1. Control of fireweed with various herbicides.

Herbicide

Rate
Product/A

Herbicide cost 1
$ per acre

% Control

2 wat2

6 wat

2,4-D amine

2 qt

6

10

0

Weedmaster, others

1.5 qt

7

33

65

Remedy, others

1 qt

14-20

93

100

Pasturegard

1.5 qt

20

92

100

GrazonNext

1 pt

8

95

100

Glyphosate

1 qt

3

70

80

Telar

0.5 oz

10

30

10

1These are approximate values taken from “Approximate Herbicide Pricing” (http://edis.ifas.ufl.edu/wg056) and do not include application costs.
2wat=weeks after treatment.

 

Figure 1.  Fireweed is characterized as a weak-stemmed plant with fluid-filled, stinging hairs.  If not controlled with herbicides, fireweed typically dies by the end of May.

 

Figure 2. Fireweed plants have fluid-filled stinging hairs that cause irritation and an intense burning sensation.  The stinging hairs are present on the stems, petioles (leaf stalks) and leaves.

 

Figure 3. Fireweed seedlings are extremely small and are typically one-half the size of an acorn at the cotyledon growth stage. 
fireweed seedling

 

Figure 4. Fireweed leaves are oppositely arranged on the stem.  Leaves are heart or triangle in shape and leaf edges are coarsely and bluntly toothed.  Stinging hairs are also present on leaf margins as well as the surface. 

fireweed leaves

 


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