published in


December 2009

Control of Spiny Pigweed (Careless Weed) in Pastures 

Brent Sellers and Jay Ferrell 
University of Florida/IFAS

For questions or comments regarding this publication contact: Brent Sellers

Spiny pigweed is a common weed in grazed pastures throughout north and south Florida. It is easily identified by its red stems and deeply recessed veins on the leaf (Figure 1). The species is most often found in areas that are heavily disturbed; high traffic areas such as holding paddocks, hay feeding areas, or in pastures that are over-grazed. This weed can grow to heights of 4 feet tall and each plant is capable of producing over 50,000 small black seeds. These seeds germinate and grow very quickly throughout the entire summer and effectively shade desirable grasses, but the main problem with this weed is the sharp spines that occur along the main stem (Figure 2). These spines deter grazing and even inhibit traffic through heavily infested areas. 

Control of spiny pigweed is essential to maintain healthy and productive forage growth. One method of control is mowing. Though mowing provides instant satisfaction by totally removing all existing plants and stimulating grass growth, it will not provide long-term control. Within a few weeks, larger plants will commonly sprout back from the cut stem and new seedlings will germinate (remember that there could easily be billions of seeds in the soil waiting to germinate). Within 4 to 6 weeks after mowing, the pasture will look exactly as it did before. 

Chemical control is the most effective means of controlling spiny pigweed. There are many herbicides to choose from and some are much more effective than others. Below is a list of commonly used herbicides and their relative effectiveness on spiny pigweed. 

Remedy, Pasturegard, and Cleanwave. These three products contain the active ingredients triclopyr, fluroxypyr, or a combination of both. Neither of these active ingredients are particularly good on spiny pigweed. Though Remedy and Pasturegard will control smaller plants, these herbicides are much weaker on larger ones. Conversely, Cleanwave (which principally contains fluroxypyr) is totally ineffective on spiny pigweed. If Cleanwave is to be used, the addition of 2,4-D is essential in order to obtain control of spiny pigweed. 

2,4-D, Weedmaster, Outlaw, and others. These herbicides are relatively inexpensive and are quite effective on spiny pigweed that is 2 feet tall, or less. For plants edging toward 3 feet tall, higher use rates will need to be employed. For plants greater than 3 feet, the overall growing conditions of the plant will dictate control. This means that rapidly growing plants with sufficient soil moisture will likely be controlled if higher use rates are applied. Conversely, larger plants growing in drought stressed conditions are liable to be injured, but not killed by the application. 

Metsulfuron (Cimarron, MSM-60, others) and Telar. University of Florida IFAS research has frequently shown that these two herbicides are incredibly effective against spiny pigweed. At rates of 0.25 oz/A or less, we have consistently achieved near 100% spiny pigweed control, regardless of plant size or growing conditions. The only drawback to metsulfuron is that it is toxic to bahiagrass and cannot be applied to bahiagrass pastures without causing severe injury. On the other hand, Telar is totally safe on bahiagrass. But, Telar has a very limited weed control spectrum. Weeds such as dogfennel, coffeeweed, and ragweed will be totally missed if Telar alone is applied for spiny pigweed control. Therefore, Telar is a great choice if spiny pigweed is the only weed targeted by the application, but will come up short if multiple species are present. 

Grazonnext. Grazonnext contains two active ingredients (aminopyralid and 2,4-D), both of which are active on spiny pigweed. Our research has shown that this herbicide is quite effective on spiny pigweed, regardless of plant size and growing conditions. Grazonnext will control many other broadleaf species as well (tropical soda apple, dogfennel (< 30”), ragweed) but is not particularly effective on woody perennials (blackberry, persimmon, etc). 

As stated previously, spiny pigweed produces thousands of seeds per year and these seeds germinate all season long. None of the herbicides we have tested have ever provided long-term soil residual control of seedlings. Therefore, depending on the density of the pigweed infestation of the forage grass, multiple herbicide applications may be required until the grass is able to out-compete the new seedlings. Spiny pigweed seeds will only persist in the soil for a few years, so your diligent control efforts will be rewarded after a couple of years if you can prevent additional seed production and encourage grass growth. 

No single herbicide will control all possible weeds in a given pasture. But, herbicide selection is much easier if you plan a control strategy early when weeds are small. Case in point, almost every herbicide listed here is effective on spiny pigweed if the application is made before the weeds are too large. Controlling large weeds often requires higher herbicide rates, more dollars per acre, and more planning to find the product (or combination of products) to achieve optimum control. 

Figure 1. The veins of spiny pigweed leaves are deeply recessed.  
Figure 1. The veins of spiny pigweed leaves are deeply recessed. 

Figure 2. The stems of spiny pigweed are armed with sharp spines that can limit grazing in heavily infested areas.  
Figure 2. The stems of spiny pigweed are armed with sharp spines that can limit grazing in heavily infested areas.