published in


October 2013

Sometimes itís the Less Common Weed

by Dr. Brent Sellers and Dr. Jay Ferrell

For questions or comments regarding this publication contact: Brent Sellers

According to the 2012 Southern Weed Science Society Proceedings, the ten most common weeds in Florida are: Dogfennel, smutgrass, tropical soda apple, cogongrass, blackberry, wild radish, broomsedge, nutsedge species, thistle, and Mexican tea (Jerusalem oak). Since these weeds do occur so frequently across Florida, a great deal of research has been collected over the years and we can provide recommendations in almost any situation.  However, these weeds are by no means the only ones you find in a pasture.  There are many other weed species that can be dominate certain pastures and be very difficult to control.  Since these less frequent weeds are often seasonal, or regional, our knowledge of them is often much less. But since we constantly test products and look to expand our control recommendations, this article will discuss some new insights about these weeds and their control. 

Maypop passionflower (Passiflora incarnata).  Maypop (Figure 1) is a climbing, deciduous perennial vine with tendrils.  Vines grow to at least 6 feet in length.  Reproduction occurs through both the creeping root system and seed.  The large, oval to round fruits contain many seeds and can be consumed by both humans and wildlife.  The flowers are purple and white.  While previous greenhouse research has shown that 2,4-D or 2,4-D + dicamba (Weedmaster, others) is effective on maypop, our research in Osceola County has shown otherwise, providing 30 to 45% control at best.  The best current treatment we have found includes GrazonNext HL + Pasturegard HL at 24 + 8 oz/A, with control around 90% at one month after treatment and 80% at two months after treatment.  The decline in control from one to two months after treatment indicates that regrowth from the root system was occurring and that this level of control will likely be temporary.  Therefore retreatment will likely be necessary.  We have also seen some promising control with treatments containing aminocyclopyrachlor (new active ingredient from DuPont).  The premixes of aminocyclopyrachlor with 2,4-D amine or triclopyr (Remedy Ultra, others) provided 80% control two months after treatment.  The interesting thing about these aminocyclopyrachlor treatments was that we did not observe as much regrowth in these plots as in the GrazonNext + Pasturegard plots.  We expect that at least one of aminocyclopyrachlor premixes will be available in 2014.

Figure 1. Maypop passionflower has large, deeply lobed leaves, with purple and white flowers. The fruits are large, nearly baseball size, that pop when stepped on. Photograph by B. Sellers.

St. John’s wort (Hypericum sp.). There are at least 25 different species of St. John’s wort (Figure 2) in Florida.  These species are either annual or perennial herbs or shrubs that can grow as tall as 13 ft in height.  Most of the problematic species in this genus are typically the shrub type and are typically found in marshes, wet pinelands, swamps, and poorly drained flatwood soils.  Except for three species where the bark is smooth and tight, the bark is usually loose.  The flowers are yellow and contain either 4 or 5 petals, depending on the species. Although these species are not common throughout the state, they can form dense canopies once established.  GrazonNext HL alone at 24 oz/A provided 100% control at one month after treatment and 95% control one year after treatment.  Pasturegard HL at 24 oz/A and triclopyr (Remedy Ultra, others) at 2 pt/A provided 100% control one month after treatment, but control dropped to 78% one year after treatment.  At this point in time, we have only investigated the control of Edison’s St. John’s wort, and we are assuming control will be similar among most of the St. John’s wort species.

Figure 2. Edison’s St. John’s wort is a woody shrub common in poorly drained sites.  It has yellow flowers with four petals.  Photograph by B. Sellers

Milkpea (Galactia spp.). Milkpea (Figure 3) species are native to Florida and several are found throughout the state, with Elliott’s milkpea (Galactia elliottii) found in central and south Florida in and around range sites.  A member of the legume family, this vining plant typically emerges in the late spring and begins flowering as early as June.  The vines are hairy, with a somewhat woody base. Flowers are either white or pink to purple, depending on the species.  Although no information can be found suggesting that the plant is toxic, we have observed that cattle do not readily graze these species.  In fact, we have observed that they will graze around the plants when abundant in pastures.  Typically, we do not observe these species in improved pastures, but we will find them in poorly managed bahiagrass pastures.  Since it is a legume species, GrazonNext HL is very effective, providing nearly 100% control during the growing season of application, and greater than 95% control one year after treatment.  Meanwhile 2,4-D and 2,4-D + dicamba also provided 80% control one year after treatment in our plots. Aminocyclopyrachlor premixes also provided control similar to that of GrazonNext HL.  Interestingly, cattle had grazed the plots where milkpea had been removed by herbicide treatment, but grass was usually taller where milkpea plants were uncontrolled (Figure 4). 

Figure 3.Milkpea is a legume vine with either white or purple flowers, depending on the species.  Photograph by B. Sellers.

Figure 4.  Cattle do not readily graze milkpea, but rather tend to graze around the plants. Photograph by B. Sellers.

Bushmint (Hyptis verticillata).  Bushmint (Figure 5) is a woody perennial species that typically grows in relatively shady areas and can grow to 10 ft in height.  Stems of this plant are typically hairless or sparsely haired, branched, and contains opposite leaves.  As it is a mint species, it has a very unpleasant mint odor when leaves are crushed or when the stems are mowed.  The most promising treatments for this woody plant include either triclopyr (Remedy Ultra, others) at 2 pt/A or Pasturegard HL at 16 to 24 oz/A.  Timing appears to be important for control of this plant as we have observed greater control in the sping when the plan is more herbaceous in nature. Retreatment will be necessary, regardless of the herbicide used, but control should range in the 70 to 80% range within one year of a single treatment. 

Figure 5.  Bushmint is a woody species with opposite leaves.  When leaves or stems are cut it has a very unpleasant odor.  Photograph by B. Sellers.

Paw Paw (Asimina spp). There are at least 10 species of paw paw (Figure 6) in the state.  These perennial shrubs are usually not overly numerous in pastures, but have become problematic in areas within a pasture. These multi-stemmed plants typically lose their leaves in the fall and have large green to white flowers in the spring prior to new leaf formation.  The taproot of these plants tends to be relatively large and extends several feet into the soil.  Control of these plants can be difficult as triclopyr (Remedy Ultra, others) and PastureGard HL typically only results in defoliation, with regrowth from the large tap root within a couple of months, but regrowth from PastureGard HL treated plants is typically slower.  The best treatment we have seen is the premix of aminocyclopyrachlor + chlorsulfuron (Telar), which provided 70 to 90% control one year after treatment.  It is possible that the chlorsulfuron portion of this premix is the active component since aminocyclopyrachlor alone does not appear to be quite as good.  Since these plants are typically not widespread throughout a given pasture, spot-treatment is probably the best method for control.  We suggest a 1-2% solution of PastureGard HL using methylated seed oil as a surfactant to help absorption into the waxy leaves.  Since the taproot is rather large, retreatment will likely be necessary.

Figure 6.  Paw paw are multi-stemmed woody shrubs arising from a large taproot.  Leaves are deciduous and green to white flowers form in the spring prior to new leaf formation.  Photograph by B. Sellers.

Whitehead broom (Spermacoce verticillata). Whitehead broom (Figure 7), also known as shrubby false buttonweed and southern larraflower, is a non-native species that appears to be increasing in south Florida pastures, and it is common along roadsides and other disturbed areas.  It is a fine-stemmed herbaceous to woody shrub with square to rounded stems.  Leaves are opposite, with two or a cluster of smaller leaves in whorls at the nodes.  The small white flowers grow in round heads in both terminal and lateral positions on the stem.  The terminal part of the stem continues to grow through the center of the flower so that the seeds develop at nodes in mid-stem. This species is a very important nectar source for the larra wasp, one of the most important biocontrol agents for the mole cricket.  However, if this plant is common on your property, care should be taken to contain it within its current location as we have yet to find a promising selective herbicide treatment. 

Figure 7.  Whitehead broom has square to rounded herbaceous to woody stems.  Leaves are opposite with smaller leaves appearing in whorls around each node.  Photograph by B. Sellers.

Ragweed parthenium (Parthenium hysterophorus). Ragweed parthenium (Figure 8) is a relatively new weed to pastures in south Florida.  It appears to be increasing, especially where cull vegetable crops have been fed to cattle in pasture.  Whereas common ragweed leaves are opposite at the base of the stem, ragweed parthenium leaves are always alternate.  Ragweed parthenium can grow to 6 ft in height.  It can be toxic to livestock, and has resulted in death of cattle when it was 5-10% of the diet.  However, it is usually sparingly eaten by cattle and other livestock. Control of this species in pastures, unlike in other cropping systems, appears to be relatively easy.  Herbicides such as 2,4-D (2 qt/A), 2,4-D + dicamba (2 qt/A), GrazonNext HL (24 oz/A), PastureGard HL (24 oz/A) have all been shown to provide 90 to 100% control of 20 to 36-inch tall plants.  Aminocyclopyrachlor premixes containing either 2,4-D or triclopyr (Remedy Ultra, others) are also effective.  Since this plant can be toxic to cattle and is known to contain allelopathic substances, it is best to get a handle on this species before it becomes a large scale problem in your pastures.

Figure 8.  Ragweed parthenium has alternate deeply lobed leaves, unlike common ragweed, which has opposite leaves near the base of the plant.  Photograph by B. Sellers. 

Sometimes it is the less common weed that is problematic in your pastures.  Thankfully, with the exception of whitehead broom, we have options to at least begin managing some of these species.  We expect that aminocyclopyrachlor premixes will also help in controlling these species in the future. However, we do not know exactly when the aminocyclopyrachlor products will be available for use in Florida pastures.  We are expecting at least one of the premixes to be available for use in 2014.  If you find that you have one of the seven weeds covered above and you have questions concerning their control, please feel free to contact us or your local county extension agent.