Weather's Impact on Pasture Weeds: What to Expect in 2014

Weather's Impact on Pasture Weeds: What to Expect in 2014

Poison hemlock (seen here), musk thistle, common chickweed, henbit, and purple deadnettle appear normal and undamaged from the recent cold temperatures.

Photo: Wikimedia Commons

Pasture managers are generally concerned about the amount of forage produced in their pastures and whether adequate rainfall will occur to support forage crops.

Since 2007, rainfall amounts varied considerably in Kentucky. In 2007, 2008, 2010, and 2012 rainfall was below normal during the peak forage production months of April through September. Forage and hay production was lower during these years, and the impact on weeds was dramatic. The drought impact was severe enough in some regions that few weeds grew or produced their normal seed numbers. However, some weedy species such as common ragweed did produce near normal seed numbers, and we noticed greater problems with this weed in the years since then. Johnsongrass is another species that has thrived over the past few years, and I believe it was the result of the very hot, dry years of 2007 and 2008 when this species also produced near normal seed production. These dry years resulted in stand losses, in some cases very severe losses, and some pastures that were not renovated continue to decline in quality.

However, rainfall in 2013 was several inches above normal throughout most of Kentucky, and many pastures contained high populations of several weeds. Common ragweed and johnsongrass were among the most troublesome. With the wet 2013 growing season and the cold temperatures in January and early February 2014, many questions are being asked about what weeds might appear this year.

Any discussion about weed emergence must begin with an understanding of these pasture plants we call “weeds.” Whether native or non-native, these plants grow only where they are supposed to grow—in an ecological niche that allows them to germinate, emerge, grow, and reproduce before a human activity prevents seed production. Also, adaptive characteristics of weeds generally will include the following: long-lived seeds; seeds that might mature after plants are uprooted or mowed; those weeds that mature in synchrony with pasture management will produce seeds or vegetative organs (i.e., seed production before mowing); and some weeds have a disagreeable odor or taste and are not eaten by animals. It is important to remember that pasture grass greatly influences the number of weeds that emerge in that pasture. A thick, rapidly growing stand of grasses can suppress almost all weed emergence and growth; conversely, poor grass stands allow weeds to germinate and thrive.

Plants native to North America are likely to be the least damaged by drastic changes in soil temperature and rainfall because these plants evolved to thrive under Kentucky’s climate. Examples of such plants include common ragweed, cocklebur, common chickweed, spiny pigweed, tall ironweed, and nimblewill, which are frequent problems in pastures. However, non-native plants are also problematic in horse pastures and include johnsongrass, common bermudagrass, musk thistle, poison hemlock, and purple deadnettle. The above mentioned weeds have the adaptive characteristics that allow them to thrive in horse pastures.

Soil temperatures at or slightly below freezing does not reduce seed viability of the weeds mentioned above. Of the non-native weeds, johnsongrass and common bermudagrass are the two that can reproduce from vegetative propagules in addition to seeds. Johnsongrass produces underground stems called rhizomes, and bermudagrass produces aboveground stems called stolons. Soil temperatures below 15 degrees F for a few days can kill these vegetative tissues and prevent them from growing. Most of the johnsongrass rhizomes occur in the top four inches of the soil and the temperatures at this depth must reach and maintain 15 degrees F for a few days. Because bermudagrass stolons develop on the soil surface, the impact of cold temperatures, especially the 0-10 degrees F experienced in January and February of this year can potentially kill stolon viability.

But how might the 2014 cold temperatures impact johnsongrass or bermudagrass? Probably not much based on soil temperatures at a four-inch depth under sod. The coldest soil temperature reported in Mayfield, Covington, and Lexington, Ky., this year was 31 degrees F and occurred on Jan. 31 and Feb. 1. These temperatures will likely not impact johnsongrass. Soil surface temperature more closely mimics air temperature and could impact bermudagrass stolon viability. Covington and Lexington reported temperatures below 15 degrees F from Jan. 15- 30, and Mayfield reported similar temperatures, except for Jan. 25 and 26. Based on these data, there is a greater likelihood of common bermudagrass being killed compared to johnsongrass. Snow cover insulates the soil, which could mitigate winter impact on bermudagrass stolons.

What weeds can you expect in pastures in 2014? The same ones that occurred in previous years will likely grow and thrive this year as well. Poison hemlock, musk thistle, common chickweed, henbit, and purple deadnettle appear normal and undamaged from the recent cold temperatures.

A frequent question pertains to controlling these weeds. There is not one, simple answer for all these species. First, determine if there is a need for removing the weeds. A poisonous plant such as poison hemlock is controlled by hand-weeding, mowing at a proper time, or herbicides applied in late fall or early spring. Regardless of the method, do not allow animals to graze dying or decaying hemlock plants. Hand weeding and removal of the plants from the pasture is the safest method. Thistles generally are too numerous to hand-weed, and herbicides are required. For most weeds, mowing is not an effective control technique. Mowing might prevent seed production of some weeds; however, to kill many weeds, the mower must cut at about two inches or lower. While this can control some weeds, it also reduces grass production.

Consult your local Cooperative Extension Service personnel for the best control tactic in your area.

William W. Witt, PhD, professor emeritus in the department of plant and soil sciences at the University of Kentucky, provided this information.

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