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Towards the end of the 2006 growing season, several regional agronomists in the south central and southwestern part of Missouri brought it to my attention that we have a "new" weed in our state that is spreading and developing into a real problem in certain environments. The areas most commonly affected are pastures, hay fields, roadsides, and other right-of-way and non-crop areas. The culprit is known as spotted knapweed (Figure 1), which really isn't a brand new weed to Missouri but one that wasn't very noticeable and much of a problem until just recently. Spotted knapweed is listed as a noxious weed in at least 15 other states but is not currently listed as a noxious weed in Missouri.

Spotted knapweed is a biennial or short-lived perennial with a pale bluish-green color that commonly grows to 3 or 4 feet in height. Spotted knapweed develops initially as a rosette, then bolts and produces flowers usually sometime in mid- to late-June in Missouri. The distinct color of this weed makes it easily identifiable in pastures or along roadsides even before flowers are produced (Figure 2, Figure 3).

Figure 1. Spotted knapweed. Photo courtesy of Kevin Bradley.

Spotted knapweed produces many white to pinkish-purple flowers at the ends of branches and each flower has spotted bracts below it that give the plant its name (Figure 4). There are several other knapweed species such as Russian knapweed and white-flowered knapweed, but so far the majority of infestations that I have seen in Missouri are in fact spotted knapweed, which is probably the most aggressive species in the family.

Regional extension agronomists who have been working in south central and southwestern Missouri for many years have indicated to me that even as little as two or three years ago spotted knapweed simply was not present. Unfortunately, these comments give us some insight into the invasive nature of spotted knapweed and how quickly it can take over these kinds of areas. Research conducted by Sheley and Jacobs (1997) indicates that after invading an area, spotted knapweed density can increase dramatically over time while simultaneously causing a decrease in the desirable forage grass in the area. One of the reasons for this is a natural defense allelopathic chemical known as catechin, which is emitted from the roots of spotted knapweed. Catechin acts as a herbicide itself, inhibiting the growth and germination of nearby weeds or desirable forage grasses. Another reason that spotted knapweed can spread and take over an area is that it can typically produce about 1,000 seeds per plant, which are able to survive in the soil for at least 8 years. Cattle also tend to avoid grazing spotted knapweed, presumably due to poor palatability and the bitter tasting compounds present in these plants.

Figure 2. Texas County, Missouri is a spotted knapweed infestation prior to blooming. Photo courtesy of Kevin Bradley.

Current research indicates that spotted knapweed can reduce grass production anywhere from 60 to 90 percent. Other researchers in Montana have observed a 63 percent reduction in cattle carrying capacity as a result of spotted knapweed infestations. In Missouri, spotted knapweed has not historically been a problem but appears to be spreading. It is clearly a species that is capable of reducing cattle carrying capacity, forage yields, and causing severe economic losses.

One of the first strategies we should adopt for spotted knapweed in this state is prevention. Unfortunately, I think we are past the point of prevention in certain regions where spotted knapweed is already established. For the remaining areas, however, successful prevention strategies can keep this weed from becoming a state-wide problem. Spotted knapweed seeds generally fall within 4 feet from the parent plant so wind dispersal is not a significant factor in the spread of this species. Seed can be dispersed long distances by wildlife and livestock, and can be carried long distances in rivers and streams. Additionally, one of the most common methods of dispersal is from contaminated hay or vehicle undercarriages. All of these factors should be considered before moving or purchasing hay, cattle, or equipment from spotted knapweed-infested areas.

If you already have spotted knapweed as a problem, cultural controls such as mowing and/or grazing alone are not likely to eliminate an existing infestation. However, sheep and goats will readily graze spotted knapweed and may reduce a stand significantly over time. Mechanical digging or pulling can be an effective method of elimination if you have a very sparse or new infestation, but the entire rootstock must be pulled from the soil in order for this strategy to be effective.

Figure 3. Spotted knapweed infestation along a roadside in Missouri. Photo courtesy of Kevin Bradley.

For those who already have infestations of spotted knapweed, spot-spraying with herbicides or broadcast sprays over the entire infestation are likely to be the most practical method of eliminating a stand. Previous research indicates that, as a broadcast spray, 1 pint of Tordon 22K per acre applied to spotted knapweed in the spring rosette, fall rosette, bolt, or bud stage of growth will provide good control one year after treatment. Similar levels of spotted knapweed control can be achieved with Milestone at 5 or 7 fluid ounces per acre. Although each of these herbicides are likely to provide 80 to 90 percent control of a spotted knapweed population the year following treatment, there are likely to be some survivors so spot-spraying spotted knapweed escapes will be critical to the overall success of the eradication program. Tordon 22K and Milestone can also be used as higher volume (1 to 2 percent) spot sprays in areas with light spotted knapweed infestations, and for these situations this approach will be much more economical than broadcast sprays over the entire acre.

Figure 4. Spotted knapweed flowers. Photo courtesy of Kevin Bradley.

Kevin Bradley
BradleyKe@missouri.edu