Spotted knapweed, Centaurea maculosa, was introduced to America via contaminated alfalfa and clover seed in the 1890s. It is considered the “most problematic invasive species in North America” (Emery et al. 2005).
All the purple blooms are for 1 plant!!
This is the plant before blooming
Aside from being an invasive species, which takes the place of native species who host our insects, spotted knapweed contributes to soil erosion, surface runoff, degradation of soil and water, reduces biodiversity, reduces wildlife habitat, and alters the ecosystem processes.
While enjoying dry, disturbed sites, Spotted knapweed is not drought tolerant. In years of low rainfall, a population will decrease, but, of course, there is an opposite effect in years of good moisture! This plant doesn’t seem too particular about the soil types and can adapt to most any type of soil.
The seeds are long-lived and durable because their hulls don’t easily decompose. In the soil, Zouhar (2001) has found buried seeds can remain viable after 5 years. They can germinate in light or dark but rarely when they are 2” or more below the soil surface. There is a wide range of temperature under which they will germinate as well, from 44°F – 93°F (Schirman 1981). While the seeds might germinate under a shady canopy, the adult plants will not survive without full sun.
Seeds are dispersed in several ways. If there is no external assistance, such as wind or mammal, the seed will remove itself from the seedhead and land 3-4 feet from the original plants; they do not overwinter on the plant (Zouhar 2001). Often they are carried to other locations by vehicles, water, shoes, and excrement of mammals and birds. Spotted knapweed seeds can remain viable 7-10 days after they are eaten by deer (Zouhar 2001).
Here’s a scary fact. Schirman’s study (1981) had 8-40 plants in a 5” hoop area, depending on the year and the placement of the hoop. He found that only 0.1% of the seed produced is required to maintain their current stand (Schirman 1981) and newly established populations will expand faster than older ones (Emery et al. 2005). Egads!!
Facts about Spotted Knapweed
- Short-lived perennial or biennial, living up to 3-9 years. This duo-classification is because the plant can live as a rosette for years before completing its life cycle or it can take only a single year. It’s plant specific.
- Stout taproot generates lateral shoots creating a fibrous mat of roots that can extend for several feet
- Produce 1,000 – 20,000 seeds per plant per year; precipitation is a major factor in this (NRCS, Sheley et al. 1998)
- About 90% of seeds are viable; can remain viable in the soil for 5-8 years
- Have spring and fall growth periods
- Older plants can have multiple rosettes and multiple blooming stems
- Allelopathic – leaves and shoots exude cnicin.
There are 29 stems for this 5″ diameter plant. This taproot is 12 1/2″ long with the longest lateral root being 6 1/2.” It’s easy to see from this photo how the plants regrow from the bud/root crown; this must be killed in order to kill the plant
A close up of one of the root crowns in this expanded plant.
Control Techniques
A combination of treatments is required to control spotted knapweed as no single treatment is the “magic bullet.” At Driftless Prairies, we have found herbicide coupled with hand pulling or using the parsnip predator is very effective.
Control of spotted knapweed requires management techniques tailored to all the plant’s life stages. No single treatment is the “magic bullet” but rather a combination of treatments needs to be part of the protocol. Two researchers found that knapweed would “increase to untreated level by the 6th year after single herbicide treatments” (MacDonald et al. 2013).
Herbicides
Herbicide is effective because it kills the root crown. This treatment must be done 1-2 times a year. Two researchers found that knapweed would “increase to untreated level by the 6th year after single herbicide treatments” (MacDonald et al. 2013). We use a 20% mix of Garlon 4 in bark oil and a handheld spray bottle (we call it the spritz method). In late May, we patrol the area either spritzing the plants, pulling them or using the parsnip predator. In late June/early July, we do it again. If we pull or predator the plants, we bag them and carry them out. The allelopathic effect of the cnicin is in the leaves and stems and large infestations can leave chemical in the soil to continue its effect on native plants.
Hand Pulling
Hand pulling was necessary for mowing and herbicide treatments to be effective but was not sufficient on its own; other studies found that persistent hand pulling could decrease the knapweed population (MacDonald et al. 2013). A “substantial” reduction of Spotted knapweed was made after three years of hand pulling coupled with mowing or herbiciding (MacDonald et al. 2013). It is imperative that when pulling that the root crown and top 3” of the taproot is removed (Panke et al. 2012). If there are flowers present at the time of pulling, these plants need to be removed from the site as they can continue to produce viable seed (Panke et al. 2012). Spotted knapweed has also been known to cause skin irritations in some folks so be sure to wear the proper protection on your hands, arms, and legs. Keep in mind that pulling can stir up seeds in the soil and cause germination.
Burning
Burning was the least effective regardless of other treatments used in conjunction with it. According to Sheley and colleagues (1998), fires may actually create the environment where Spotted knapweed can thrive rather than knocking it back as it might with other invasive plants. A literature review found that “in 80% of the studies prescribed burning either increased or had no effect on” knapweed (Emery et al. 2005). Early spring and fall burns were ineffective; fall burns actually increase germination success. Summer burns when the plant was in flower would reduce an infestation if the fire were over 320 degrees F (MacDonald et al. 2013). But…stands of spotted knapweed do not carry fire well making it doubtful the fire would reach the heat intensity required (Emery et al. 2005). If there was a small patch, possibly a handheld propane torch would produce an intense enough fire to kill the plant.
Mowing
Mowing isn’t effective but can be used to triage. The season of mowing is important. Mowing during the bud or flowering stage reduced the number of plants and reduced seed germination (Rinella et al. 2001). A single mowing when the plant was in bud or bloom was ineffective, two years of fall mowing did not have much effect but three years of fall mowing decreased the adult population (MacDonald et al. 2013, Rinella et al. 2001). If you have a monoculture of spotted knapweed, this type of mowing is possible, but if it is interspersed with natives, this mowing regime will harm those natives.
If mowed early in the season, they will regrow and bloom.
This photo was taken in early July and shows the cut stems of a spotted knapweed and its regrowth. It was cut in early June as a triage method for other plants in the same area.
Grazing
Grazing can have a negative effect on the plant. The rosettes are tasty and nutritious but they are so low growing which makes it difficult for cattle to eat. Sheep will eat these at the young stage, reducing recruitment but because of cnicin, a bitter tasting sesquiterpene, they avoid the 2nd year and older plants (Olson et al. 1997). Collateral effects could be the increase of non-native cool season grasses and grazing of natives, which could set them back or kill them.
Bio Control
Biological control seems to be well known for this plant but, I am skeptical of introducing insects that are not native to this area. One study shows that after the release of several species, the seed output was reduced and roots were damaged but the density of the plants were not altered (Rinella et al. 2001). There are 12 known insect species that are used for biocontrol; their purpose is to stress the plant with the hope the plants around it can outcompete it (Sheley et al. 1998). Caution is urged in using biocontrol as it means introducing a non-native insect. As I’m fond of saying, two non-natives don’t make a native.
Spotted knapweed is a very problematic invasive. Plan to spend some considerable time controlling this plant. But know that you rewards will be plentiful. Below are a couple of photos of our remnant where we’ve been removing spotted knapweed since 2018.
Resources
Carpinelli, Michael. Spotted Knapweed. Plant Conservation Alliance’s Alien Plant Working Group Least Wanted.
Czarapata, Elizabeth J. 2005. Invasive Plants of the Upper Midwest. Madison, WI: University of Wisconsin Press.
Davis, Edward S., Peter K. Fay, Timothy K. Chicoine, and Celestine A. Lacey. 1993. Persistence of Spotted knapweed seed in the soil. Weed Science 41: 57-61.
Emery, Sarah M. and Katherine L. Gross. 2005. Effects of timing of prescribed fire on the demography of an invasive plant, Spotted knapweed. Journal of Applied Ecology 42: 60-69.
Jacobs, James S. and Roger L. Sheley. 1998. Observation: Life history of spotted knapweed. Journal of Range Management 51(6): 655-673.
NRCS Pest Management – Invasive Plant Control. 2007. NRCS Conservation Practice Job Sheet MN-797.
MacDonald, Neil W., Laurelin M. Martin, Corey K. Kapolka, Timothy F. Botting, and Tami E Brown. 2013. Hand pulling following mowing and herbicide treatments increases control of Spotted knapweed. Invasive Plant Science and Management 6: 470-479.
Panke, Brendon, Ryan deRegnier, Mark Renz. 2012 Management of invasive plants in Wisconson: Spotted knapweed. University of Wisconsin Extension Publication A3924-13.
Rinella, Matthew J., James S. Jacobs, Roger L. Sheley, and John J. Borkowski. 2001. Spotted knapweed response to season and frequency of mowing. Journal of Range Management 54(1): 52-56.
Schirman, Roland. 1981. Seed production and spring seedling establishment of Diffuse and Spotted knapweed. Journal of Range Management 34(1): 45-47.
Sheley, Roger L., James S. Jacobs, and Michael F. Carpinelli. 1998. Distribution, biology, and management of Diffuse knapweed and Spotted knapweed. Weed Technology 12: 353-362.
Zouhar, Kris. 2001. Centaurea maculosa. In Fire Effects Information Ssytem, U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station.