Spotted Knapweed

Spotted Knapweed (Centaurea stoebe ssp. micranthos) Susan Farrington Natural History Biologist, Ozark Region Missouri Department of ConservationSpotted knapweed was first introduced to North America from Eurasia as a contaminant in alfalfa seed inBritish Columbia in 1893. It’s suspected that there were multiple accidental introductions, not just one.Slide 2Spotted knapweed is a short lived perennial which reproduces solely from seeds. It is called “spotted” because it has dark spots on the floral bracts.

It typically forms a rosette in its first year, sending down a very taproot.Slide 4It flowers in subsequent years, typically flowering in June in our area.

It’s good to be able to recognize the seedheads, which retain the spotted bracts. These seedheads are visiblethroughout the winter.Slide 6The leaves are usually highly lobed and grey green in color, whitened with tiny hairs. They resemble a thistleleaf somewhat, but

Native field thistle (Cirsium discolor) Exotic bull thistle (Cirsium vulgare)there are no prickles on the leaves or stems. Here you see native field thistle on the left, with its whitenedundersides, and exotic bull thistle on the right, with its green undersides.Slide 8 Native Grey headed coneflower (Ratibida pinnata) Spotted knapweedIt also resembles native grey headed coneflower, but the coneflower leaves are a darker green and have fewerand larger lobes.

Spotted knapweed Grey headed coneflowerConeflower also has spreading hairs on the stems and leaves, whereas knapweed’s hairs are very tiny and fineand lay tightly against the leaf and stem.Slide 10Knapweed seeds remain viable in the seed bank 5-8 years, and germinate from spring through fall. Mowing itback does not prevent it from flowering or going to seed: I have seen it in a close-cropped lawn in upstate NewYork, blooming at 2” in height!

Spotted knapweed is extremely adaptable, found at elevations up to and over 10,000 feet, and in areas thatreceive from 8 to 80 inches of rain annually.Slide 12It prefers well-drained, light-textured soils that receive summer rainfall. In MO, it likes roadcuts, gravel barsalong rivers and streams, pastures, glades and prairies.

It can be easily spread when it is mowed after going to seed. It is also spread in gravel from infested gravel pits,or from ATV travel.Slide 14 Photos provided by MoDOT Route 133 Pulaski County May 2010This is Highway 133 in Pulaski County in May 2010. You can see knapweed showing up as blue-green patches amidst the fescue.

Fall 2011Here it is in fall of 2011, and the patches have taken over nearly the entire highway interchange.Slide 16Although it typically starts in a disturbed area, it is very capable of moving into healthy pastures andundisturbed areas of high natural integrity. It exhibits allelopathic activity, suppressing the growth of otherplants. It does not compete well with vigorously growing grasses in moist areas, but in sunny dry areas, it canrapidly become a monoculture, and I’ve seen it displace sericea lespedeza!

Because it forms monocultures and crowds out other forage foods, this is a great threat to cattle producers.However, I had always thought knapweed was bad for cows, and have since learned it is actually reasonablygood forage. No one in their right mind would PLANT it in a pasture, but when faced with an infestation, cattleCAN be trained to eat it, and if they graze it BEFORE flowering, they can help keep it from going to seed.Slide 18Knapweed is highly branched above, and retains very little foliage below. In winter, this means there is verylittle vegetation on the ground except small rosettes.

Spotted knapweed increases runoff and sediment (Lacey 1989) - harming aquatic wildlifeAs a result, spotted knapweed infested hillsides increased runoff 56% and sediment yield 192% overneighboring hillsides covered with native bunch grass (Lacey 1989). This can negatively affect our aquaticwildlife, including the Ozark hellbender, which has recently been listed as federally endangered.Slide 20 Thru 1955In Missouri, knapweed was first collected in 1933 in Boone County and in Oregon County in 1955.

1960’sTexas, Christian, McDonald and Holt Counties were added in the 1960’s. Keep in mind the counties shown arethose where someone bothered to collect a specimen to submit to the MO Flora herbarium at the MOBotanical Garden.Slide 22 1970’sIn the 70’s, we added Stone, Carter, Reynolds, St. Francois, Franklin and St. Louis City to the list.

1980’sIt spread more in the 80’s…Slide 24 1990’sAnd the 90’s…

2000’sAnd by the 2000’s, it was established across much of southern Missouri, with additional populations scatteredareas around the state.Slide 26 Distribution in MOComparing the list of counties where it has been collected to the distribution listed on the EDD MappingSystem website, there are 5 more counties where it has been reported, but not collected for MBG. It ispossible that it is now in EVERY county in MO, but it hasn’t been reported there yet.

So what can we do to stop the onslaught? Small infestations of knapweed CAN be pulled, but it is difficult. Theplants have very deep tap roots, and it thrives in heavy clay and rocks, conditions that make it tough to pull.Make sure the ground is well moistened before you attempt pulling it. There is a persistent rumor thathandling it can cause cancer, but I’ve never found anything to substantiate this claim. Gloves a good idea,though, as it may well be a skin irritant for some folks.Slide 28Spraying is far more practical. Currently, the best selective herbicide for it is aminopyralid, which is sold underthe brand name Milestone. It came out in 2005, and is selective mostly for forbs in the aster family. This is alarge family, unfortunately, so there are a number of forbs that are weakened or killed by the herbicide, but itis far more selective than previous herbicide recommendations for this noxious weed.

Milestone tolerance: Native forb Common name Tolerance to Milestone: Allium stellatum Glade onion Tolerant Asclepias syriaca Common milkweed Tolerant Echinacea purpurea Common name coneflower to Milestone: Native forb Allium stellatum Purple Glade onion Tolerance Tolerant Tolerant Fragaria virginiana Common milkweed Asclepias syriaca Wild strawberry Tolerant Tolerant Fragaria virginiana Wild strawberry Tolerant Monarda fistulosa Monarda fistulosa Wild bergamot bergamot Wild Tolerant Tolerant Verbena stricta Vervain Tolerant Verbena stricta Z. aptera GoldenVervain Zizia aurea and alexanders Tolerant Tolerant Aster laevis Zizia aurea and Z. aptera wild indigo alexanders tolerant Baptisia alba Smooth aster White Golden Moderately Moderately tolerant Tolerant Aster laevis helianthoides GoldenSmooth aster Moderately tolerant Chrysopsis villosa Heliopsis aster Ox-eye sunflower Moderately tolerant Moderately tolerant Baptisia alba Liatris aspera Rough blazing starwild indigo White Moderately tolerant Moderately tolerant Silphium perfoliatum Cup plant Moderately tolerant Chrysopsis villosa Solidago gigantea Golden aster Moderately tolerant Late goldenrod Moderately tolerant Solidago missouriensis Missouri goldenrod Moderately tolerant Heliopsis helianthoides prairie cloversunflower susceptible Dalea purpurea Purple Ox-eye Moderately Moderately tolerant Desmodium canadense Showy tick trefoil Liatris asperacaptiata Lespedeza Rough blazingModerately susceptible star Round-headed bush clover Moderately susceptible Moderately tolerant Silphium perfoliatum CanadaCup plant Solidago canadensis Aster simplex goldenrod Moderately susceptible Moderately tolerant White panicle aster Susceptible Solidago gigantea Dalea candida Late goldenrod White prairie clover Susceptible Moderately tolerant Helianthus maximilliani Maximillian sunflower Susceptible Solidago missouriensis Lobelia spicata Spiked Missouri goldenrod lobelia Susceptible Moderately tolerant Oenothera biennis Common primrose Susceptible Ratibida pinnata Gray headed coneflower Susceptible Rudbeckia hirta Black eyed Susan SusceptibleForbs that are tolerant or moderately tolerant of Milestone.Slide 30 Milestone tolerance: Native forb Common name Tolerance to Milestone: Dalea purpurea Purple prairie clover Moderately susceptible Native forb Common name Tolerance to Milestone: Desmodium canadenseonion Allium stellatum Glade Showy tick trefoil Tolerant Moderately susceptible Asclepias syriaca Common milkweed Tolerant Lespedeza captitataWild strawberry Fragaria virginiana Round-headed bush clover Tolerant Moderately susceptible Monarda fistulosa Wild bergamot Tolerant Solidago canadensis Verbena stricta Vervain Canada goldenrod Tolerant Moderately susceptible Aster simplex Z. aptera Golden alexanders panicle aster Zizia aurea and White Tolerant Susceptible Aster laevis Smooth aster Moderately tolerant Dalea candida Baptisia alba White wildWhite prairie clover indigo Moderately tolerant Susceptible Chrysopsis villosa Golden aster Moderately tolerant Helianthus maximilliani sunflower Heliopsis helianthoides Ox-eye MaximillianModerately tolerant sunflower Susceptible Liatris aspera Rough blazing star Moderately tolerant Lobelia spicata Silphium perfoliatum Cup plant Spiked lobelia Moderately tolerant Susceptible Oenothera biennis Late goldenrod Solidago gigantea Common primrose tolerant Solidago missouriensis Missouri goldenrod Moderately Moderately tolerant Susceptible Ratibida pinnata Purple prairie clover Dalea purpurea Gray headed coneflower Moderately susceptible Susceptible Desmodium canadense Showy tick trefoil Moderately susceptible Rudbeckia captiata Lespedeza hirta Round-headed bush eyed Susan susceptible Black clover Moderately Susceptible Solidago canadensis Canada goldenrod Moderately susceptible Aster simplex White panicle aster Susceptible Dalea candida White prairie clover Susceptible Helianthus maximilliani Maximillian sunflower Susceptible Lobelia spicata Spiked lobelia Susceptible Oenothera biennis Common primrose Susceptible Ratibida pinnata Gray headed coneflower Susceptible Rudbeckia hirta Black eyed Susan SusceptibleForbs that are likely to be harmed by spraying Milestone.

Milestone: • Active ingredient is aminopyralid • Selective to certain genera in the Asteraceae and some legumes • Provides pre-emergent control • Does not harm grasses • In addition to knapweed, treats thistle, burdock, crown vetch and others • 3 to 7 fl oz per acreMilestone will not harm grasses, which is important in maintaining some vegetation at the site since knapweedwill quickly colonize any bare places. Milestone also provides preemergence control of germinating seeds oremerging seedlings following the application. Milestone is very pricey, averaging about $100 a quart!Fortunately, a little goes a long way, as 3 to 7 ounces treats an acre. I typically use a quarter ounce per gallonwhen using a backpack sprayer.Slide 32Spotted knapweed often occurs in areas where sericea lespedeza can also be found, and unfortunately, there isnot a good chemical that treats BOTH at the same time. But you can mix Milestone with Pastureguard, which isvery effective for sericea. Obviously, this will increase the number of forbs that can suffer collateral damage. Idon’t like to hurt native forbs when spraying, but if knapweed and sericea are not controlled, you won’t haveany native forbs left to protect.

Prescribed burning will not typically hurt spotted knapweed, and on the contrary, infrequent fire is likely toencourage it by releasing nutrients and creating bare ground for new seedling establishment. Fire can,however, hem knapweed back if it is used annually during the later spring or summer.Slide 34If the knapweed is thick, it won’t burn well, if at all. But fire is useful if combined with spraying: if you sprayshortly after green-up following the burn, it is much easier to find and target the plants, and by encouragingseedling emergence, it may help to deplete some of the seed bank.

A final method of control, and perhaps our best hope given the overwhelming problem that is looming in manyareas of our state, is biological control. Let me preface my remarks with the comment that I am always a littleleary about introducing any exotic pest, even though it is introduced for the good cause of killing a nasty plant.There are many possibilities for unintended consequences, and we need to be as sure as we can be that theexotic pest won’t cause problems for our native species and ecosystems.Slide 36 Reduction in spotted knapweed from root weevil and seedhead weevil Minnesota Dept of AgricultureThat said, biological controls have been used for spotted knapweed for 20 years in some parts of the country,and with very good results. This shows a large reduction in spotted knapweed in Minnesota. It’s important torealize that biological controls are not a panacea: you have to wait a number of years to see results, and theywill never completely eradicate the plant, but they can greatly reduce large invasions to a manageable size thatcan then be eradicated through spraying efforts.

Urophora quadrifasciataOne biocontrol agent that has been used a long time, and which is frequently found wherever spottedknapweed is found is a seedhead fly Urophora quadrifasciata. This fly is not terribly effective, however, as asingle bioagent.Slide 38 Seedhead weevils: Larinus obtusus and L. minutus Copyright © 2008 Karl VolkmanThe best biocontrol seems to be achieved by a combination of a seedhead weevil, Larinus species, and a rootweevil, Cyphocleonus achates. Larinus was first released in the US in 1991 and 93. It feeds on the seedhead,effectively eliminating much of the seed produced by the plant. Larinus weevils are strong fliers and can travelmiles between patches.

Root weevil: Cyphocleonus achatesCyphocleonus was released first in 1988. The weevil larvae feeds on the root of the plant and weaken or killsthe plants. They don’t fly much if at all, but they are good walkers and have been found up to a mile away fromtheir release site. Both Larinus and Cyphocleonus have been extensively tested. They do occasionally feedlightly on other species in the thistle tribe, but only lay eggs on plants within the genus Centaurea.Slide 40Both of these species were first released in Missouri in 2008 by MoDOT in a few selected locations. This mapshows the roads that MoDOT mapped as having spotted knapweed, and the original release locations.

In 2009, MDC and the MU Extension office also released these biological controls, and MoDot released awhole lot more. MDC released them at one location at Peck Ranch, at Tingler Prairie Natural Area, CoverPrairie, a private property, and along several roadsides.Slide 42We obtained the Larinus weevils for free from the Colorado Dept of Agriculture and purchased theCyphlocleonus using a USDA APHIS grant and a Wildlife Diversity Grant.

We collected baseline information at each of the sites, including one control site, counting stems in quadratsalong permanent transects. We plan to repeat the sampling in 2014, which will be five years post release.Slide 44We don’t expect quick results: in Minnesota, they have found that in small infestations (less than ½ an acres,they it took 4-5 years for the bioagents to control the infestation. In large infestions (greater than 10 acres), ittook a decade to see the bioagents having a visible effect.

Tingler Prairie Natural Area Knapweed biological control study area Knapweed spread along trails by mowingWe are also not just waiting for the weevils to do their magic. At Tingler Prairie, for example, we are spotspraying at locations along the path and further away from the release area, where we have seen theknapweed spread by mowing activities. We are NOT spraying, however, within the release zone. We also havecertain restrictions for burning: we can’t burn during the active growing season when the weevils are activeabove ground. We CAN burn during the dormant season, when they are safely underground.Slide 46In 2010, MDC released more weevils at each of the sites, as well as in 2011. Meanwhile, MoDot released moreweevils in new roadside locations in 2011. I look forward to watching knapweed die along our highways in thecoming years!

Questions?

Spotted Knapweed

by MO Dept. of Conservation on Jun 01, 2012

Accessibility

Categories

Upload Details

Usage Rights

Statistics

Spotted Knapweed Document Transcript