Download - Spotted Knapweed

Spotted Knapweed(Centaurea stoebe ssp. micranthos)

Susan FarringtonNatural History Biologist, Ozark RegionMissouri Department of Conservation

Spotted knapweed was first introduced to North America from Eurasia as a contaminant in alfalfa seed in British Columbia in 1893. It’s suspected that there were multiple accidental introductions, not just one. Slide 2

Spotted 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 4

It 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 visible throughout the winter. Slide 6

The leaves are usually highly lobed and grey green in color, whitened with tiny hairs. They resemble a thistle leaf 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 whitened undersides, and exotic bull thistle on the right, with its green undersides. Slide 8

Native Grey headed coneflower (Ratibida pinnata) Spotted knapweed

It also resembles native grey headed coneflower, but the coneflower leaves are a darker green and have fewer and larger lobes.

Grey headed coneflowerSpotted knapweed

Coneflower also has spreading hairs on the stems and leaves, whereas knapweed’s hairs are very tiny and fine and lay tightly against the leaf and stem. Slide 10

Knapweed seeds remain viable in the seed bank 5-8 years, and germinate from spring through fall. Mowing it back does not prevent it from flowering or going to seed: I have seen it in a close-cropped lawn in upstate New York, blooming at 2” in height!

Spotted knapweed is extremely adaptable, found at elevations up to and over 10,000 feet, and in areas that receive from 8 to 80 inches of rain annually. Slide 12

It prefers well-drained, light-textured soils that receive summer rainfall. In MO, it likes roadcuts, gravel bars along 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

Route 133 Pulaski County May 2010

Photos provided by MoDOT

This is Highway 133 in Pulaski County in May 2010. You can see knapweed showing up as blue-green patches amidst the fescue.

Fall 2011

Here it is in fall of 2011, and the patches have taken over nearly the entire highway interchange. Slide 16

Although it typically starts in a disturbed area, it is very capable of moving into healthy pastures and undisturbed areas of high natural integrity. It exhibits allelopathic activity, suppressing the growth of other plants. It does not compete well with vigorously growing grasses in moist areas, but in sunny dry areas, it can rapidly 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 reasonably good forage. No one in their right mind would PLANT it in a pasture, but when faced with an infestation, cattle CAN be trained to eat it, and if they graze it BEFORE flowering, they can help keep it from going to seed. Slide 18

Knapweed is highly branched above, and retains very little foliage below. In winter, this means there is very little vegetation on the ground except small rosettes.

Spotted knapweed increases runoff and sediment

(Lacey 1989) - harming aquatic wildlife

As a result, spotted knapweed infested hillsides increased runoff 56% and sediment yield 192% over neighboring hillsides covered with native bunch grass (Lacey 1989). This can negatively affect our aquatic wildlife, including the Ozark hellbender, which has recently been listed as federally endangered.

Slide 20

Thru 1955

In Missouri, knapweed was first collected in 1933 in Boone County and in Oregon County in 1955.

1960’s

Texas, Christian, McDonald and Holt Counties were added in the 1960’s. Keep in mind the counties shown are those where someone bothered to collect a specimen to submit to the MO Flora herbarium at the MO Botanical Garden. Slide 22

1970’s

In the 70’s, we added Stone, Carter, Reynolds, St. Francois, Franklin and St. Louis City to the list.

1980’s

It spread more in the 80’s… Slide 24

1990’s

And the 90’s…

2000’s

And by the 2000’s, it was established across much of southern Missouri, with additional populations scattered areas around the state. Slide 26

Distribution in MO

Comparing the list of counties where it has been collected to the distribution listed on the EDD Mapping System website, there are 5 more counties where it has been reported, but not collected for MBG. It is possible 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. The plants 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 that handling 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 28

Spraying is far more practical. Currently, the best selective herbicide for it is aminopyralid, which is sold under the brand name Milestone. It came out in 2005, and is selective mostly for forbs in the aster family. This is a large family, unfortunately, so there are a number of forbs that are weakened or killed by the herbicide, but it is far more selective than previous herbicide recommendations for this noxious weed.

Milestone tolerance:

Native forb Common name Tolerance to Milestone:Allium stellatum Glade onion TolerantAsclepias syriaca Common milkweed TolerantFragaria virginiana Wild strawberry TolerantMonarda fistulosa Wild bergamot TolerantVerbena stricta Vervain Tolerant

Zizia aurea and Z. aptera Golden alexanders TolerantAster laevis Smooth aster Moderately tolerantBaptisia alba White wild indigo Moderately tolerantChrysopsis villosa Golden aster Moderately tolerantHeliopsis helianthoides Ox-eye sunflower Moderately tolerantLiatris aspera Rough blazing star Moderately tolerant

Silphium perfoliatum Cup plant Moderately tolerant

Solidago gigantea Late goldenrod Moderately tolerantSolidago missouriensis Missouri goldenrod Moderately tolerantDalea purpurea Purple prairie clover Moderately susceptibleDesmodium canadense Showy tick trefoil Moderately susceptibleLespedeza captiata Round-headed bush clover Moderately susceptible

Solidago canadensis Canada goldenrod Moderately susceptibleAster simplex White panicle aster SusceptibleDalea candida White prairie clover SusceptibleHelianthus maximilliani Maximillian sunflower SusceptibleLobelia spicata Spiked lobelia SusceptibleOenothera biennis Common primrose SusceptibleRatibida pinnata Gray headed coneflower Susceptible

Rudbeckia hirta Black eyed Susan Susceptible

Native forb Common name Tolerance to Milestone:

Allium stellatum Glade onion Tolerant

Asclepias syriaca Common milkweed Tolerant

Echinacea purpurea Purple coneflower Tolerant

Fragaria virginiana Wild strawberry Tolerant

Monarda fistulosa Wild bergamot Tolerant

Verbena stricta Vervain Tolerant

Zizia aurea and Z. aptera Golden alexanders Tolerant

Aster laevis Smooth aster Moderately tolerant

Baptisia alba White wild indigo Moderately tolerant

Chrysopsis villosa Golden aster Moderately tolerant

Heliopsis helianthoides Ox-eye sunflower Moderately tolerant

Liatris aspera Rough blazing star Moderately tolerant


Solidago gigantea Late goldenrod Moderately tolerant

Solidago missouriensis Missouri goldenrod Moderately tolerant

Forbs that are tolerant or moderately tolerant of Milestone.

Slide 30

Milestone tolerance:

Native forb Common name Tolerance to Milestone:Allium stellatum Glade onion TolerantAsclepias syriaca Common milkweed TolerantFragaria virginiana Wild strawberry TolerantMonarda fistulosa Wild bergamot TolerantVerbena stricta Vervain Tolerant

Zizia aurea and Z. aptera Golden alexanders TolerantAster laevis Smooth aster Moderately tolerantBaptisia alba White wild indigo Moderately tolerantChrysopsis villosa Golden aster Moderately tolerantHeliopsis helianthoides Ox-eye sunflower Moderately tolerantLiatris aspera Rough blazing star Moderately tolerant


Solidago gigantea Late goldenrod Moderately tolerantSolidago missouriensis Missouri goldenrod Moderately tolerantDalea purpurea Purple prairie clover Moderately susceptibleDesmodium canadense Showy tick trefoil Moderately susceptibleLespedeza captiata Round-headed bush clover Moderately susceptible

Solidago canadensis Canada goldenrod Moderately susceptibleAster simplex White panicle aster SusceptibleDalea candida White prairie clover SusceptibleHelianthus maximilliani Maximillian sunflower SusceptibleLobelia spicata Spiked lobelia SusceptibleOenothera biennis Common primrose SusceptibleRatibida pinnata Gray headed coneflower Susceptible


Native forb Common name Tolerance to Milestone:

Dalea purpurea Purple prairie clover Moderately susceptible

Desmodium canadense Showy tick trefoil Moderately susceptible

Lespedeza captitata Round-headed bush 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


Forbs 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 acre

Milestone will not harm grasses, which is important in maintaining some vegetation at the site since knapweed will quickly colonize any bare places. Milestone also provides preemergence control of germinating seeds or emerging 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 gallon when using a backpack sprayer. Slide 32

Spotted knapweed often occurs in areas where sericea lespedeza can also be found, and unfortunately, there is not a good chemical that treats BOTH at the same time. But you can mix Milestone with Pastureguard, which is very effective for sericea. Obviously, this will increase the number of forbs that can suffer collateral damage. I don’t like to hurt native forbs when spraying, but if knapweed and sericea are not controlled, you won’t have any native forbs left to protect.

Prescribed burning will not typically hurt spotted knapweed, and on the contrary, infrequent fire is likely to encourage 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 34

If the knapweed is thick, it won’t burn well, if at all. But fire is useful if combined with spraying: if you spray shortly after green-up following the burn, it is much easier to find and target the plants, and by encouraging seedling 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 many areas of our state, is biological control. Let me preface my remarks with the comment that I am always a little leary 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 the exotic 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 Agriculture

That 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 to realize that biological controls are not a panacea: you have to wait a number of years to see results, and they will never completely eradicate the plant, but they can greatly reduce large invasions to a manageable size that can then be eradicated through spraying efforts.

Urophora quadrifasciata

One biocontrol agent that has been used a long time, and which is frequently found wherever spotted knapweed is found is a seedhead fly Urophora quadrifasciata. This fly is not terribly effective, however, as a single bioagent.

Slide 38

Copyright © 2008 Karl Volkman

Seedheadweevils: Larinusobtusus and L. minutus

The best biocontrol seems to be achieved by a combination of a seedhead weevil, Larinus species, and a root weevil, 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 travel miles between patches.

Root weevil: Cyphocleonusachates

Cyphocleonus was released first in 1988. The weevil larvae feeds on the root of the plant and weaken or kills the plants. They don’t fly much if at all, but they are good walkers and have been found up to a mile away from their release site. Both Larinus and Cyphocleonus have been extensively tested. They do occasionally feed lightly on other species in the thistle tribe, but only lay eggs on plants within the genus Centaurea.

Slide 40

Both of these species were first released in Missouri in 2008 by MoDOT in a few selected locations. This map shows 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 a whole lot more. MDC released them at one location at Peck Ranch, at Tingler Prairie Natural Area, Cover Prairie, a private property, and along several roadsides.

Slide 42

We obtained the Larinus weevils for free from the Colorado Dept of Agriculture and purchased the Cyphlocleonus 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 quadrats along permanent transects. We plan to repeat the sampling in 2014, which will be five years post release.

Slide 44

We 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), it took a decade to see the bioagents having a visible effect.

Tingler Prairie Natural AreaKnapweed biological control study area

Knapweed spread along trails by mowing

We are also not just waiting for the weevils to do their magic. At Tingler Prairie, for example, we are spot spraying at locations along the path and further away from the release area, where we have seen the knapweed spread by mowing activities. We are NOT spraying, however, within the release zone. We also have certain restrictions for burning: we can’t burn during the active growing season when the weevils are active above ground. We CAN burn during the dormant season, when they are safely underground.

Slide 46

In 2010, MDC released more weevils at each of the sites, as well as in 2011. Meanwhile, MoDot released more weevils in new roadside locations in 2011. I look forward to watching knapweed die along our highways in the coming years!

Questions?