Apidays New York 2024 - Scaling API-first by Ian Reasor and Radu Cotescu, Adobe
Spotted Knapweed
1. Slide 1
Spotted Knapweed
(Centaurea stoebe ssp. micranthos)
Susan Farrington
Natural History Biologist, Ozark Region
Missouri 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.
2. Slide 3
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.
3. Slide 5
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
4. Slide 7
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.
5. Slide 9
Spotted knapweed
Grey headed coneflower
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!
6. Slide 11
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.
7. Slide 13
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 2010
This is Highway 133 in Pulaski County in May 2010.
You can see knapweed showing up as blue-green patches amidst the fescue.
8. Slide 15
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!
9. Slide 17
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.
10. Slide 19
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.
11. Slide 21
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.
12. Slide 23
1980’s
It spread more in the 80’s…
Slide 24
1990’s
And the 90’s…
13. Slide 25
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.
14. Slide 27
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.
15. Slide 29
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 Susceptible
Forbs 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 Susceptible
Forbs that are likely to be harmed by spraying Milestone.
16. Slide 31
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.
17. Slide 33
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.
18. Slide 35
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.
20. Slide 39
Root weevil:
Cyphocleonus
achates
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.
21. Slide 41
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.
22. Slide 43
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.
23. Slide 45
Tingler Prairie Natural Area Knapweed 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!
