Spotted knapweed [Centaurea
stoebe L. ssp. micranthos (Gugler) Hayek][CENMA] [CDFA list: A][Forestry, Ornamental, Turf] Photographs
Map of Distribution Biocontrol
Diffuse knapweed [Centaurea
diffusa Lam.][CENDI] [CDFA list: A] Photographs
Map of Distribution Biocontrol
Squarrose knapweed [Centaurea
squarrosa Willd.][CENSQ][CDFA list: A] Photographs
Map of Distribution Biocontrol
Purple starthistle [Centaurea
calcitrapa L.][CENCA][Cal EPPC list: B][CDFA list: B][Ornamental] Photographs
Map of Distribution
Iberian starthistle [Centaurea
iberica Spreng.][CENIB][CDFA list: A][Forestry] Photographs
Map of Distribution
DESCRIPTION:Noxious bushy weeds with
spiny or comb-like phyllaries and white, pink, or purple flowers.
Plants exist as basal rosettes until erect, highly branched flowering stems
with are produced late spring/summer. Centaurea species produce allelopathic
effects and are highly competitive with other plants, often displacing desired
vegetation. Centaurea is a large genus comprised of about 500 species,
none native to California. Thirteen species occur in California as introduced
weeds and escaped ornamentals. Numerous biocontrol agents are known to parasitize
Centaurea species in their native habitats. For the knapweeds, only
the banded gall fly (Urophora affinis) and UV knapweed seed head fly
(Urophora quadrifasciata) are currently established in California at
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SEEDLINGS:Cotyledons spatulate to oval. Rosette leaves pinnate-divided.
Purple and Iberian starthistles develop straw-colored
spines at the centers of rosettes.
PLANT:Upper stem leaves not winged. Foliage variously covered
with short to medium interwoven gray hairs. Leaves alternate. Lower stem leaves
deeply 1- or 2-pinnate-lobed, ~ 10-20 cm long. spotted knapweed: Leaves
resin-dotted. Upper leaves mostly pinnate-divided.
and UNDERGROUND STRUCTURES:Taprooted long
in diffuse knapweed, stout in spotted knapweed, squarrose
knapweeds, and purple starthistle and Iberian starthistles.
FLOWERS:Flower heads consist of few to many fertile disc flowers
interspersed with long bristles on the receptacle. Phyllaries overlapping
in several rows, with tips variously spiny or comb-like. Phyllary characteristics
are important for species identification. Refer to the Flower Head Comparison
of White, Pink, or Purple-flowered Centaurea and Related
Species Table for a comparison.
and SEEDS:Achenes (1-seeded fruits)
oblong, 2.5-3.5 mm long, apex flattened, tapered to a rounded, laterally
notched base. Pappus (when present) +/- whitish, composed of unequal, stiff,
minutely barbed bristles or tiny, flat scales.
CHARACTERISTICS:Old flower stems can persist
for an extended period after senescence (less common for diffuse knapweed).
Phyllaries and achenes remaining on old stems can aid with species identification
when plants are overwintering as rosettes.
HABITAT:Fields, roadsides, disturbed open sites, grasslands, overgrazed
rangelands, and logged areas. Plants seldom persist in shaded places and colonize
most soil types with a disturbed A horizon.
PROPAGATION/PHENOLOGY:Reproduces by seeds, except where noted. These species
have variable dispersal mechanisms described below. However, most seeds or
seed heads of all Centaurea species fall near the parent plant, and
some can disperse to greater distances with human activities, vehicles, heavy
machinery, water, soil movement, and by clinging to shoes, clothing, tires,
and feet, fur, or feathers of animals. Germination can occur over a broad
range of environmental conditions. Seedling emergence is typically highest
after the first fall rains. Mortality of seedlings that emerge in spring can
be high when conditions become dry after emergence. Most seedlings emerge
from seeds at or near the soil surface. Plants produce fewer viable seeds
in dry years. Infestation density correlates with the age of the population
and degree of disturbance. Spotted and diffuse knapweed seeds
exhibit 3 germination patterns: non-dormant seeds that germinate with or without
light exposure, dormant seeds that germinate in response to red light, and
dormant seeds that are not light sensitive. All germination types occur on
each plant. For spotted and diffuse knapweeds, optimal germination
is between 10-28º C (50-82º F).
applications and poorly timed mowing can encourage survival. Rosettes are
usually too low to be affected by mowing. Mowing mature flower stems disperses
seed and can stimulate re-growth of stems.Burning removes current growth,
but may enhance seed germination. Hand pulling 2-4 times per year or severing
plants at least 2 inches below crowns can control small infestations (less
effective for spotted knapweed).
SPECIES:Other knapweeds with fringed
or comb-like phyllaries that lack spines include black knapweed
[Centaurea nigra L.][CENNI], meadow knapweed [Centaurea pratensis
Thuill.], and annual cornflower or bachelors buttons [Centaurea
cyanus L.][CENCY]. Black and meadow knapweeds are less common
perennials found in the Klamath Ranges, North Coast, North Coast Ranges, and
San Francisco Bay region; to 500 m (1650 ft). Cornflower is an escaped
garden annual with showy blue, purple, or white flowers found throughout California,
except deserts; to 2000 m (6600 ft). Unlike Centaurea species, Russian
knapweed [Acroptilon repens (L.) DC.] and bearded creeper
[Crupina vulgaris Cass.] have phyllaries that are ovate
and narrowly lanceolate respectively, with papery margins and
seeds 3-4 mm long that lack a lateral notch near the base. In
addition, only bearded creeper has leaf margins with stiff hairs
barbed at the tips (glochidiate hairs) and flower receptacles with
flattened, scale-like (chaffy) bracts. Refer to the Flower Head
Comparison of White, Pink, or Purple-flowered Centaurea
and Related Species Table for comparison.
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Diffuse and spotted knapweed are excellent pioneer species and rapidly establish
in disturbed roadside, rangeland, wildland, or recreation areas. Seed production
may range from 1,000 to 18,000 seeds per plant and dispersal occurs by numerous
methods. For diffuse knapweed, wind dispersal occurs by a "tumbleweed
effect" and seed may be released over long distances. Spotted knapweed
primarily spreads by peripheral colonization of infested areas. Both species
have spiny seedheads which may be transported on animal hair or fur. Plant
fragments also become attached to vehicles and equipment and spread seed for
hundreds of miles. Off-road or recreational vehicles may damage existing vegetation
and increase soil disturbance. Hikers and hunters can spread seed by picking
the flowers or transporting contaminated hay for horse feed. These human dispersal
mechanisms can be controlled. Avoid driving vehicles or equipment through
infested areas and clean vehicle undercarriages when necessary. Use pelleted
feed or certified weed free hay for horses in backcountry areas. Minimize
recreational vehicle use in sensitive areas near infestations. For ranchers
and rangeland managers, good grazing practices and early detection of new
infestations are essential to reduce diffuse knapweed invasion. Avoid overgrazing
and allow competitive forage to recover by implementing grazing rotations.
Use weed free certified hay to winter feed animals and rotate feeding areas
to prevent soil compaction and trampling of newly emerged grasses and forbs.
Where intensive management is economical, pasture fertilization and irrigation
will greatly reduce the chances of knapweed invasion.
Mechanical: While not a
significant problem in agricultural cropping areas, knapweeds proliferate
in non-crop areas of soil disturbance. Therefore, tillage in non-crop areas
is not recommended. Hand pulling of small infestations of diffuse knapweed
has shown considerable success. Since resprouting from the crown can occur,
the entire plant must be removed. Hand pulling must be repeated 2-4 times
a year and is easiest when the plants have begun to bolt in the late spring
and the soil is still moist. Hand pulling of large infestations is very labor
intensive and may not always be feasible. Proper disposal of removed plants
is important to prevent spread. Piling and burning in a hot fire is a proven
method of disposal. Hand pulling spotted knapweed may be less effective, since
vegetative reproduction from short lateral roots can occur for several years.
Mowing will reduce, but not eliminate seed production of either species. Timing
of mowing is critical. Rosettes are robust to mowing and generally too low
to be successfully cut. A single mowing in the bud to early flower stage has
been most effective, reducing seed production by greater than 75%. Mowing
more mature plants will facilitate seed dispersal and is not recommended.
Fire: The use of fire has
demonstrated mixed results for managing diffuse knapweed. Fire followed by
vigorous grass regrowth can reduce knapweed stands. However, crown resprouts
and increased seedling germination may eliminate any benefits from burning.
An integrated approach using fire and herbicides may be more successful than
herbicides alone. Applying the correct herbicide to newly emerged plants following
a burn is an effective approach.
Biological: The bronze knapweed
root borer (Spenoptera jugoslavica) and the banded gall fly (Urophora affinis)
are the only two insects currently established in California for control of
diffuse and spotted knapweed. They are both compatible for dual release in
infested areas. There are several other insects which have been released in
the United States for knapweed control.and are listed in Table 1. These biocontrol
agents greatly vary in their ability to reduce knapweed seed and vegetative
reproduction. Successful establishment may take from one to several years.
They will not eradicate knapweeds. However, they may be effectively utilized
to reduce knapweed populations over time.
Table 1: Insect species which have been introduced into the United States
for the control of C. diffusa, C. stoebe, and C. virgata, and certain other
related weedy species.
Chemical: Herbicides can
be used to control existing stands of C. diffusa and C. stoebe and substantially
reduce seed production. However, since the seed of both species is viable
in the soil for up to seven years, retreatment will be necessary. Long term
reductions in the seed bank must be the goal for effective knapweed management
with herbicides. Proper timing of herbicide applications is critical to effective
control. In California, there are three herbicides important for knapweed
management: 2,4-D, dicamba, and clopyralid. All three are most effective when
applied in the spring, when plants are beginning to bolt. 2,4-D should be
applied at 2 lb ae/A; dicamba at 1 lb ae/A; and clopyralid at 0.25 lb ae/A.
Clopyralid and dicamba are the most effective treatments. Both will provide
some residual control, particularly clopyralid, and retreatments may be necessary
in the second, third, or fourth years. Dicamba will injure or kill most other
broadleaves it contacts, including desirable forage and native broadleaf species.
Clopyralid is more selective, but will injure legumes such as clovers. 2,4-D
is the least expensive treatment, but is less effective than dicamba and clopyralid,
and retreatment will be required every year. 2,4-D will also injure other
broadleaves, similar to dicamba. The most effective knapweed treatment is
picloram applied at 0.25 lb ae/A. However, it is not labeled for use in California.
Picloram will provide 100% residual control of C. diffusa and C. stoebe
for 2-5 years after treatment. This herbicide will kill or severely injure
most other broadleaf species it contacts in the soil or on the foliage including
trees, forbs, and shrubs. No resistance to these herbicides has been documented
in C. diffusa or C. stoebe. However, any herbicide management program should
integrate rotation between herbicides to prevent the development of resistance.
Integrated management strategies:
Seed production by C. diffusa or C. stoebe is typically 1000 times greater
than required to maintain infestations. Thus, relatively few plants per acre
are needed for rapid reinfestation. Management must be continuous or reinfestation
is inevitable. Reseeding and establishment of competitive grasses or other
native species is critical. Native bunchgrass communities are generally very
resistant to knapweed invasion. However, almost any form of disturbance (including
inclement weather such as hailstorms) may open a niche for invasion. Research
has also shown that areas receiving 10-14 inches of precipitation annually
are most susceptible to knapweed invasion, even in established perennial bunchgrass
communities. The severity of knapweed infestations in other states such as
Montana and Wyoming should serve as an indicator of the potential economic
and environmental problems that knapweeds pose to California.