Diffuse Knapweed Management
Cultural Management
Seeding
and maintaining aggressive grasses will help in competing with diffuse
knapweed and slow its spread. However seeding alone in infected
areas will not provide adequate control. Proper fertilization, grazing,
and supplemental irrigation of grasses are always effective cultural
control methods.
Supplemental control methods, such as judicious use of herbicides,
may be needed to give grasses a chance to compete. Additional herbicide
treatments after grass establishment may be needed to keep diffuse
knapweed populations reduced to an acceptable level.
It produces early spring growth that is hard to compete against.
Mechanical Management
Mowing (or other mechanical efforts) diffuse knapweed when it is
in the bud stage, and again when it flowers, will significantly
reduce seed production. Preventing seed set for a number of years
will eventually eliminate an infestation, once the seed source in
the soil is depleted. This assumes no seed enters the area from
an outside source. This weed has been known to flower at a plant
height below mower level.
Diffuse knapweed can be easy to pull by hand. Get at least 2/3
of the root. Start in May as plants bolt. Leave plants on the ground
if they haven't flowered; otherwise bag them and dispose. Wear gloves
and long-sleeved shirt. Handpulling is effective on small patches,
but is not recommended for large infestations.
Biological Management
Two species of seedhead flies (Urophora affinis and U. quadrifasciata)
reduce the seed production of this weed. When combined with a root-boring
beetle (Sphnoptera jugoslavica), diffuse knapweed control
is increased. Sheep will graze young plants from spring to early
summer. Cattle will eat diffuse knapweed young growth. Goats will
also eat it.
Herbicide Management
Few Herbicides are labeled for use on diffuse knapweed. Those that
have been effective when used independently or in combination with
each other are: picloram (Tordon), dicamba (Banvel), clopyralid
(Stinger, Transline), clopyralid + 2,4-D (Curtail) and 2,4-D. The
latter used alone works poorly. Refer to the table below for more
information.
