NODDING THISTLE AND ITS CONTROL IN GRASS PASTURES
J. D. Green
Department of Agronomy
Nodding Thistle (Carduus nutans)
is the most widespread thistle species found in Kentucky. Its name describes
the large flowers because they bend over and appear to nod or droop. Musk
Thistle is another common name for this plant.
Like other weeds, nodding thistle lowers
the productivity of pastures. Also, the plant discourages livestock from
grazing close by because of its spiny nature. An understanding of the nodding
thistle's growth and life cycle is useful for controlling it.
Identification and Life Cycle
The plant is easiest to identify when
it is in the flowering stage. Nodding thistle produces a red to purplish
flower about 2 inches in diameter. The flower is attached to an erect stem
that can be 3 to 6 ft tall and the flower frequently droops or nods. The
plant's lower portion is branched with 1 to 3 flowers per branch. Large
plants are capable of producing 50 or more flowers.
Nodding thistle, like other biennial
plants, usually requires 2 years to complete its life cycle. However, it
can develop as an annual or winter annual depending on environmental conditions.
Its only method of reproduction is by seed. Seed germinate in the fall
or early spring and the seedling forms a rosette. The rosette is a circular
cluster of leaves with the growing point close to the ground. A young rosette
can range from 4 to 8 inches in diameter.
The plant remains in the rosette stage
throughout its first year of growth. The second spring after emergence,
the rosette size increases to about 12 to 18 inches across, the stem begins
to elongate and flower stalks develop. Flowers begin to form in late May
to early June.
One reason nodding thistle can be difficult
to control is that one plant can produce several thousand seed. A large,
mature plant produces over 4,000 seed that can become new plants next year.
Some of the seed remain viable for at least 10 years. Also, the seed are
easily carried or spread by wind, water, animals, farm equipment and in
hay. Since nodding thistle only reproduces by seed, effective control programs
depend on preventing seed production and spread.
Cultural and Mechanical Control Practices
Nodding thistle becomes easily established
on areas with bare or exposed soil. It is frequently found in overgrazed
pastures, areas with poor fertility and disturbed sites. Under these conditions,
desirable grasses are unable to grow vigorously and compete effectively
against this weed's emergence and growth.
A control program for nodding thistle
should involve good management practices that will help establish and maintain
productive pastures. These practices include maintaining good soil fertility
and proper soil pH and managing pastures to prevent over grazing. It is
also important to avoid the spread of nodding thistle seed by keeping fence
rows and adjacent lots weed-free and by preventing transport of seed by
Mechanical methods for control consist
of mowing or hand removal. These methods may not eliminate this weed problem
but should reduce seed production. With mechanical controls the plant should
be mowed or removed as close to the ground as possible before it begins
to flower. Some regrowth will probably occur, but seed production will
be noticeably less than if no mechanical control practices were used.
Control Using Herbicides
When to Spray
The optimum time for nodding thistle
control with herbicides is while the plant is in the rosette stage. Apply
herbicides in the spring during late March or early April before thistles
produce a flower stalk. A fall application in late September, October or
early November will also kill many of the rosettes of first-year plants.
Herbicide treatment may need to be repeated the following spring.
The rosette must be actively growing
for effective control. Plants absorb more herbicide and move it to the
roots when sod moisture conditions are good and air temperatures are above
50 F for several days. Applying herbicides after flower stalks begin to
elongate can result in inconsistent control.
Herbicides to Apply
Herbicides that can be used in grass
pastures include the 2,4-D Amine or 2,4-D Ester formulations, and Banvel.
Apply 2,4-D Amine or 2,4-D Ester at 1 to 2 qt/A (i.e. assuming a 2,4-D
formulation containing 3.8 lb acid equivalents/gal). Note: when applied
at 1 qt/A a 2,4-D formulation containing 3.8 lb ae (acid equivalents)/gal
as stated on the label equals approximately 1 lb ae/A of 2,4-D. The lower
rate of 2,4-D should be used only at temperatures above 60 F and/or before
flower stalk elongation. Use the higher rate when air temperature is 40
to 60 F or after large rosettes develop. Apply Banvel at 1 to 2 pt/A.
Good plant coverage with the herbicide
is essential for best results. When making broadcast applications use a
spray volume of 20 to 40 gal of water/acre and spray pressures less than
40 p.s.i. This spray gallonage and low pressures help provide good spray
coverage and also reduce the potential for off-site movement of spray particles.
Spot treatment of thistle plants may
be adequate on areas containing a few scattered plants and along fence
lines. Use a spray mixture of 2 or 3 TBS of 2,4-D concentrate (i.e. assuming
a 2,4-D formulation containing 3.8 lb acid equivalents/gal) for each gallon
of water. Apply enough to moisten the nodding thistle leaves without run-off.
No herbicides are available to control
nodding thistle in grass pastures interseeded with legumes such as red
clover or alfalfa. A thistle control program in grass-legume mixtures must
rely on cultural and mechanical practices.
Read and understand precautions on
the herbicide label before you spray.
•Red clover, alfalfa and lespedeza
interseeded in pastures are likely to be killed by 2,4-D or Banvel.
•Avoid spray drift and spraying near
sensitive crops. Do not spray when spray particles and vapors may be carried
by air currents to areas where sensitive crops are growing. Crops sensitive
to both 2,4-D and Banvel include tobacco, soybeans and some vegetables.
•Do not use spray equipment which has
contained 2,4-D or Banvel for spraying other pesticides on susceptible
•Check the label for animal grazing
restrictions following herbicide use. These restrictions may vary depending
on the herbicide used and the amount applied. Most 2,4-D herbicide formulations
require a waiting period of 7 to 14 days after application before livestock
can graze treated pastures.
Integrating Control Strategies
In most situations, no single weed
control practice alone will eliminate or maintain thistle-free pastures.
Best control comes from an integrated approach using various control strategies.
Effective thistle control programs
begin with preventing establishment and spread of the weed. In situations
where severe nodding thistle problems exist, combine timely chemical controls
with pasture improvement. Doing so promotes competitiveness of the desirable
forage grasses. A persistent effort for several years is generally required.