| It is simple to kill the aboveground shoots of Canada thistle of field
bindweed throughout the year, but what about preventing new shots in the
coming year?
The key to Canada thistle and field bindweed management is to kill the
root system. Management of field bindweed and Canada thistle requires
attention throughout the growing season, but fall is generally the best
time to apply herbicide for perennial weed management.
As winter nears, perennial weeds such as Canada thistle and field
bindweed start to pump food reserves to increase root mass, the number of
reproductive buds and the ability to over-winter. Herbicides that are most
effective on perennial weeds follow the flow of food reserves in the
phloem from the leaves down to the roots. Thus, fall is a good time to
manage perennial weeds as it maximizes the amount of herbicide movement
from the leaves to the root system and vegetable buds of perennial weeds.
Herbicides with activity on Canada thistle and field bindweed include:
2, 4-D dicamba (Banvel, Clarity and numerous other trade names), picloram
(Tordon), clopyralid (Curtail), quinclorac (Paramount) and glyphosate
(Roundup, Glyphmax and numerous other trade names). All of these
herbicides move down the root system of the plant. Herbicide selection
will depend on the cropping system, specific weed targeted, the rotational
crops and other factors. Herbicides are often recommended in combination
for increased activity. Although some herbicides have residual activity in
the soil, herbicides must be applied to actively growing foliage for
maximum results.
TORDON: Tordon has good to excellent activity on both Canada thistle
and field bindweed. Combinations of Tordon with 2,4-D amine applied in the
fall have been the most effective treatments for field bindweed control
until recently. Use rates should not exceed 6 to 8 oz. In fallow
(depending on timing) or 1 1/2 oz. in small grain to minimize the
potential for injury. This rate is recommended for control of certain
annual weeds only. Tordon has a long residual life in the soil and should
only be used on land that will be planted to small grains or grass the
following year (check label rates for recommended planting intervals). Do
not use Tordon on fields that will be seeded to plus crops, oilseed crops,
sugarbeets, potatoes, or other broadleaf crops in the foreseeable future
due to potential crop injury from Tordon carryover.
PARAMOUNT: Paramount is a new product with good to excellent activity
on field bindweed, but limited activity on Canada thistle. Paramount has
been as effective for controlling field bindweed one year after
application as Tordon. For long-term control of field bindweed, the label
recommends paramount be applied in the fall for three consecutive years at
5.3 oz./A the first year and 3 to 5.3 oz./A each consecutive year.
Paramount has the potential to injure follow crop winter wheat, but the
initial data suggest it is less injurious to small grain than Tordon.
Rotation restrictions for Paramount are 24 months plus a field bioassay
for alfalfa, flax, clover, pulse crops, potatoes, and sugarbeets, and 10
months for all other crops except wheat and sorghum. Paramount must be
applied prior to fall frost. Paramount requires addition of methylated
seed oil (preferable) or oil concentrate to spray solutions. Addition of
ammonium sulfate (AMS) or area ammonium nitrate solutions (UAN) may
enhance performance of Paramount.
CURTAIL (Stinger + 2,4-D): Curtail has good to excellent activity on
Canada thistle, but limited activity on field bindweed. Curtail has been
shown to be at least as effective as Tordon for managing Canada thistle in
cropland conditions. Curtail has also been shown to be less injurious to
follow crop wheat and barley that Tordon. Curtail can be used at rates up
to 4 pt/A in fallow and 2 to 2 2/3 pt/A in small grain, depending on
application timing. Rotation restrictions for Curtail are 5 months to
sugar beets and 18 months to all other broadleaf crops. For best control
of Canada thistle, Curtail should be applied in mid-September to allow for
complete translocation.
DICAMBA (clarity, Banvel and numerous other trade names): Dicamba can
have fair to good activity on bindweed, but limited activity on Canada
thistle depending on timing and rate. Combinations of glyphosate of 2,4-D
with dicamba can improve its activity on field bindweed in fallow. Dicamba
rates should not exceed 4 oz/A in small grain or 1 pt/A in fallow. Dicamba
is best for use in crop rotation that include broadleaf crops in order to
minimize herbicide carryover concerns. Dicamba does have limited soil
activity and has caused injury to fallow crops during the growing season
after late September or October application. Pulse crops, specifically
lentils, have shown the most injury to dicamba carryover. Cooler
temperatures over the fall season and drought conditions increase the
potential for dicamba carryover.
2,4-D: 2,4-D has fair to good activity on both Canada thistle and field
bindweed. 2,4-D is most effective in combination with other herbicides for
control of perennial weeds. 2,4-D amine is generally most effective for
perennial weed control because its slower action allows or greater
translocation of herbicide form foliage to roots. However, in dusty or
drought conditions when weeds are stressed, 2,4-D may improve absorption
of herbicide into the plant and improve overall activity. 2,4-D has little
or no residual period so it does not pose any injury threat to follow
crops.
GLYPHOSATE (Roundup, glyphomax and numerous other trade names):
Glyphosate has fair to good activity on bindweed and fair activity on
Canada thistle, depending on application time and rate. Glyphosate
activity can be greatly improved by tank mixing with 2,4-D or dicamba.
Glyphosate combinations are most effective if applied near or just after
frost. Glyphosate should be applied at or near 1 qt/A for best results
with fall applications on Canada thistle or field bindweed. Glyphosate has
no soil activity and no follow crop concerns.
Montana State University Weed Research Results
Several trials have been conducted to assess the activity of various
fall herbicide applications on Canada thistle and field bindweed. In each
research trial, Canada thistle and field bindweed were managed through the
fallow season then allowed to regrow. Herbicides were applied to weed
regrowth in either September or October. Canada thistle and field bindweed
control was assessed in spring wheat during the subsequent cropping
season.
Canada thistle
Herbicide treatments were applied in both October and September at the
same experimental site. Both Canada thistle control and spring wheat
injury were determined in May and June of the following year. Delaying
herbicide application until October did not improve activity on Canada
thistle.
Curtail at 4 pt/A was the most effective treatment in June. Tordon
combined with Curtail at 2 pt/A was neatly as effective at 4 pt/A of
Curtail. Tordon at 12 oz/A has caused up to 35 percent wheat injury in
other research trials. Tordon must be applied at least 45 frost-free days
prior to seeding wheat in order to avoid injury.
Banvel and 2,4-D amine tank mixes with 2 pt/A of Curtail increased
thistle control in May. However, the benefit of adding Banvel and 2,4-D
amine to Curtail at 2 pt/A for Canada thistle control had diminished by
the end of June. Landmaster did not improve the activity of Curtail on
Canada thistle.
Field Bindweed
Herbicide treatments were applied in September at two separate
experimental sites onto barley stubble (post harvest). Field bindweed was
determined the following June or July in a fallow field. Field bindweed
was moderately drought at the time of herbicide application.
Tordon provided 80+ percent control in June and July the following year
at both sites. Adding 1 pt/A of 2,4-D amine or Landmaster to Tordon at 12
oz/A improved the field bindweed activity. 2,4-D amine applied alone did
not effectively manage field bindweed, but when combined with Banvel gave
65+ percent control. Paramount was the most effective bindweed product.
These results confirm that herbicides alone are not the answer to field
bindweed and Canada thistle management. A combination of management
efforts will be required for several years to bring Canada thistle and
field bindweed patches under control.
When considering fall herbicide application as a weed control strategy,
the risks and benefits must be weighed carefully. One major advantage to
fall herbicide applications is that it may free up time in the spring for
the producer. Finishing an herbicide application in the fall eliminates an
operation in the spring and may allow producers to initiate planting
sooner.
Spring application of herbicide to perennial weeds are much less
effective over the long-term. In spring, the herbicide follows the flow of
stored food reserves to aboveground shoots of perennial weeds, rather than
the roots, decreasing the impact on vegetative belowground buds.
Fall applications of herbicide should be delayed until after a
significant rainfall (rain that results in renewed growth of plants).
Also, there should be adequate vegetation present; 4-6 inches of regrowth
is generally required. Drought conditions occurring at numerous locations
around Montana this summer will greatly decrease herbicide activity and
eliminate any potential benefit of fall herbicide applications on
perennial weeds.
Finally, herbicides should be recognized as a part of perennial weed
management systems. Fall herbicide applications will be much more
effective if combined with cultural practices such as timely tillage or
establishment of competitive crops that stress weeds throughout the
growing season and minimize their reproduction over time. |