North Dakota State University Extension Service - Burke County
Burke County AgAlert
September 23, 2008
Canada Thistle On The Increase - One of the most
significant perennial weed problems in this area for many years has been
canada thistle. I doubt if you can find a quarter of land in Burke county
that does not have at least a few plants of canada thistle
Research trials and experience through the years has found that the best
control of canada thistle in cropland is achieved with a glyphosate
application made either pre-harvest or post-harvest. Glyphosate has no soil
residual and gives full flexibility for crop rotation the next spring. A
minimum of 1 quart per acre based on the 3 pound per gallon formulation or
the equivalent is recommended plus the addition of ammonium sulfate.
Pre-harvest applications generally provide more consistent canada thistle
control than post-harvest applications, although post-harvest applications
can be equally effective under the right conditions. For optimum control,
thistle plants in harvested cropland need to have 4 to 6 inches of regrowth
when sprayed. Spraying old mature stems of plants cut off at harvest without
any regrowth will result in poor absorption and translocation of the
herbicide and poor weed control. Recent rains and the delay of any killing
frost makes this year the best potential canada thistle and quackgrass
control year we have seen for several years.
When
spraying thistle in the fall temperatures should be at least 50°F and
ideally, you want to spray on a nice, bright, sunny day when temperatures
are in the 50's to 60's or higher. Spraying when temperatures are colder or
on overcast days will result in less absorption of the herbicide by the
plant as it is not actively growing and less control. If the herbicide dries
on the leaves before being adsorbed, little additional absorption will occur
even if it warms up the next day.
Curtail at 1 quart per acre is another option, but consult the label for
crop rotation restrictions. Curtail has been more inconsistent when applied
in the fall compared to an in-crop treatment. Curtail needs 21 frost free
days after application and no tillage after application to allow ample time
for translocation.
While Canada thistle is a problem in cropland, it is getting to be an even
bigger problem in some of the non-crop areas such as ditches and around
sloughs. A more concerted effort is needed to control thistle populations in
these areas by landowners and managers. Canada thistle often gets overlooked
in these areas, but when it is left unchecked, it serves as a seed source to
re-infest cropland. Plus, Canada thistle is on the state’s noxious weed list
and North Dakota’s noxious weed law specifically states that it shall be the
duty of every person in charge of or in possession of land to eradicate or
to control the spread of noxious weeds on those lands.
Glyphosate generally isn’t an option for the control of Canada thistle in
these areas due to the presence of desirable grasses, which means that other
herbicides need to be considered. Among the options would be herbicides such
Curtail, Tordon, or the newer aminopyralid products from Dow AgroSciences,
which include Milestone and ForeFront R&P.
Research from across the Midwest has shown that aminopyralid provides
equivalent or better control of thistles than current standards such as
Tordon and Curtail. In addition, the aminopyralid products are effective
against a number of other hard to control weeds such as Absinth wormwood,
knapweeds, and star thistle.
Aminopyralid is a growth regulator herbicide in the same class as picloram
(Tordon), but has a significantly shorter half-life and is relatively
immobile in the soil, especially when compared to picloram, making it safe
to use near water. Even though it
has a shorter half-life, it still has soil residual activity to control
later-emerging weeds.
The active ingredient in Milestone is straight aminopyralid. ForeFront is
basically Milestone pre-packaged with 2,4-D for broader spectrum weed
control.
There are no restrictions on grazing or haying following applications of
Milestone, however, livestock should not be moved from treated grazing areas
to areas containing sensitive broadleaf crop areas without first allowing 3
days of grazing on an untreated area. This is because urine may contain
enough aminopyralid to cause injury to sensitive broadleaf plants such as
alfalfa. Because of soil residual, Milestone does have rotational
restrictions that need to be considered before applying to areas that may be
converted back to annual crop production anytime in the near future. The
label provides more information on rotational restrictions.
Aminopyralid is not the answer to all weed problems, but it does appears to
be a good option for the control of several hard to control perennial weed
problem such as thistle and wormwood in non-cropland areas and CRP acres.
Although closely related to Tordon it is not effective on leafy spurge.
Common Tansy
Common tansy, a bush-like perennial with a fern type leaf and a cluster of
yellow flowers on top of a 3 to 4 foot stem is another perennial weed which
can be controlled in the fall. Commly seen in the road ditches and wetland
from Bowbells to Portal, it is now finding its way into cropland where it is
extremely difficult to control in a no-till cropping system. The best
control we’ve seen in non-crop areas has been with 1 quart of picloram
(Tordon) or 1.5 ounces of metsulfuron (Ally or Escort). We’ve seen excellent
fall control when using one of these products on regrowth of common tansy
where it had been mowed earlier in the year or where there are a lot of
young plants. Control of older plants has been less than perfect but can
still be good if there are lush green leaves near the bottom of the plant.
The key on these more mature plants seems to be using plenty of water. Both
picloram and metsulfuron have considerable soil activity to kill next years
growth but you need to use enough water to get the product to the soil.
The Burke county weed board offers cost share on control of common tansy and
has both picloram and metsulfuron available at reduced cost.
Dan Folske
Extension Agent/Burke County
Soil Conservation Technology Specialist
PO Box 280
Bowbells, North Dakota 58721
dan.folske@ndsu.edu
701-377-2927