The best timing for perennial weed control is the point where these plants enter the bud stage. For Canada Thistle that will be over the next week or two. Whether it is an unplanted field or CRP area if you need to get this species under control plan to act soon. The picture at the left shows the bud stage just prior to bloom opening. In the next couple days the pink flowers will begin to open and the plant shifts into a mode where sugars go into building root reserves. We can take advantage of this flow to get systemic herbicides into the root system that will give an entire plant kill.
Control recommendations from the Ohio/Indiana Weed Control Guide Problem Weed Section page 179-180 are:
Canada thistle is a perennial weed that spreads both by seed
and creeping roots. Canada thistle plants emerge early in the
spring and tend to grow in dense, spreading patches. This weed
is extremely competitive and can be a problem in all crops.
Noncrop/Fallow Areas. The most effective treatment for control
of Canada thistle is glyphosate applied in late spring or early fall
when thistles are in the bud‑to‑flower stage. Fall treatment will
be most successful when thistles have been mowed or clipped off
earlier in the season and allowed to regrow to the bud stage (or to
a height of at least 10 to 14 inches). This method may be used in
noncrop areas and fallow fields, or following small‑grain harvest.
For fall treatment, apply 0.75 lb ae/A of glyphosate plus AMS
(17 lbs per 100 gallons water) in a spray volume of 5 to 10 gallons
per acre, or 1.5 lb ae/A of glyphosate in spray volumes greater
than 10 gallons per acre. For most effective control of thistle,
do not mix other herbicides with glyphosate. High rates (1 to 2
quarts/A) of dicamba or 2,4‑D are generally less effective than
glyphosate, but a low‑cost application of 2,4‑D will provide some
long‑term control of thistle. Alternatively, any of these herbicides
may be applied as a spot treatment using a 2 percent solution.
Treatment with glyphosate, dicamba, or 2,4‑D in this manner
should be delayed until thistles regrow enough to begin producing
buds, or applied as late in the fall as possible. Thistle plants
will survive a few light frosts, but apply before the first freeze.
Wheat. Stinger (1/3 pint/A), Curtail (2 to 2 2/3 pts/A), and
combinations of 2,4-D with high rates of Express/Nuance controls
or suppresses Canada thistle, preventing harvest problems.
Stinger provides the most complete thistle kill, but is more
expensive than the other three herbicides. Huskie, or dicamba
+ 2,4‑D will also often suppress Canada thistle to the point that
it does not interfere with harvest.
Corn. Most effective postemergence control of the entire
plant results from application of Stinger (2/3 pint/A), Hornet
(4 to 5 ounces/A) plus a few ounces of Stinger, or glyphosate
(1.1 lb ae/A – glyphosate-resistant corn). Apply Stinger/Hornet
when thistles are at least 4 inches tall and before the bud stage.
Glyphosate should be applied when thistles are in at least the
bud stage for best results; earlier applications will control thistle
through the growing season.
Status (5 oz/A) or Hornet (4 to 5 ounces/A) will effectively
control the above-ground part of the plant. Postemergence application
of Basagran (1 quart/A), Laddok (2.3 pints/A), or Liberty
plus atrazine (Liberty Link corn) will control above-ground growth
of actively growing Canada thistle in the 8-inch to bud stage.
Other treatments with activity on thistle include Buctril/atrazine,
Marksman, dicamba, dicamba + 2,4-D, NorthStar, Yukon, and
combinations of Spirit, Steadfast, Lightning (Clearfield corn
only) or nicosulfuron with dicamba. Most of these treatments
will control or suppress the above‑ground part of the plant, which
will help prevent further spread.
Soybeans. Glyphosate (1.1 lb ae/A – Roundup Ready soybeans)
is the most effective postemergence treatment. Late post applications,
when plants are in the bud to flower stage, will provide the
most complete control of underground plant parts. To prevent
yield loss where thistle populations are high, apply when thistles
are small and retreat regrowth as necessary. Postemergence
applications of Basagran (2 pints/A) will control above‑ground
parts of the plant or suppress growth of Canada thistle. Regrowth
usually occurs, but this treatment will reduce competition from
Canada thistle in soybeans and help prevent production of more
rootstock. Apply when thistle plants are from 8 inches tall to
the bud stage. COC should be included in the spray mixture. A
second application at the same rate may be made 7 to 10 days
later, if necessary. Other products and mixtures with activity on
thistle include Flexstar and mixtures of Basagran with Reflex,
Flexstar, Ultra Blazer, or Cobra. Postemergence application of
Pursuit (1.44 ounces/A), Classic (2/3 to 3/4 ounce/A), FirstRate
(0.3 oz/A), and Synchrony XP (3/4 ounce/A) may also suppress
thistle growth, but results have been variable.