Post harvest glyphosate applications are a very effective
method to gain control of perennials and winter annuals before heading into the
winter. Controlling them in the fall is effective because weeds such as Canada thistle,
dandelions etc are beginning to shut down for winter and therefore actively
translocating sugars to their roots to over winter, the glyphosate gets
translocated efficiently down as well, killing the plants at their roots. This
leaves a much cleaner field come spring to seed into and reduces the headache
of trying to get rid of these difficult to control weeds (Note: fields should still
be considered for pre burn applications in the spring).
With all of this said, the temperature fluctuations and
potential frost threats in the fall make spray timing a more difficult task, so
it is best to follow some simple guidelines when it comes spraying at this time
of the year.
Glyphosate (like most non-residual actives) is a product
that can be touchy if sprayed at a time when it is not readily absorbed and
translocated within the plant. This means to watch time of day spraying
(glyphosate is notorious for being less effective sprayed in the late evening
or early morning) and conditions before and after the frost, on top of the weed
species you are targeting and the severity and length of the frost. Lots of
growers believe that if a killing frost has already hit, their post harvest
spray season has ended, but that isn’t necessarily the case.
If you have a hard frost of (-4 degrees C or colder) then
you must wait before going into spray any weeds. It is best to wait anywhere from
48-72 hours before going into assess the severity of the frost damage. If you
go out and see the plants are still remaining over 50-60% green, there are
signs of active growth and the weather conditions have begun to be more
favourable, then aiming to spray the next day once temperature have risen to
above 10 degrees C for over 2 hours would be your next opportunity to hit that
field. Remember, there shouldn’t be frost in the forecast for the next couple
days either.
If there was simply a light frost (0 to -3 degrees C), this only
slightly affects perennial and winter annual weed control. With that said, time
below freezing can come into play and it is still good to go out the next day
and assess the plants. Typically, if it was a light frost like this and you go
out and spray the following day once temperatures have reached 10 degrees for a
couple of hours (and no frost in the forecast) then you should be safe. Giving
the plants an extra 24 hours after a light frost can be an effective strategy
though.
It is best to spray glyphosate in the middle of the day,
especially late in the season like this. If you can catch a warmer, sunnier
day, then it is even better. To further it even more, if you get a nice shower,
go in the next day and you will see an increase in efficacy as the fall
moisture gives the plants a big perk and from what I have seen significantly
helps fall glyphosate effectiveness. As for rates, I personally don’t like to
see less than a 1L equivalent of glyphosate used when trying to take down any perennials
or winter annuals.
I always recommend putting a second chemical in with the
glyphosate such as 2,4-D, Express, Dicamba, Florasulam etc. as having a second
mode of action is effective in decreasing the potential of glyphosate
resistance and also because these products all have a residual in some
capacity. I am not going to touch on these to much as the date they are used,
rates, soil texture, soil moisture, soil temperature, organic matter and more
come into play when determining what can be seeded there next year, if you have
any specific questions feel free to ask or talk to your local agronomist as
they will be aware of what products will be effective for your situation in
your area. I will note that florasulam and Express (tribenuron) typically have
strong activity on asteraceae family weeds which is what family our thistles
fall into.
Dandelions are generally considered to be the most frost
hardy, while Canada thistle, sow thistles and perennial/winter annual grasses
are thought to be less frost tolerant. Also, keep in mind if you were late
harvesting you must assess the plants regrowth and you should see new leaves
being put out by the plant before spraying.
Spraying in the fall is the most effective way to clean up
the perrenniels/winter annuals in your field so if you can take advantage of
that you’ll be happy you did come the next season.
