Canada thistle in strawberry plantings.

Posted: May 8, 2012

Got Canada thistle? In strawberries? It’s not unusual. Canada thistle is one of Pennsylvania’s worst perennial weeds and often infests strawberry plantings.
Canada thistle at bud stage.

Canada thistle at bud stage.

Perhaps you planted into a field where it was already established. Or maybe your farm is adjacent to unmanaged sites that “go to seed”…. literally. Canada thistle seeds float long distances on fluffy parachutes. Maybe your straw mulch was infested. Regardless of how it got there, Canada thistle is a weed that you can’t ignore. It is a creeping perennial that will continue to expand its real estate if left un-managed.

Effective herbicide use.

(For non herbicide options scan down.)

Two herbicides, glyphosate (Roundup, etc) and clopyralid  (Stinger, etc) are very effective in controlling Canada thistle but timing is critical. Late spring and very early fall are the two time periods where these translocated herbicides are most effective on this weed. With both herbicides your target is healthy, actively growing thistle foliage.
There is a special local needs label for Stinger on strawberries in Pennsylvania. See it for details. Stinger applications need to be made at least 30 days before strawberry harvest and that time may have passed by the time you read this. Thirty days pre-harvest will usually be about early May for most of Pennsylvania, but in this unusually warm spring everything is advanced. 

Stinger is very useful because it is selective. That is, it can be applied over the actively growing berry plants without damage.  Note that you will not find an “ounces per gallon” rate when looking at the Stinger label. This product has soil residual activity and the correct rate is very important. You’ll have to calibrate the backpack sprayer for spot treatments. A simple way is to determine how much area you treat with one gallon of water. Figure it up from there. Or, measure how much water is required to treat 1000 square feet. This is 1/44 th of an acre. The maximum  Stinger rate in strawberries is 10.5 ounces per acre! Over application is likely if you are not careful.

Ok, let’s say you are inside the 30 days to harvest interval and clopyralid is not an option.  Glyphosate products are label up to 14 days before harvest and this timing is still very good for control of control of thistle with this herbicide. Sometime after thistle flower buds are visible but before bloom is ideal. Here you can simply use a 2 percent solution of the 41% a.i. glyphosate products (or the equivalent when using other products). Again, spot treating Canada thistle colonies is usually all that is necessary in most plantings. Note that glyphosate is non-selective and will kill or severally injure any strawberry plants that are contacted. It may be necessary to sacrifice some parts of a strawberry planting when using glyphosate. Wiper applicators can be very precise.

The second key time for Canada thistle control with these herbicides is very early fall… before a hard frost. Canada thistle makes substantial foliage (not flowering) re-growth in late summer. This foliage will absorb and translocate glyphosate or clopyralid very well, knocking out thistle root systems. Don’t ignore those low growing colonies. They are the tip of  the iceberg. They are a prime target in the battle against this weed. Plan to apply either clopyralid or glyphosate about two weeks before your first killing frost for maximum effect. This will be sometime in September for most of Pennsylvania.

Note that the maximum amount of Stinger that can be applied to strawberry plantings per year is 10.5 ounces. If you make spring and fall applications, do not exceed this rate. Make a split application of five ounces each time. Or consider using clopyralid at one timing and glyphosate at the other. Note that Stinger is not recommended in the spring of the planting year. Delay applications on new plantings until fall.


Not using herbicides?


A timely mowing or weed whacking in the early bud stage, before flowering, is effective in reducing the plant’s energy reserves. Continue to remove all thistle foliage every two to three weeks throughout the season and until hard freezes kill top growth. Don’t ignore that late summer growth. Your strategy is to totally deplete the plant’s energy reserves by forcing it to use stored energy for re-growth, and at the same time, not allowing it to recharge the system.