Weed Control - Musk and Scotch Thistles
and Musk Thistles
respond much the same way to cultural and chemical control methods. Bottom line is don't let these voracious seed producers set mature flowers. Live seven foot tall scotch thistle when sprayed will become dead seven foot tall scotch thistle which is still a nuisance. If possible, apply herbicides to rosettes or very young plants. Our recommendations are just that ..... but they have been proven out in the field for years. Once again, it is extremely important that you READ AND FOLLOW THE LABEL with any herbicide you chose to use. In Weber County, the preferred weapon when targeting Musk Thistle is Biological Weed Control (weed feeders). The "time line" featured near the bottom of the page represents a generalized guide to the most effective timing of chemical weed control here in Northern Utah.
Residual herbicides labeled for these species such as Tordon® 22K will do a fine job and keep working when you have packed up and left. Apply at rosette stage when able. Because of it's tenacity, Tordon® will work well on the next years weed crop at any growth stage. HOWEVER, you will have a near impenetrable thicket of thistle to work on if you don't get these weeds while they are young. 2-4,D ester or 2-4,D amine + dicamba have good action on these thistles in the rosette stage. With appropriate adjustments in rates, these herbicides also work very well on pasture or rangelands. Nevada research has shown that 2,4-D alone is not effective on adult Scotch thistle, permitting the plant to set viable seed even after the thistle appears dead.
For Bare Ground, we like Arsenal® or Oust® tank mixed with diuron for most all bare ground jobs. For us, it is very cost effective. Your results may vary. See labels for rates on your soils. CAUTION watch sloping ground for potential runoff in the event of heavy rain or overhead irrigation!
Roundup® can be used on very young Musk or Scotch Thistle for general clean-up but is not cost effective on the giant adult weeds. If herbicide in not an option and the thistle is just too tall, mow or swath the thistle, let them re-green a couple of weeks and then apply the appropriate herbicide. Clean off that implement before using it in another field!
Rights of Way
Scotch & Musk thistle next to irrigation ditches and streams must be treated with care. There are some amine formulations of 2-4,D that are labeled for aquatic use where grasses under or near the noxious weeds are not desired to be killed. If grasses are of no consequence, we like Rodeo® and the appropriate surfactant.
Along Roadsides, these weeds are just another target for the normal tank mix of the day. 2-4,D & Banvel® (in weed districts of course) are among our favorites. We use Campaign® a lot and see that young thistle rosettes are taken out. We really get great chemical mowing from the Campaign® also, but with this herbicide, timing is EVERYTHING. The applicator must watch and wait for as many species of target broadleaf weeds to germinate while not letting the roadside grasses get over about 12 inches tall.
Most Labels for herbicides used in crops do not specifically mention Scotch or Musk Thistle. If rowcrops or alfalfa are being considered for a piece that has been infested with these broadleaf weeds, might be a good idea to plant cereal grains the first year, make the young grain weed free with your favorite chemical, and follow up after harvest when thistle rosettes again appear with Roundup® or 2-4,D. Then the following year, the tillage and/or chemical treatments you would normally use on rowcrops or alfalfa should keep any remaining invaders in check.