Figure 1. Musk thistle rosettes December 2012.
Figure 1. Musk thistle rosettes December 2012.

Fall is an excellent time to spray musk thistle, and it’s not too late now. Musk thistles are all in the rosette stage during the fall. It is much easier to control thistles in the rosette stage compared to treating blooming plants. Another advantage for treatment in the fall is reduced risk of off-target risk due to drift. Most deciduous trees have lost their leaves and most crops are harvested. A wider window of opportunity for treating musk thistle exists in the fall. The spraying window probably extends until the ground is frozen and the musk thistle plants have shut down activity until warmer temperatures in the spring.

Freezing temperatures will start to damage musk thistle plants, with some yellowing and curling of leaves. However, the plants are susceptible to herbicides as long as green tissue exists.

Data presented in Table 1 were collected in July 2013 following treatment on December 6, 2012. Conditions at the time of treatment were 50o F air temperature, 66% relative humidity, and 6-8 mph wind speed. Skies were overcast and cloudy. All treatments provided excellent control of rosettes present at the time of spraying (data not shown).

The data in this table reflect residual control of rosettes that germinated during spring 2013. The number of rosettes on untreated plots increased 92% between December 2012 and July 2013, indicating spring germination. The only treatment not providing nearly 100% residual control was 2,4-D LVE applied at 64 fl oz/acre. The active ingredient in Milestone is aminopyralid. Tordon 22K contains 2 lbs/gallon picloram. Chaparral contains aminopyralid and metsulfuron. These products are all labelled for use on range and pasture. Milestone, 2,4-D, and Tordon 22K are also labeled for use on non-cropland sites including roadsides, right-of-ways, and industrial sites. Opensight was not included in this test, but is a product similar to Chaparral that can be used on non-cropland sites.

If you need to treat musk thistle this fall, herbicides exist that will not only control the rosettes at the time of application, but will carryover and control new emerging rosettes next spring. Select a warm sunny day if possible when spraying musk thistle in the fall.