Spring is here and with it comes the emergence of weeds- especially problematic perennials like Canada thistle (Figure 1). Many weeds in perennial fruit crops are controlled during site preparation, but can become problematic again after crop establishment. Below is some information about Canada thistle and methods to manage it. Keep in mind two things: 1) many of these strategies will work for other weeds, and 2) management of perennial weeds often requires persistence and an integrated approach.
Scientific name: Cirsium arvense (L.) Scop.
Legal status: Canada thistle is considered a noxious weed in 46 states including Indiana. It is a non-native invasive species from Europe, and landowners with Canada thistle on their property are obligated to take measures to control it.
Growth habit: Deep-rooted and colony-forming perennial. Plants form a low-growing rosette in the spring prior to bolting in mid-to-late May.
Reproduction: By seeds carried up to 1/2 mile by wind and through adventitious shoots that develop from root buds.
Control: Often multiple types of control measures are needed. Consider a combination of the following:
- Exclusion and sanitation: Avoid spreading thistle roots, shoots, and seeds by implements such as mowers between fields. Control Canada thistle in roadsides, field roads, and fencerows. Use tested seed when planting row middles to minimize the risk of contamination with weed seeds (Figure 2). If you bring hay on-farm, use a reliable source.
- Herbicides: There are essentially two herbicide strategies. Many herbicides registered in fruit crops may burn back the top growth of the weed, but do very little to suppress the extensive root system. This type of herbicide application is sometimes referred to as a “chemical mowing”. A second option is to use effective systemic herbicides. Two with good efficacy are glyphosate (ie. Roundup) and clopyralid (ie. Stinger). At the moment, clopyralid is labeled for use in apple, stone fruits (peach, nectarine, plum, cherry), and blueberry. Avoid contact of either herbicide with the crop. Target Canada thistle from the rosette to the bud stage. For instructions on how to use each herbicide, consult the Midwest Fruit Pest Management Guide (https://ag.purdue.edu/hla/Hort/Documents/ID-465.pdf) and the product labels.
- Mowing: Mowing just before flower buds open can stop the development of Canada thistle seeds. However, timing is critical as viable seeds can be produced within 8 to 10 days after flowers open. Using multiple mowings, one at bud stage and one targeted toward fall regrowth, will help deplete root energy reserves.
- Cultivation/plowing: Cultivation may be used during site preparation and early crop establishment. Use caution when using tillage around Canada thistle. Standard, light tillage will cut the roots into fragments and move them throughout the field. Tillage is not a stand-alone treatment. If tillage is used as the primary method of control, it should be done often and deeply to deplete root reserves. Do not use tillage prior to applying systemic herbicides, which could reduce their efficacy.
- Cover crops: There is some evidence that shows cover crops can be useful for suppressing Canada thistle growth. Depending on the fruit crop grown, cool-season crops like cereal rye can be planted in row middles in the late summer/early fall and will compete with Canada thistle as it emerges in spring.
- Biological controls: There are limited options. Although some insects feed on foliage, stems, crowns, and developing seeds, control is often highly variable. Seeds at or near the soil surface are often consumed by birds, rodents, and insects.
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- Figure 1. Overwintered Canada thistle shoots emerge in April in central Indiana.
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- Figure 2. Seeds, like this cereal rye, can contain other crop and weed seeds
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