The weed pictured here is Canada thistle (Cirsium arvense). Canada thistle is a creeping perennial, reproducing largely by rhizomes (i.e. underground stems). Canada thistle can also produce seed, though this is not the main means of reproduction and spread. Canada thistle has lobed leaves with smooth upper surfaces and sharp-tipped spines on the margins. The roots of Canada thistle have been measured down to 6 ft, with the majority found between 8-16 inches. Most other thistles such as bull thistle are biennials, meaning they spend one year as a rosette and then send up a shoot with flowers in the second year. Bull thistle (Cirsium vulgare), has rough hairs on the upper leaf surface and has a taproot, usually unbranched, and no rhizomes like Canada thistle.

Removing the aboveground growth can eventually starve the root/rhizome system, but this often takes several years (multiple efforts per year) depending on the level of establishment.

Impermeable mulch (plastic mulch) may help, but if there are any holes in the mulch for desirable vegetation, it is likely Canada thistle will exploit those. Organic mulches (e.g. wood chips, bark, etc. ) are not effective.
Herbicides are often the fastest option to gain control over a Canada thistle population. The product selected may depend on your intentions for this area and nearby vegetation. Herbicides that contain glyphosate or triclopyr are often good option. Both can be sprayed on the thistle foliage or you can treat cut stems with the concentrated form of the herbicide to reduce the potential for herbicide movement. 

Glyphosate-  This active ingredient is getting harder to find in the residential market. The Roundup brand in particular has recently changed a lot of their active ingredients but has not changed the overall look of the labels, so be sure to check the list of active ingredients, often on the front of the label in very small print. You should still be able find it as Roundup Weed & Grass Killer Super Concentrate or in generic forms at places like Tractor Supply Company (TSC). These formulations will need to be mixed in a hand sprayer if you are treating the foliage. 

When using products containing glyphosate there are a few important points to consider. First, as with any pesticide, remember to read and follow all labeled instructions. Second, glyphosate is a broad-spectrum herbicide, meaning it will injure or kill other plants contacted during application, so care is needed to avoid green plant material, exposed roots, and injured bark of desired plants. Third, glyphosate is relatively safe in the environment when used as labeled. It adsorbs strongly to soil in most cases (i.e. clay and organic matter), allowing even sensitive crops to be planted shortly after application; meaning no carry over issues are expected. Fourth, glyphosate alone can take up to 14 days to show full activity under ideal growing conditions. Retreatment of the area may be needed depending on the degree of infestation. Glyphosate is most effective for perennial control in the fall but can be applied anytime the plants are actively growing (temperatures consistently above 50F). Finally, be sure that the product you choose has only the active ingredient glyphosate or glyphosate + pelargonic acid. Products with additional active ingredients may have other unwanted effects and may delay the planting of other plants in the coming season(s).

Triclopyr- This active ingredient is only active on broadleaf plants and will not impact grasses when used as labeled. It can present some issues with replanting the area in the current growing season (see label for details). Examples of products that contain triclopyr include Bioadvanced Brush Killer Plus, Ortho Poison Ivy & Tough Brush Killer, Ortho Weed B Gone Chickweed, Clover, and Oxalis Killer, etc. 

Please let me know if you have any questions. 

On 07/01/2024 11:59 AM EDT Ask Extension <<personal data hidden>> wrote:

 

 

Both of the herbicides mentioned can/will damage boxwood if contacted. I would recommend treating cut stems or covering the boxwood that could be contacted with something impermeable (I sometimes use old mulch bags) until the spray has dried (or longer if the label states a longer reentry time) and then dispose of the covers.

On 07/02/2024 9:37 AM EDT Ask Extension <<personal data hidden>> wrote: