How do you eradicate Canada thistle in midst of perennials?

Asked October 30, 2013, 10:28 AM EDT

In our town home association, we have a pond area that serves as a kind of rain garden for water run-off from the entire area plus the neighboring condo building. The area is fenced off. Within the pond area the ground slopes fairly steeply toward the pond area at the bottom. Along the sides there are various plantings, including high bush cranberry, birch clusters, lilacs, dogwoods, and other shrubs. There is also a good number of native flowers (bergamot, rudbeckia, aster, etc.) that grow around the sides.

Along one of the sides we are noticing a growing encroachment by Canada thistle. It is fairly widespread along the one side. We want to control it, but hesitate to use an herbicide, because it could easily wash down or infiltrate into the pond itself. Also, we don't want to do a general broadleaf herbicide application because it would also kill the perennials and natives that we want to grow there. Thus far we have "controlled" the thistle by cutting it down in June before it blooms, or by pulling out those we can.

Our question: Is there some way (up to and including use of chemicals) to eradicate the thistle, or at least a way to keep it under control, without having to slog our way through the growth to cut or pull it? I have read that applications of 2-4-D can control Canada thistle, but I am not sure how specific that chemical is; that is, what effect it would have on desirable perennials. We would be grateful for your advice.

Ramsey County Minnesota

1 Response

As I'm sure you will attest, Canada Thistle is on the DNR's noxious, invasive list and is a challenging plant to control:

It seems that much of the literature on control is aimed at larger scale agricultural situations but most the basic recommendations can be scaled down for residential use.
Here is a link that explains the growth cycle of Canada Thistle, and indicates the best times in that growth cycle to apply the proper herbicides:

The U. of Minnesota recommends the following:

Selective - Postemergence (Non-lawn)
Thistles can be controlled with selective herbicide applications of 2,4-D or triclopyr at the pre-flower bud stage of growth. May require multiple applications. Non-selective herbicides can also be used at that same stage. This too may require multiple applications. Fall treatment of the rosette (i.e., circular basal cluster of leaves) stage of growth with either of the selective herbicides mentioned or the non-selective herbicide glyphosate can also be done.
Non-selective - Postemergence (Non-lawn)
Non-selective, postemergence herbicides are most commonly used in non-lawn areas. Common postemergence, non-selective herbicide active ingredients for home use include glyphosate and glufosinate ammonium. As the term non-selective implies, these products will destroy all of the vegetation, grasses and broadleafs, that they contact. Therefore, be very careful, when using these materials in and around desirable broadleaf plants such as garden flowers, wildflowers, trees and shrubs as they can cause serious injury and even death to those plants should they come in contact with the herbicide. The herbicide Rodeo is the glyphosate product labeled for use in or near water areas. Always explicitly follow product label directions for use.

There is a biological control for Canada Thistle, but the results seem to be getting mixed reviews. You should realize that even an effective biological control does not eliminate the pest plant, rather it simply knocks it back to a more "acceptable" level. This is from the University of North Dakota:
Biological – Canada thistle was among the first 19 weed species selected as targets for biological control in 1959. The painted lady butterfly (Vanessa cardui L.) is believed to be the only native insect that attacks Canada thistle. Larvae of Larinus planus, a seedhead weevil, feed on developing seedheads of the plant. Larvae of a stem gall fly, Urophora carduii, produce galls in the stem of Canada thistle that act as a nutrient sink. Cassida rubiginosa, a leaf-feeding beetle, reduces biomass and survival of Canada thistle. These biological control agents have been largely unsuccessful due to biological agents
not being host specific and/or doing little to reduce Canada thistle infestations. However, Ceutorynchus litura, a stem-boring weevil, was recently released in North Dakota. Larvae hatch in the mid-veins of rosette leaves and mine down through the vein into the base of the stem and upper taproot, thus stressing the plant. Weevils exit the Canada thistle stems leaving holes that make the plant vulnerable to pathogens.
Several pathogens have been examined for Canada thistle control. Sclerotinia sclerotiorum causes wilting and death of shoots and roots of Canada thistle. The rust fungus, Puccinia punctiformis, causes etiolation of systemically infected shoots and necrosis of stems of leaves. Pseudomonas syringae pv. tagetis is a pathogen that causes apical chlorosis and leaf spot on the plant. These pathogens may or may not be successful in controlling Canada thistle.

I'm afraid that whatever method or methods you chose to use you will have to "slog through the growth". Timely cutting out of the growth, and carefully applying chemicals to the thistle - only - are the best ways to do the least amount of damage to the rest of your plantings.

Persistence is the key here. It will take some years to eradicate this nasty plant. In the meantime the rest of the plantings will grow and their shade will make a reinfestation less likely.

I hope this information is helpful. Please contact AaE again if you have further questions.