Is Lontrel safe? - Ask Extension
I have an enormous amount of Canadian thistle in a very large flower bed—essentially my whole front yard. It has been recommended that Lontrel be us...
Knowledgebase
Is Lontrel safe? #875616
Asked July 02, 2024, 9:59 AM EDT
I have an enormous amount of Canadian thistle in a very large flower bed—essentially my whole front yard. It has been recommended that Lontrel be used to control the thistle and that it can be sprayed on top of the bed without hurting my perennials and Japanese maples that are planted there. Is that correct?
Baltimore City County Maryland
Expert Response
No, that recommendation doesn't sound accurate. The active ingredient in the product Lontrel, based on the product label we looked up, appears to be clopyralid. This is a very long-residual herbicide chemical, meaning that its residues do not degrade quickly and it can interfere with plant growth long after it was applied. (A useful trait for weeds in certain settings, but otherwise a risk for use around desirable plants. It can even contaminate compost if clippings from treated plants were included, affecting the plants that compost is applied around. It's such a problem that it is generally no longer available for use on home lawns.) Given that, we do not recommend its use in this setting, and while the label does allow for overspray (applying over the top of) on certain ornamental plants, it would not be low-risk in your situation. Even if you didn't mind sacrificing the ornamental plants already in the flower bed invaded by the thistle, the treatment residues might interfere with planting for months or years afterward.
The more suitable option among herbicide choices may be to use glyphosate, the ingredient used in many systemic products. If its residues enter the soil, they bind tightly to soil and don't leach or generally affect other plants in the area. You would still need to carefully apply it to only the weeds, as contact with the foliage of desirable plants would still cause damage to them, but that will be true for most herbicides. Canada Thistle is a very aggressive, tenacious weed, so even glyphosate may require multiple treatments before it's fully successful. Follow all product label directions, including about how long to wait between applications. Treatment of the thistle just as it's nearing flowering stage is one of the ideal times to apply systemic herbicide.
Baltimore City regulates herbicide use, though if you are treating a flower bed and not a lawn, they may not apply to your situation. If any of the thistle is growing into a lawn as well, you may need to check with local government to clarify whether you are allowed to treat the thistle there with glyphosate (such as if there's an exception for this particular weed).
Miri
The more suitable option among herbicide choices may be to use glyphosate, the ingredient used in many systemic products. If its residues enter the soil, they bind tightly to soil and don't leach or generally affect other plants in the area. You would still need to carefully apply it to only the weeds, as contact with the foliage of desirable plants would still cause damage to them, but that will be true for most herbicides. Canada Thistle is a very aggressive, tenacious weed, so even glyphosate may require multiple treatments before it's fully successful. Follow all product label directions, including about how long to wait between applications. Treatment of the thistle just as it's nearing flowering stage is one of the ideal times to apply systemic herbicide.
Baltimore City regulates herbicide use, though if you are treating a flower bed and not a lawn, they may not apply to your situation. If any of the thistle is growing into a lawn as well, you may need to check with local government to clarify whether you are allowed to treat the thistle there with glyphosate (such as if there's an exception for this particular weed).
Miri
