Lillies of the Valley WON'T DIE!!!
We have used Round Up on large patches of Lillies of the Valley twice in the past week and they don't even look like we sprayed. Only a couple of plants even show a spot or two on the leaves. What can we do? We're trying to get rid of Giant Ferns and the Lillies of the Valley so we can plant a new front lawn. The ferns Are turning brown. Will that ensure that that particular plant will not come back next year?
Is it possible that the lillies will come back next year even if we can finally kill them this year?
Eaton County Michigan
This is the second lily of the valley control question I've had this year.
I'm not sure how long ago you treated the plants, but it can take up to two weeks to see the full extent of control following a glyphosate (Roundup Weed and Grass Killer) application under ideal conditions (i.e. >50F air temperatures). Many persistent perennials require multiple applications of glyphosate due to their extensive root systems. The best time to treat perennials is in the fall when the natural flow of photosynthates to the root system facilitates better movement of the glyphosate within the plants. You are seeing control on the ferns, and it could be that they are just a more sensitive species than the lily of the valley. I would suggest making another application of glyphosate if it has been more than 2-3 weeks since the first application. Be sure to read and follow all labeled instructions and check the rate also if you are using a concentrate.
Once you have killed above ground plant material, it is possible that you will have reemergence from the roots. Its kind of a wait-and-see process. Herbicides that are more potent could have long-lasting activity and prevent you from seeding your new lawn, so I would be reluctant to try other products at this point. Lily of the valley is a monocot, similar to grasses, so it is unlikely that your typical lawn herbicides that target broadleaf weeds (e.g. 2,4-D, dicamba, mecroprop, etc) will be effective for control.
If you do have reemergence next year in your new lawn, there are no herbicide products labeled specifically for this plant in a lawn. You could try a product with the active ingredient sulfentrazone, like Ortho Nutsedge Killer for Lawns. This product has activity on other monocots, like nutsedge, wild garlic, and star-of-bethlehem...so an educated guess would say that it may be effective on lily of the valley as well.
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