Herbicide for Weeds Under Conifers - Ask Extension
We have an area of 12 mature (40 years old) conifers of various varieties. Six 6 years ago I limbed up the branches to about 8 foot high. 3 years ago ...
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Herbicide for Weeds Under Conifers #937765
Asked July 02, 2026, 4:28 PM EDT
We have an area of 12 mature (40 years old) conifers of various varieties. Six 6 years ago I limbed up the branches to about 8 foot high. 3 years ago I moved the existing needle straw underneath, put weed paper down and returned the needle straw.
The past couple seasons we have areas with high weed growth while other areas are not highly infested. It looks like the needle straw has broken down and created a soil like base. Pulling them is easy but the amount has become overwhelming.
What herbicide can I use to kill actively growing spreading pellitory, chickweed, pigweed, and speedwell under the various conifers without also being absorbed by the conifer roots?
Oakland County Michigan
Expert Response
Yes, unfortunately, weed mats are only good for things coming up from below, and many times we see seeds deposited after the placement of mulch, needles, rocks, etc. Weeds are very resourceful, so many don't need much "soil."
Currently, post-emergence products that would be labeled for use above a tree root zone include the following (though there are likely more):
When using non-selective products, such as those listed, there are a few important points to consider. First, as with any pesticide, remember to read and follow all labeled instructions. The label is the law. Second, non-selective herbicides will injure or kill other plants contacted during application, so care is needed to avoid direct or drifted contact with green plant material, exposed roots, and injured bark of desired plants. As we move away from using glyphosate, it is important to note that many of the currently available products will prevent replanting in the area for some period of time (see “Not your mother’s Roundup” from Michigan State University Extension). Look for this replant time on the label, as it varies by product.
Wait at least 2 full weeks after the application to see the full activity before deciding whether to retreat.
Once you get this year's infestation under control, there may still be dormant weed seeds, or more may be deposited. You might consider a pre-emergence product, such as Preen Garden Weed Preventer (a.i. trifluralin), to kill them as they are germinating. For most of these weeds you've mentioned, you'd want to make a spring application (like the time crabgrass preventer gets put on), but if you're seeing, over time, some fall-emerging, winter annual weeds, then a mid-August application would also be helpful.
Please let me know if you have any questions.
Currently, post-emergence products that would be labeled for use above a tree root zone include the following (though there are likely more):
- Roundup Weed & Grass Killer Exclusive Formula (active ingredients: fluazifop, triclopyr, diquat)
- Ortho Groundclear Weed & Grass Killer (a.i.s ammonium nonanoate)
- Spectricide Weed & Grass Killer (a.i.s diquat, fluazifop, dicamba)
When using non-selective products, such as those listed, there are a few important points to consider. First, as with any pesticide, remember to read and follow all labeled instructions. The label is the law. Second, non-selective herbicides will injure or kill other plants contacted during application, so care is needed to avoid direct or drifted contact with green plant material, exposed roots, and injured bark of desired plants. As we move away from using glyphosate, it is important to note that many of the currently available products will prevent replanting in the area for some period of time (see “Not your mother’s Roundup” from Michigan State University Extension). Look for this replant time on the label, as it varies by product.
Wait at least 2 full weeks after the application to see the full activity before deciding whether to retreat.
Once you get this year's infestation under control, there may still be dormant weed seeds, or more may be deposited. You might consider a pre-emergence product, such as Preen Garden Weed Preventer (a.i. trifluralin), to kill them as they are germinating. For most of these weeds you've mentioned, you'd want to make a spring application (like the time crabgrass preventer gets put on), but if you're seeing, over time, some fall-emerging, winter annual weeds, then a mid-August application would also be helpful.
Please let me know if you have any questions.
