Hi Neil,

Thank you for reaching out about using herbicide in your rocks under your deciduous trees to control the weeds. This is a tricky question and one that we as Master Gardeners are prescribed from recommending any brand names. However, I see from your email that you have done research into the top herbicide chemicals because you are concerned for your trees, and potentially any pollinating insects, well done. Herbicides can be complicated and come with warning for proper use which you also noted in your email. It sounds like you would like to use a non-selective (for all weed types broadleaf and grass) post emergent (after they germinate and sprout) herbicide. This type is systemic and can be taken up through the roots of trees and other plants near by.

It is important to know what type of weeds you would like to manage (broad leaf versus grasses) and if you would like a pre or post emergent, first.

I have attached a fact sheet from the Oklahoma State Extension that defines the types of herbicides and what they do.

https://extension.okstate.edu/fact-sheets/controlling-weeds-in-home-lawns.html

It is as important as it is lawful, as you know, to use the herbicide as directed on the container. Once you have found the type of herbicide to fit your needs, you will then need to examine how to use them. Here is a fact sheet from Penn State Extension outlining caution when using glyphosate around trees, because of that systemic problem I mentioned above. Do not spray the suckers of the trees and it is best to hand pull weeds near the trunk.

https://extension.psu.edu/use-glyphosate-with-care-near-trees

Now to the meat of your question, which is better? That is a tough question, so I have listed a few pros and cons of both here.

Roundup is a post-emergent herbicide only.
RM43 works as both post-emergent and pre-emergent herbicide. This will give longer lasting weed control for all types of weeds and any other type of vegetation grown in that soil. RM43 binds to the soil killing seed and plants.

Pure glyphosate is low in toxicity to fish and wildlife. 
(National Pesticide Information Center Website: 
http://npic.orst.edu/factsheets/glyphogen.html)

RM43 requires more care when applying than Roundup. This is because RM43 is residual in the soil. The herbicide is prone to “drifting” over time, due to water runoff. As the RM43 leaches from one area to another, it will kill new plants and grass it comes into contact with, as it is absorbed by the plant roots.

This is an important note to keep in mind when using RM43. The extra firepower comes at a cost—you could put nearby desirable plants in danger. Glyphosate will not attack nearby plants after entering the target weeds.

Apologies for the long answer, but it is a tricky question.
I hope this helps, please reach out with any further questions.

Happy Gardening!

Sincerely,
Amy S.
Colorado Master Gardener-Boulder County


                

Hi Neil,

Thank you for reaching out about using herbicide in your rocks under your deciduous trees to control the weeds. This is a tricky question and one that we as Master Gardeners are prescribed from recommending any brand names. However, I see from your email that you have done research into the top herbicide chemicals because you are concerned for your trees, and potentially any pollinating insects, well done. Herbicides can be complicated and come with warning for proper use which you also noted in your email. It sounds like you would like to use a non-selective (for all weed types broadleaf and grass) post emergent (after they germinate and sprout) herbicide. This type is systemic and can be taken up through the roots of trees and other plants near by.

It is important to know what type of weeds you would like to manage (broad leaf versus grasses) and if you would like a pre or post emergent, first.

I have attached a fact sheet from the Oklahoma State Extension that defines the types of herbicides and what they do.

https://extension.okstate.edu/fact-sheets/controlling-weeds-in-home-lawns.html

It is as important as it is lawful, as you know, to use the herbicide as directed on the container. Once you have found the type of herbicide to fit your needs, you will then need to examine how to use them. Here is a fact sheet from Penn State Extension outlining caution when using glyphosate around trees, because of that systemic problem I mentioned above. Do not spray the suckers of the trees and it is best to hand pull weeds near the trunk.

https://extension.psu.edu/use-glyphosate-with-care-near-trees

Now to the meat of your question, which is better? That is a tough question, so I have listed a few pros and cons of both here.

Roundup is a post-emergent herbicide only.
RM43 works as both post-emergent and pre-emergent herbicide. This will give longer lasting weed control for all types of weeds and any other type of vegetation grown in that soil. RM43 binds to the soil killing seed and plants.

Pure glyphosate is low in toxicity to fish and wildlife. 
(National Pesticide Information Center Website: 
http://npic.orst.edu/factsheets/glyphogen.html)

RM43 requires more care when applying than Roundup. This is because RM43 is residual in the soil. The herbicide is prone to “drifting” over time, due to water runoff. As the RM43 leaches from one area to another, it will kill new plants and grass it comes into contact with, as it is absorbed by the plant roots.

This is an important note to keep in mind when using RM43. The extra firepower comes at a cost—you could put nearby desirable plants in danger. Glyphosate will not attack nearby plants after entering the target weeds.

Apologies for the long answer, but it is a tricky question.
I hope this helps, please reach out with any further questions.

Happy Gardening!

Sincerely,
Amy S.
Colorado Master Gardener-Boulder County