How does the weed killer you use on your lawn not kill the grass? It can kill hundreds of variety of weeds, but not the grass. How? Why?
Broadleaf lawn weeders are commonly a mixture of dicamba and MCPA both of which are synthetic mimics of the plant hormone auxin. They kill plants because natural auxin controls growth in plants in part by softening the wall of the cells to allow them to stretch. Synthetic auxins have the same effects, but they never allow the cell walls to harden again. As a result plants treated with these herbicides become twisted and deformed as the cells grow out of control and eventually the plant dies because the hard walled cells needed to conduct water never develop in the new tissues.
Grasses and broadleaf weeds are not closely related, one being a dicot and one a monocot. That means they are as responsive to the hormone auxin. They need a much larger dose over a prolonged time period to produce the same effect. At the rates of application used on lawns there just isn’t enough of the hormone available to cause the death of grasses. If you apply these herbicides at 4X the label strength or if you apply them every6 week for a month or so you will kill your grass.
Simply speaking it’s the same reason why aerosol flysprays are safe to use. People are just more resistant to the chemical. At large enough doses they would kill people but we don’t use them at those doses. The same is true of grass and broadleaf weeds. Provided the dose is kept low and given time to wear off the effect of the poison on grasses is small.
Are there weeds that are similar enough to grass that they aren’t affected by weed killer?
“Weed” is such a relative term. Broadly defined, a “weed” is something that grows where you don’t want it. Daisies in your lawn are “weeds”. Grass in your flowerbed is a “weed”. In my lawn, anything that even remotely resembles grass is fine by me, whether you define it as a “weed” or not.
<I> Are there weeds that are similar enough to grass that they aren’t affected by weed killer?</I>
Plenty. Many of the sedges such as the ubiquitous nutgrass/nutsedge are very closely related to the grasses and almost immune to broadleaf herbicides.
I have wild violets in my yard. The leaves are broad, but they have a semi-tuberous root. A single application of weed killer just slows them down as they reach into the root for more resources to grow again. Supposedly, if you respray on the right schedule you can kill them by exhausting the root. But then (esp. in my case) the grass is more likely to die.
Ditto a lot of other stuff with tuber-like roots. E.g., “rance” (a type of wild onion).
There are also a lot of invasive grasses that of course are not harmed. E.g., quackgrass.
To pesticide companies, a “weed” is something their product works on.
My strategy is to try and keep the grass healthy enough that it “out grows” the weeds. (Hard to do with all the trees I have.)
But in those cases ftg the lants aren’t resistant because they are closely related to grass and have the same resistance mechanism, which was the question asked. Violets aren’t closely related at all. Onions are more closely related but from in tis case resistance isn’t caused by that relationship.
For those tuberous weeds a single application of glyphosate via a wick wiper is the way to go. Glyposate translocates far better than the synthetic auxins.