how does weed killer know to kill the weed but not the grass?

Answer 1

Selective herbicides....which you're asking about, are designed to kill dicots OR monocots, but not both. Dicots include all the broad-leaveed plants...and also most of the weeds you find on our lawns such as oxalis and dandelion. If used in the correct dosage, it will not harm monocots such as lawn grass.

The way they work is to interfere with growth, either by blocking photosynthesis and protein production or destroying or inhibiting root formation, or the plant will essentially grow itself to death (such as in the case of 2, 4-D).

2,4-D is an auxin or plant hormone that, when applied to broad- leaved plants, simply confuses the plant to death. When auxin, the hormone that makes plants grow, is sprayed over the entire plant, the plant becomes deformed, then dies.

Hope this answered your question.

(certified professional crop consultant with a degree in plant science)

Answer 2

You could try a systemic weed killer.There are a few on the
market ,its a long time since i used weedkillers i dont bother with
them as i dont want the stuff in the soil. What you do is put the
weedkiller on the top of Weed and side ,just enough to cover it.
It soaks into Weed without killing Grass.