Eli5, how do selective herbicides such as broadleaf weed killer work? Why do they only kill certain things?

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Eli5, how do selective herbicides such as broadleaf weed killer work? Why do they only kill certain things?

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By targeting something that only that species or group of species have.

Because they are broadleaf, that means they are all related. Much like humans, chimps, gorilla’s all share many characteristics because we all primates.

So we find something that all broadleaf has in common, that other plants don’t and target that.

It could be a chemical that inhibits their growth, prevents cell division, something like that.

Anti biotics work in the same way. You take a drug that only hurts bacteria and not you, because it targets things only bacteria have.

There’s a couple of different ways that herbicides and weed killers are made to target certain things.

1. Most weed killers are root poisons. Weeds are short lived and don’t need extensive root structures for long term energy storage; they only grow enough roots to hold themselves up. So you make a poison that’s strong enough to take out their limited roots, but not strong enough to do significant damage to grass or other plants with more extensive root structures.

2. Most poisons are absorbed thru the roots, but can be designed to be applied to the leaves instead. Weeds and grass tend to have different types of leaves, so you make a poison that is better absorbed by one type of leaf. Everything gets the same dose of poison, but certain kinds of weeds absorb a lot more of it, so they’re the ones to die. Your broadleaf weed killer is probably this type of poison.

I use herbicides for work so I can actually help here

Plants have families, genus, and species and beyond (sbsp var etc) but are also split into various groups such as dicotyledonous and cotyledon to name some, known as broad lead vs narrow lead. However this isn’t botanically correct as there are more botanical features which tell which of these groups a plant falls into such as;

Root types
Flower type/shape
“organs” (the inner workings of how it)
Leaf shape or leaf replacement

The list goes on

The most important one is the inner workings of how plants respond to stress or how plants actually work to live. General herbicide like glyphosate attach to the circulatory cycle and just pump toxic plants using their general systems all plants have, but specific targeted herbicides don’t, they attach to the parts of plants that the other groups either lack or are underdeveloped.

Not really a 5yo level but hope it helps