It depends on what type of weedkiller you choose to use over your weeds.
And the above question also depends on what method you apply, the amount of time it takes, the activity, and weeds on what family it belongs to.
After applying weed killer over the soil, you can't grow anything over that sprayed area because chemicals act over the ground. It won't allow seeds to germinate.
That's why, nowadays, weedkillers design to evaporate within 24 to 78 hours. To ensure the safe growth of the plant, scientists have discovered these types for easy cultivation.
Generally, weed killers work in different ways. the main three ways are
1) The way they act
2) The way they're used
3) By how they killed
1. The way they act
The way the weedkiller acts is basically of two types. One is contact; another one is a systemic weedkiller.
Contact weedkiller kills only the part where the chemical is treated; thus, it is considered the fastest-acting weedkiller type.
The other one is systemic, destroying the targeted plant entirely where the chemical acts through the roots.
2. Then the way they're used
The weedkiller typically used in three ways: pre-plant incorporated weedkiller,pre-emergent weedkiller, and post-emergent weed killer.
let us know these ways in detail,
i)pre-plant incorporated weedkiller
ii)pre-emergent weed killer
iii)post-emergent weed killer
i. Pre-plant incorporated weedkiller
From the name itself, it is clear that it is applied on the ground before the crops are planted. Means sprayed over the soil; before the plant cultivated, it won't allow weeds to grow; it stops the growth.
ii. Pre-emergent weed killer
Pre-emergent means these types of weedkiller is applied over the soil before the weed emerges. Which means it is killing before it grow.where the chemical is absorbed by the soil.
iii. Post-emergent weed killer
In the post-emergent type, weed killer applied overgrown weeds. it directly reacts with roots and stems and causes tissue to get damaged.
3. And then by how they killed
For an effective way of killing, first, apply over plants. When you apply, it retains on the leaf.
It penetrates the waxy cuticular layer, found over the leaf's surface.
It moves to water-filled space .Once moved; it must again pass to a lipid-like membrane. When it passes through the membrane, it has reached its target.
Target is that it binds a targeted enzyme. Finally, when the enzyme is damaged, the whole plant comes to an end.
Even though weedkiller targets the enzyme function, it has not clearly understood why plants die and lose their lives after applying the weed killer.
For ordinary people to understand it better, plants die by stopping one of a commonly known process called photosynthesis.
Generally, plants suck carbon dioxide and energy during photosynthesis to make their food, called sugar molecules and oxygen. The plant releases oxygen.
Chlorophyll only gives plants a green color because it fails to absorb the green wavelength of white light.
Thus chlorophyll is affected when weed killers are applied to plants, which stop the process and make leaves drier. And finally, cause plants to die.
Misuse and misapplication
Weedkiller gives the best result only when applied correctly. However, people sometimes add more water to get more amount of weed killer to spray over their weeds.
When you misuse like this, it won't affect your weed because adding too much water reduces its effectiveness.
Misapplication like spraying over rainy days which wash out your weedkiller and nothing remains in the soil to act. You can find the result as more grown weeds only.
How glyphosate in weed killer works
At first, it starts to build up to toxic substances. It harmfully stays only at a low level in the starting stage by inhibiting enzymes (targeted site), causing the substances to build up.
Finally, it damages the plant. This is how the herbicide glyphosate works in the plant.
Most of the weedkiller works when weeds are a little wet in a condition so that it quickly enters into the weeds and finally starts to kill. If it is not wet, you have to water it slightly to wet those weeds for the best result.
But if you apply weed killer on rainy days, it gets to wash out, so at that time, use a waterproof weedkiller. that would be a perfect choice.
Almost all weed killers are manufactured to kill weeds within 2 to 4 weeks. You can blindly trust those weedkillers; it gives the best result, but the way you use makes the difference.
Weed Killers work great only when you follow all the procedures provided in the label of the product. When doubt arises, how to use, turn the product, read it twice, and then start to use it.
If you do this, it indeed gives the expected result.