When should I use pesticides to kill my weeds forever?

where are the weeds?

First start with boiling water, then wait until they die, 1-2 days. Next soak the areas with the garden hose, wait 24 hrs. Pick your weeds, the'll come out by the root if you grab them at the bottom. No pesticides, no smell for you to breathe. They will not grow back if you get the root. Good Luck!

Firstly pesticides do not get rid of weeds, you mean weedkiller.

Weed killer will not get rid of weeds permanently, new weeds will just grow or if they are perennial weeds & you don't kill the roots they will grow back.

Annual weeds are easy to pull up but they seed so widely you will be pulling for years.

Depending on where they are there are several methods to help.

If they are in a flower border then pull them up & apply a mulch, this works best for annuals. Perennials are best either dug up or you can use Roundup Gel. this is NOT a spray & will not touch any other plants, therefore only killing the one you want, several applications may be needed & it may take a few weeks before it completely dies. Some people do not like using chemicals they view to be harmful to the environment.

If they are in a path or on open ground you can use boiling water or even a weed wand (burning), vinegar mixed with salt & water can also be sprayed but none will stop new seeds from germinating & will not necessarily kill the root.

If you are trying to clear a large area to plant other things you can lay a membrane down & mulch over the top, then plant through it.

Hope that helps