I come in peace lol

I expect to be ridiculed for this question lol, however, if i place a planter over a couple of weeds or small clumps of grass in my garden they will die. Will the roots of them also die?

 

It depends on what the weeds are. Some will die, some will spread sideways and grow up around the base of the pot, some will find their way into the pot, nice new soil, food etc. and sprout up beside the plant growing in the pot.

 

It's never usually a good idea to put pots/planters directly on grass if you have something in them you want to keep. Apart from the weed problem described, you risk having drainage problems, and also roots growing through the bottom into the grass, which isn't ideal.

 

Can't you just dig them up?
By the way ...

 

If there’s nothing in the planter @Pipcas and you just want to use it to cover the weed and grass, it will block out light and water and weaken them, causing the foliage to fade but the roots may remain viable and send up shoots elsewhere. It depends what the weed is. A photo would help to identify it.