You need the one you paint on the leaves but can't remember name. Garden Centre will know. Is it Roundup?

Roundup does now make a gel formulation - you paint it on the leaves and stems of the offending plant, but don't get it anywhere else. It's not a selective weedkiller though, in that its not fussy, get it on the leaves of one your prize plants and it'll kill that too.

The only selective weedkillers I know of are those which you use on grass because they don't kill grass but kill other weeds in the lawn.

as others have said you need to paint a weed killer onto weeds keeping it off the plants you want to keep.

You can use Round up on weeds around trees as it won't kill a tree through it's bark