Well, pamelagrace, are these annoying weeds in your flower beds or in your lawn?
In the flower beds, if you have annuals that reseed themselves or even perennials that are close to coming up you might be hurting them with the weed preventer.

If in the lawns it's nearly time to start putting down the first dose of crabgrass preventer.

But, back to the flower gardens which(here) are full of sprouted weed seeds and chickweed I believe the instructions on my Preen container said weed the bed,plant the annuals and potted perennials before you sprinkle the bed with preen.
It's a seed sprouting inhibitor I think.
I don't recall if there are any special instructions to adding plants to a bed that has been so treated.
Maybe you can find the instructions on-line.