They do say that if you leave an annual weed to grow, you will have seven more years from the seeds! Perennials are less prolific, but you have to get all the roots out, or the tiniest bit will regrow. No plant can live without photosynthesising through its leaves though so, as I garden organically, I am an ardent hoer! I go out and tip the tops of all the weeds with a sharp hoe, every so often. Do that often enough and perennials will give up, and annuals will never seed so will only live for that year. Mulching thickly (3- 4") with bark will work for a couple of years, getting less effective as it becomes a seedbed itself. It is quite an expensive weed suppressant if you have anything more than a tiny patch. I am weeding today and whilst sometimes I find it soothing (like peeling wallpaper off), today I am cheesed off with how quickly weeds grow, and how tenacious they are!!!!!