Established perennial weeds take more work. If you can’t pull or dig up the roots, cut the plant down and cover the crown with a thick layer of mulch. Cut down any new shoots and don’t allow the plants to flower, produce new leaves, or go to seed. With no leaves to photosynthesize energy, the roots will eventually use up their reserve energy, stop sending up new shoots, and die.
Sheet mulching with organic matter such as compost, leaves, and wood chips over cardboard, newspapers, or weed control fabric can take up to two years to enrich the soil and kill all the weeds in a large area. Placing drip emitters in a 12” grid on the soil under newspaper and cardboard mulch speeds up the process and provides waters roots of plants you want to keep. While you wait for the layers of mulch to decompose, you can plant vegetables in holes dug out of the cardboard.
All organic mulches break down over time, some quicker than others. You’ll need to re-mulch every two or three years.