You should add 1 to 4 inches of mulch, depending on your garden type and needs. Keep in mind that no amount of mulch can completely prevent weeds, but a heavier coverage (closer to 4 inches) can suppress weeds enough that you are only seeing them sprout every few weeks. Finer mulch prevents weeds more effectively than large mulch, so if you’re using mulched leaves or shredded bark, 1 to 2 inches will likely suffice, while 3 to 4 inches is best for larger mulch, like bark chips or straw.

Mowing your grass regularly, especially if you’re mowing close to the ground, can help kill Johnsongrass. Frequent mowing helps limit the spread of Johnsongrass and can even deplete carbohydrate reserves in rhizomes over time. The more time you leave in between mowing, the more likely Johnsongrass is to regrow and take over your lawn. 

Purchase a selective herbicide that targets clover and similar weeds but doesn't take out your grass. You can also keep it from growing with corn meal gluten or remove clover from your lawn manually at the start of the season as part of your lawn maintenance checklist.

Dandelions, while hard to control and widespread across the country, are not typically categorized as noxious. Yellow starthistle, however, looks very similar to a dandelion and is highly damaging to local plants and invasive across the country.