Landscape fabric and a few inches of gravel should be enough to put under a deck to prevent weeds from flourishing. If you don’t want to invest the time and money on a project like this, you can regularly spray the area underneath your deck to kill weeds.

The most effective way to eliminate quack grass without killing the grass nearby is to overseed the area, keep the seed wet until it grows, mow often, and regularly fertilize the area to promote healthy grass that can choke out the invasive weed.

Mowing is one of the most important ways to maintain your lawn because it encourages grass to grow horizontally, creating a fuller, more uniform yard. On top of mowing your lawn, you’ll want to routinely fertilize your lawn, water it regularly, and take care of pests the moment they arise. 

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.