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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.

Yes, Johnsongrass is a hard weed to kill. It has the ability to grow fast and spread even quicker, so it requires a detailed plan and thorough application to eliminate it. The best plan of action is to kill Johnsongrass while the weed is establishing itself by hiring a lawn care professional or following the steps above. 

Technically, dandelions aren’t bad for your lawn, and they don’t directly kill grass. In fact, they appear in areas that are nutrient-deprived, and their long taproots can loosen the soil and bring nutrients closer to the surface. 

However, dandelions do spread quickly, and they can take over your lawn, eventually crowding out grass. This is why a healthy lawn is the best defense against dandelions.

Glyphosate and other non-selective herbicides can kill orchard grass, but they will also kill the surrounding plants. Also, not all areas allow the use of glyphosate due to its environmental impact—which the EPA describes as a "potential risk to terrestrial and aquatic plants and birds, and low toxicity to honeybees." Consider mechanical removal to target the plants specifically.