Crabgrass and reed sweetgrass are both common grassy weeds. Crabgrass largely invades unkempt lawns and abandoned lots. While crabgrass is unattractive, most species are not aggressive enough to be categorized as noxious weeds. Reed sweetgrass, however, is a perennial aquatic grass that can take over a large area in a relatively short period of time. It can be found throughout North America, and in most areas, it is classified as a Class A noxious weed, which means that landowners who find it on their property are legally required to eradicate it. Each grassy weed requires different control methods. However, as with any weed, you stand the best chance of getting rid of crabgrass and sweetgrass by catching the infestations early.
Dig up small stands of sweetgrass. First, use a pair of lopping shears to cut back (and dispose of) the majority of the grass. Then use a shovel to dig up the roots. Be sure to remove even small pieces of the sweetgrass' rhizomes to prevent it from regenerating. Keep a close eye on the patch for the next few months and immediately uproot any seedlings that may emerge from overlooked seeds or rhizomes.
Repeatedly mow or cut back larger stands of sweetgrass to slowly starve their rhizomes. If you keep the sweetgrass mowed to only a few inches in height, you can control its spread and allow other vegetation to establish itself and hopefully outcompete the sweetgrass. However, this is unlikely to get rid of the entire stand.
Spray crabgrass with a preemergent herbicide prescribed for use on crabgrass and listed as safe to use on the grass in your lawn. Crabgrass is an annual weed, and the best way to get rid of it is to prevent next season's seeds from germinating. Crabgrass seed germinates in spring when temperatures reach between 55 and 60 degrees Fahrenheit and will continue to do so for the next week or so. Therefore, the best time to apply preemergent herbicide is in early spring before the crabgrass has a chance to germinate. Follow the manufacturer's instructions when applying the herbicide. For the best results, reapply the herbicide at the rates dictated by the manufacturer, until summer when all threat of seed germination has passed.
Replant all cleared areas. Crabgrass thrives in thin lawns. After the residual preemergent herbicide has disappeared, seed your lawn to fill in any bare patches. Replace reed sweetgrass with a competitive ground cover or another, less-invasive aquatic grass or plant.