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There are organic herbicide options that will kill grass but not flowers, including white vinegar or a dish soap solution. Commercial selective herbicides can also target grass over other plants, but read the instructions carefully to avoid damaging your garden. Herbicides that kill grass but not flowers usually include ingredients like clethodim and sethoxydim. Otherwise, your best bet will be to weed by hand.

Due to its slow growth, Bahia grass does not generally take over weeds. In fact, the opposite can be true. Young Bahia grass is susceptible to being taken over by weeds. 

Likewise, in the cool season, Bahia grass goes into a dormant state that can allow weeds that thrive in early spring to get the upper hand. If you want to prevent weeds from taking over your yard, remove them by hand or apply a Bahia grass-friendly pre-emergent herbicide to your lawn before weeds have a chance to appear.

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.

Some of the particular herbicides that work best to get rid of nut grass without killing the grass include Halosulfuron-methyl, mesotrione, sulfentrazone, and metsulfuron. A herbicide like glyphosate would kill nutgrass, but it will kill nearby plants, too.