Whether you face Johnsongrass in your pastures, crab grass in your vegetable gardens, or lawn fescue creeping into your perennial beds, killing grass in an ecologically friendly manner is an ongoing challenge for every gardener. Chemical herbicides can damage adjoining plants and potentially harm downstream aquatic environments; opt for non-chemical methods, or the old-fashioned "elbow grease" of hand-weeding, to kill grass and protect the environment.
A flame weeder can be an effective way to kill patches of grass. Farmer Jake Guest, in a video posted by the Vermont Cooperative Extension Service, recommends using a backpack flame weeder on grasses when they are a few inches tall. Although flame weeders are effective on other weed species just as they emerge from the soil, Guest notes that grass flamed at this early stage tends to re-emerge. Use caution any time you use a flame weeder; keep an ample water source handy to douse any thatch or dry vegetative matter that might catch fire.
Boiling water is a simple, inexpensive way to kill grass. Large patches of grass can be killed by pouring on the contents of a boiling soup kettle--the leftover water from cooking corn-on-the-cob or shellfish at a summer barbecue can even be recycled as grass killer. The Audobon Society recommends using boiling water, poured in a stream from a tea kettle, to kill weeds like grass in driveway or walkway cracks. Be very careful not to splash the boiling water on yourself; wear closed-toe shoes and long pants, and hold the vessel containing the boiling water close to the ground while pouring. A day or two after killing the grass with boiling water, hand-weed or dig out the dead grass to discourage re-sprouting from any remaining live roots. Treat the area again any time new grass shoots emerge.
Mulch can be an effective way to preclude weed seeds, including grass, from germinating, but organic mulches such as straw or pine bark chips applied over growing grass may not be effective at killing it; the grass will re-emerge through the mulch. The Audobon Society recommends covering the area with black plastic, staked tight to the ground, and left in place for four to six weeks; the increased temperature under the plastic mulch will kill the underlying weeds and grass. Once the grass has been killed back with black plastic mulch, hand-weed or dig out the dead turf, then plant your desired plants and mulch around them heavily with a layer of wet newspapers followed by an aesthetically pleasing organic mulch.