Agave Americana proves tough to kill for many gardeners. These plants grow to a six to 12 feet in size, according to the Johnson Wildflower Center at the University of Texas. Their spiked leaves make them a hazard for growing along walking paths. Since they grow by rhizomes in the ground, unless you kill these roots or dig them out completely, the agave Americana will return, but you can effectively kill these tough desert plants with some diligence.
Dig up any roots around the agave plant with a sharp shovel. Dispose of these roots in a sealed trash bag and do not compost them.
Saw the agave Americana plant down to the base stem coming from the ground.
Soak a sponge brush with undiluted glysophate herbicide and immediately coat over the cut portion of the agave root.
Repeat digging up and coating the roots of any more agave Americana plants that emerge.