"she found a blade of nut-grass in her yard"
nut-grass
Public Domainnut-grass - Credit: australianinsects.com

Nut-grass, also known as nutsedge (Cyperus rotundus), is one of the most invasive weeds in the world, being widespread in tropical and temperate regions. It has been called "the world's worst weed" as it is known as a weed in over 90 countries, and infests over 50 crops worldwide.

Its existence in a field significantly reduces crop yield, both because it is a tough competitor for ground resources, and because it is allelopathic (meaning that its roots release substances harmful to other plants).