Pigweed (Amaranthus retroflexus) goes by many names, including redroot pigweed, redroot amaranth and many others. Pigweed thrives in garden settings and grows wild in untended fields. Young pigweed can be eaten in salads and its seeds are often used as a flavoring. Small amounts of pigweed can also be ued as an animal feed. Left on its own, pigweed will invade and crowd out almost all other plants. Older pigweed plants are not edible and become true weeds. Pigweed is generally resistant to herbicides such as Roundup. And because pigweed has such a deep taproot it is difficult--but not impossible--to kill.
Dig up pigweed in early spring while plants are small. Dig down and remove as much of the tap root as possible. It will be easy to dig up young pigweeds, but older, established plants will be more difficult.
Recheck the area in two weeks. Dig up as much pigweed as possible, once again digging deep down into the soil to free the taproot whenever possible.
Use a hoe to cut pigweed down to ground level in places where it is not possible to dig out the taproot. Continue cutting pigweed down to ground level every week during the growing season. This will make the roots use most of their energy trying to sprout new growths--and will eventually cause the roots to die.
Search for any mature pigweed plants in fall and either dig down, removing the taproot, or hoe the plant down level with the ground in order to prevent the plants from seeding. Preventing the plants from seeding will prevent the weed from spreading any farther next year and will make your job of destroying new pigweed growth in the following season that much easier.