Knotweed is a difficult to control weed that has gotten more widespread every year. There are two species here that are both in the buckwheat or polygonum family. Other weeds in this family are curley dock red sorrel and skeltonweed. The two species that are a problem here are silversheath knotweed and prostrate knotweed. Although they are normally described as annuals, both of these species persist as short lived perennials in the low desert and can be found year round. They grow almost anywhere that has moisture and can thrive in poor soil. Silversheath knotweed gets its name from the long white to silver sheath that is at the base of each stem. Both knotweeds form wiry branched matts that can extend for 3 or 4 feet. Prostrate knotweed has a slightly broader leaf and is less upright . The stems are solid and look like bamboo. Numerous species of knotweed can be found worldwide. It is often the first weed to colonize volcanic slopes and is very prolific everywhere that it grows. Few herbicides, including glyphosate are very effective in killing it.