Knotgrass is a perennial plant that has a very distinct look that makes it easy to identify. The plant's official name is Japanese knotgrass which is how it's often referred to in the gardening world. Though the plant itself is relatively harmless, it tends to grow quickly and can easily spread from one site to another. For this reason it's important to contain Japanese knotgrass and to dispose of it properly if you choose to remove it from your premises.
Look at the shape of the stem. Knotgrass has a tall, thin cane that is similar in appearance to bamboo. This bamboo-like stem is hollow and can grow up to nine feet tall.
Examine the colour of the plant in question. Sometimes the Japanese knotgrass is mostly green, but there are also thousands of tiny purple or red dots covering the plant to look for. On occasion, the purple specks can overwhelm the plant, especially in early spring. This causes the knotgrass to appear mostly purple or green.
Look at the leaves. Knotgrass leaves are big, green and shaped like an acorn. The underside of the leaves should be lighter than the top side.
Note the characteristics of the branches. Japanese knotgrass has distinctive, bamboo-like branches that come equipped with tiny thorns. Creamy white flowers grow on knotgrass branches during early fall months (August to October). In the winter, the only part of the plant visible is the branches and stems. They are hard, dry and orange/brown in appearance.