Palmer amaranth (Amaranthus palmeri), a member of the "pigweed" family, is one of the most troublesome weeds in many southern row crops. Seed can germinate all season and plants can grow to over 6 feet in height. Plants have either male flowers that shed pollen or female flowers that can produce up to 600,000 seed per plant. One Palmer amaranth per 30 foot of row can reduce cotton yield by 6 to 12%.

Invasive exotic species are plants that are not native to a given area and have the ability to out-compete indigenous plant species. Invasive exotics are often brought into their non-native surroundings by humans with good intentions.

Plants can be defined as multicellular, photosynthetic organisms with reproductive structures that are more complex than single cells. By this definition, algae are not considered plants because they are either unicellular or their reproductive structures are essentially unicellular. Fungi, too, are excluded because they are not photosynthetic. At least 400 million years of diversification have resulted in a wide diversity of taxonomically distinct major groups of plants. Some of the most important groups of plants found in Virginia are described below.

Grassy weeds in pastures and hayfields compete with desired forage species and reduce the productivity of forage systems. Lack of selective herbicides makes grassy weed species, such as Johnsongrass, Japanese stiltgrass, broomsedge, and foxtail species difficult to control. Proper soil fertility, grazing management, and correct timing and placement of herbicide application can effectively control these species.