Chapter 2: Why Are Weeds So Successful? Basic Ecology Concepts
2.5 Weed Classification – Taxonomy
Tissue Type
Weeds can also be classified based on their tissue type. The first tissue type classification is called herbaceous, and these plants can be simple or creeping as well. Herbaceous plants do not have stems that make it through the winter, rather all the above ground tissue will die back. There are herbaceous annuals and herbaceous perennials.
In contrast to herbaceous types, plants can also be woody types. With these, the stem tissue do remain above ground and overwinter. This would include shrubs and trees, which can be considered weedy in some situations.
Practice Activity
Look at the images below. Determine if the weed pictured is considered a woody or herbaceous plant.
Monocots and Dicots
Weeds can be further characterized as either dicots, which are broadleaf plants, or monocots, which are a grass-like type of plant. Soybeans are a common dicot crop, while corn is a common monocot crop.
Practice Activity
Complete the drag and drop activity below to practice identifying monocots and dicots.
Common burdock (Arctium minus) image is from the Ohio Perennial and Biennial Weed Guide. Green foxtail (Setaria viridis (L.) Beauv) image is from Howard F. Schwartz, Colorado State University, Bugwood.org. Yellow nutsedge (Cyperus esculentus) image is from the Ohio Perennial and Biennial Weed Guide.
Photosynthesis Type
Finally, we can think of weeds in terms of physiology properties. In general, weeds can carry out photosynthesis in two different ways, just like our crop plants, either as a C3 or C4 mode. Both C3 and C4 plants complete photosynthesis to create sugars, but C4 plants contain additional cell structures to maintain photosynthesis during times of heat stress. Common C3 crop examples include soybeans, wheat, and alfalfa. Some C3 weed examples include curly dock, common lambsquarters, cocklebur and jimsonweed. Common C4 crop examples include corn and sorghum. Some C4 weed examples include yellow nutsedge, johnsongrass, barnyard grass, foxtail, crabgrass and pigweed.
Pictured below from left to right are redroot pigweed (Amaranthus retroflexus L.) and curly dock (Rumex crispus). Redroot pigweed is an example of a C4 weed, and curly dock is an example of a C3 weed.
Reedroot pigweed image is from Phil Westra, Colorado State University, Bugwood.org. Curly dock image is from the Ohio Perennial and Biennial Weed Guide.