The following questions and answers were developed with help from these weed scientists: Kevin Bradley, University of Missouri; Aaron Hager, University of Illinois; Bill Johnson, Purdue University; Daniel Stephenson, Louisiana State University; and Bryan Young, Southern Illinois University.
Where do weeds come from?
Most weeds are plants that spring up in habitats altered by human activity or simply grow where they are not wanted. Even crop plants can become weeds—volunteer corn, for instance. Many weeds were introduced as sources of food by Native Americans or European settlers and moved across the country. The most problematic weeds tend to be those that are non-native or introduced species. Kudzu, for example, was introduced from Japan as a miracle vine that could control erosion.
How are weeds spread?
Some weed seeds have special anatomical features that allow them to be blown by the wind (dandelion) or attach to animals or humans (common cocklebur). Sometimes animals or birds eat and excrete seeds far from the point of consumption (pokeweed). Machinery, especially tillage, mowing and harvesting equipment, can distribute seeds. So does water, especially when it floods. Some perennial species spread by creeping roots (Canada thistle) or rhizomes (quackgrass).
What is an annual weed?
Annuals complete their life cycle within one year and reproduce by seed. Summer annuals germinate in spring, set seed in late summer or fall and compete with crops such as corn, soybeans and cotton. Winter annuals germinate in the fall, complete their reproductive cycle in the spring or summer and are typically found in no-till crops or pastures. Common ragweed is a summer annual. Common chickweed is a winter annual.
What is a biennial?
The life cycle of a biennial spans two growing seasons. Biennials germinate and grow vegetatively during the first year (often in the rosette stage), then flower, make seed and die during the second year. Typically found in waterways, pastures and fence rows, they need two years to complete a life cycle. Poison hemlock is an example of a biennial weed.
What is a perennial?
Plants that live more than two years are perennials. They can reproduce by seed, but they often reproduce by tubers, rhizomes, bulbs, stolons (aboveground runners) or roots. Managing perennial weeds can be a challenge because they have multiple reproductive systems. Johnsongrass is one of the most troublesome perennial grasses in the U.S.
Are there some basics to help identify weeds?
Weeds are generally classified as broadleafs (dicot), grasses (monocots) and sedges. In broadleaf seedlings, the first pair of leaves that open after emergence (called the cotyledons) can vary in shape from linear, egg, round or butterfly, but are always opposite. The true leaves, those emerging after the cotyledon, and stem are broad and offer the main clue to the weed’s true identity. Grass leaves are narrow and arranged in sets of two. Look at the collar region where the leaf meets the stem (culm) to tell the difference between grasses. Sedges appear grasslike from a distance, but up close you’ll find a triangular stem. The leaves on sedges are glossy or shiny, hairless and occur in sets of three.
How are weeds named?
Swedish naturalist Carl Linnaeus (1707–1778) developed what is known as the binomial system to develop scientific names for plants. "Binomial" means two words are used for classification purposes; both are generally Latin, but they are sometimes Greek. Many species names refer to color, size or shape. Some names refer to special plant characteristics—pubi, hirti, villi and barbi all indicate that the plant is hairy, for example. Plants can sometimes be named to honor a person. Most weeds have enough nicknames to make your head spin. Their common names generally describe something about the plant. For example, stinkgrass emits a disagreeable odor when crushed.