How Weeds are Classified
There are many ways in which weeds can be classified. Two of the most common ways are by gross morphological features, and by their lifecycle. When classified by their gross morphological features, weeds are broken into three major categories: grasses, sedges, and broadleaf weeds. When classified by their life cycle, they are broken into annual, biennial, and perennial. The latter classification method can have a profound impact on the effectiveness of control measures.
Classifying Weeds by Gross Morphological Features
Weeds considered true grasses are monocotyledons having only one seed-leaf when seedlings emerge from the soil. They have leaves that are long and narrow with a parallel venation pattern. Leaves arise in an alternate pattern on each side of a hollow stem or culm. Each leaf has two parts: the lower portion called the sheath, which is wrapped around the culm, and the upper portion called the blade. Most have fibrous root systems. Some examples of weeds that are grasses include crabgrass, sandbur, and dallisgrass.
Weeds considered sedges are also monocotyledons, but not true grasses, though they exhibit a lot of the same characteristics as grasses. They are different from grasses in that their stems are solid, triangular, and have no nodes. Their leaves have a three-ranked arrangement (instead of an alternate arrangement) with each leaf one third the way around the stem from the one below it. The basal portion of each leaf forms a tube around the stem with no clear division between sheath and blade. Examples of weeds that are sedges are nutsedge and green kyllinga.
Broadleaf weeds are different from grasses and sedges in that their leaf blades are expanded. Their leaves have netted venation as they are dicotyledons. Their stems branch as the plant grows, and they often have flowers. When broadleaf weed seedlings emerge from the soil, they have two seed-leaves. They often have tap roots or coarsely branched roots. Because of the diversity of dicots, it is important to first identify broadleaf weeds accurately before developing a management plan. Some common broadleaf weeds include dandelion, clover, burdock, and thistle.
Classifying Weeds by Their Life Cycle
Annual weeds complete their life cycle in one year or less. During that time, they germinate, complete their growth cycle, flower, produce seeds, and dia. Some develop prostrate stems or adventitious roots. If the stems are cut, they may develop into new plants.
Annual weeds can be further divided based on the time of year their life cycle begins and ends. Summer annual weeds or warm-season annual weeds like crabgrass germinate in spring and die in fall when weather turns starts to turn cold. The seeds they produce during the summer growing season lay dormant in the soil during winter months and germinate the following spring. Winter annual weeds or cool-season annual weeds like henbit, annual bluegrass, and common chickweed germinate and develop in fall. They overwinter and mature the following spring. Then they flower, seed, and die during the summer.
Biennial weeds have life cycles that span two years (i.e. two growing seasons). During the first year, biennials germinate, and the plant focuses on growth. The plant overwinters, and then during the second year or growing season, it flowers, produces seeds, and dies.
Perennial weeds have life cycles that span more than two years. They reproduce from seeds or vegetative parts of the plant like rhizomes, bulbs, tubers, and stolons. Like annuals, perennial weeds can be classified into cool-season perennial weeds and warm-season perennials based on the time of year when they grow.