Weeds can be classified according to their gross morphological features, their life cycle and their habitat.
Grasses - Grass seedlings have only one seed-leaf (monocotyledons). Their leaves are generally long and narrow with parallel veins. Leaves arise alternately from the nodes, have two parts: a lower portion called the sheath, and an upper portion called the blade. The sheath is wrapped around the culm. At the junction of the blade and the sheath is usually a papery membrane called the ligule. Note that not all grasses have ligules. They have stems or culms that have nodes and internodes. Nodes are well-defined solid swellings at regular intervals on the stem, from which leaves arise. The internodes, portions of the culm between nodes, are usually hollow. Most grasses have fibrous root systems.
Sedges - Stems are usually solid and triangular. Have a three-ranked leaf arrangement. Each leaf arises one-third of the way around the stem from the one below. The basal portion of each leaf is fused to form a tube around the stem and there are no distinct sheath-blade divisions.
Broadleaved weeds - This group can be distinguished from grasses and sedges by the presence of expanded leaf blades. Venation of the leaves may be parallel as in monocots or netted as in dicots.
Annuals - Annual weeds complete their life cycle in one year or less. Most reproduce only by seeds. Some may produce adventitious roots on prostrate stems. If stems are cut or damaged, they may root and form new plants. Most common field weeds are annuals.
Biennials - Weeds that live more than one year but not more than two years.
Perennials - Perennial weeds are weeds that live more than two years. Perennials reproduce from seeds and vegetative parts of the plant, such as creeping roots, rhizomes, bulbs, tubers (below-ground root-like stems), or stolons (above-ground stems that produce roots).
Aquatic - Weeds that can emerge and grow in very wet or submerged soils.
Semi-aquatic - Dry land plants that have some tolerance to submergence.
Terrestrial - Plant that grow on dry land.