Throughout this section, and throughout the resources on this website, we use specific terms to quickly describe a weed’s life cycle, growth form, leaf arrangement and margination, root structure, and flower structure, all of which can help you determine the key traits that set the weed apart from other plants. Below we give you definitions and examples of key words frequently used in plant identification. All technical terms used throughout this website are also defined on the glossary page, or you can hover your cursor over select words to see definitions immediately.
Annual species complete their entire life cycle from germination to the production of seed within one year and then die. Summer annuals germinate during spring/early summer and mature by fall of the same year. Winter annuals germinate during fall and mature during the spring or summer of the following year. Biennial species take two years to complete their life cycle. In the first year, the plant is a rosette. During the next spring or summer, the plant bolts, sets seeds, and dies. Perennial species live for more than two years.
Plants typically grow as annuals, like yellow starthistle; biennials like houndstongue; or perennials like Brazilian peppertree.
The first recognizable stage of many plant life cycles is a seedling, when a plant has one to a few small leaves. Many perennial and biennial plants then grow into rosettes, which are clusters of basal leaves typically of the same height and length. Annual plants and twining vine species frequently do not have an obvious rosette stage. Many plants then shoot up one or a few flowering stems in a stage called bolting. In the bud stage, immature, closed flowers appear on flowering stems and branches. These open during flowering and then set seed upon maturation. At senescence, a plant has typically released its seeds and dies back for the winter or permanently.
Plants develop through different stages in their life cycle. Many plants, such as Canada thistle, begin as seedlings before forming rosettes, bolting, producing buds, flowering, and then senescing.
Although there is often some overlap, many plant species can be categorized by their overall habit or growth form. Forbs are herbaceous plants, meaning they don’t have woody tissue at or above the ground surface. Grasses are herbaceous plants with jointed and hollow stems and sheathed leaves. Shrubs are woody perennial plants typically growing less than 13 feet tall and often with multiple stems arising from or near the ground. Subshrubs are similar, but usually less than 2 feet tall. Trees are woody perennial plants typically growing taller than 13 feet and often with a single stem (trunk) arising from the ground. Vines are twining or climbing plants with long stems; they can be woody or herbaceous. Terrestrial plants live completely on land. Aquatic plants live in water and can grow fully submersed, emergent above the water surface, be anchored in the hydrosoil, or be free-floating on the water surface.
Although not always clearly delineated, many plants can be categorized according to their general growth form, including forbs, grasses, shrubs, subshrubs, trees, and vines.
Plants can also be categorized by being terrestrial or aquatic. Aquatic plants can grow fully submersed, emergent above the water surface, be anchored in the hydrosoil, or be free floating on the water surface.
Nodes are parts of a plant stem from which a leaf, branch, or root grows. Plant stems have axillary buds at their leaf nodes. Axillary buds are embryonic shoots located in the axil of each leaf. Each bud has the potential to form shoots. Once formed, a bud may remain dormant for some time, or it may form a shoot immediately. Some leaves are called compound in that they are further divided into smaller leaflets. You can tell the difference between a compound leaf’s leaflet and a simple leaf by checking where the leaf attaches to the stem. If there is no axillary bud, then it’s a leaflet and not a leaf.
Plants produce axillary buds at their leaf nodes that can form new leaves or shoots. Compound leaves are further divided into smaller leaflets. A leaflet differs from a leaf in that leaflets do not have axillary buds where they attach to the stem.
The way true leaves are arranged along the stem is an important diagnostic feature. Alternate leaves appear singly at stem nodes, on alternate sides of the stem. Opposite leaves appear in twos at stem nodes, on opposite sides of the stem. Whorled leaves are when three or more leaves radiate outward from a single stem node.
Most leaves have margins that are smooth, lobed, or toothed. Smooth margins are fairly straight and smooth along their entire length. Lobed margins have shallow or deep rounded segments. Toothed margins (also called serrated) are saw-like with teeth on the edge that may be different in size. Some margins are doubly toothed where the individual teeth each contain smaller teeth.
Many plants have either taproots or fibrous root systems. Taproots are large, central, and dominant roots from which other roots sprout laterally. Typical taproots are somewhat straight and very thick, tapering in shape, and grow directly downward. Fibrous roots do not have a primary root like the taproot. They grow downward and outward, with repeating branches to form a mass of small roots. Some plant root systems also have rhizomes or stolons which shoot up new stems, allowing the plant to spread vegetatively. Rhizomes are horizontal underground stems that resemble roots and send out new roots and shoots. Stolons (also called runners) are stems which grow at the soil or water surface or just belowground that form new roots at nodes and new plants from buds.
A typical flower has four main parts: sepals, petals, stamens, and a pistil (see diagram). Sepals are the outer parts of a flower that protect the interior flower while it emerges. Sepals are typically green and leaf-like, because they are modified leaves, but they can be any color depending on the type of plant. Some sepals are even more showy than petals. Not all flowers have sepals, and in some cases, the sepals are modified into bracts that surround the flower. Often the function of petals is to attract pollinators, so many petals are brightly colored, showy, and of interesting patterns and sizes. Not all flowers have obvious petals. Stamens are the male reproductive organs of a flower. Each stamen contains two main parts. The filament is a long cylindrical stand on which sits the anther, a sac that contains pollen. The pistil is the female reproductive organ of a flower. Each pistil is often shaped like bowling pin. At the base of the pistil is the ovary, which produces and contains developing seeds. Several flowers are often grouped together in an inflorescence.
Many of the weeds in North America are members of the sunflower family (Asteraceae). Members of this family produce flower heads that are an aggregation of many individual flowers. These flowers, called florets, are clustered together and attached to a receptacle. There are two types of florets: disc and ray. Some species produce only one type, while others produce both. The receptacle and florets are enclosed by modified leaves called bracts. The type, color, and shape of florets and bracts can help in weed species identification. Each floret produces one seed from mid- to late summer. Some species produce seeds with a tuft of fine hairs on one end, similar to those on seeds of dandelion.
Though grasses share a lot of features with other flowering plants, they have several unique traits that can greatly aid in their identification. Grass inflorescences are seed heads composed of spikelets. Each spikelet consists of two glumes and one or more florets. Each floret contains reproductive parts surrounded by a lemma and palea. The glumes, lemma and palea are modified leaves that are roughly equivalent to the sepals and petals of more conventional flowers. In some grass species, one or more awns extend from the tip of the lemma. These can be long or short, straight or curved, or absent. The way grass leaf blades attach to the stem can also be diagnostic. A leaf sheath is the part of the leaf which encircles the grass stem, before it opens out into the leaf blade. At the junction of leaf blade and leaf sheath, there is sometimes a small extension called a ligule. These vary from absent to several millimeters in length and are often membranous or consisting of a short ring of hairs. Auricles are small outgrowths just below the ligule that clasp the plant stem. They are often finger-like, hairy, or absent.
