By: Paul A. Baumann
Weed identification is necessary to the success of any weed control program. Frequently, simple plant keys or “picture book” identification guides are used to identify weeds. However, some plant taxonomy keys can be quite complex and require an advanced understanding of plant structure and taxonomic terminology. While more difficult W to use, a detailed key or guide allows for the precise identification of weeds. This handbook is intended for use as a companion piece to better understand the plant part terminology used in less complicated identification guides.
Weeds can be classified into one of three different categories: broadleaves, grasses or sedges. Broadleaf weeds often are referred to as dicots; the seedlings produce two cotyledons (seed leaves) that are evident as the plant emerges through the soil surface. Broadleaf plants generally have wider leaves than grass plants and the stems are often branched. Leaf veination is netlike or has a branched appearance.
Grasses and sedges are monocots; their seedlings produce only one cotyledon (sometimes referred to as the coleoptile in grasses). Grasses have bladed leaves that are parallel veined and this veination extends onto the sheath. In cross section, the stems in grasses are usually round or somewhat flattened. The stems of sedges, in cross-section, are usually triangular.
A second, relatively simple method of weed classification is by the categories of annuals, biennials or perennials. Annual plants live for one growing season and are often referred to as either summer annuals or winter annuals. Summer annuals germinate from seed in the spring, produce vegetative growth, set flower and produce fruit mid- to late summer, and die back in the fall. Winter annuals germinate in the late summer to early fall, produce vegetative growth, flower and produce seed in mid- to late spring, and die off in the summer.
Biennials are plants that live for two years; seeds germinate in the spring, summer or fall of the first year and the plants overwinter usually as a rosette of leaves. During winter, the shoot tips are exposed to cold temperatures for a period of time. This vernalization promotes normal development of flowering and seed production during the summer of the second year. The plants then die in the fall.
Perennials are plants that produce vegetative structures that allow them to live for three or more years. These plants are classified as simple or creeping perennials. Simple perennials spread primarily by seed; they cannot spread vegetatively from underground roots. Creeping perennials, on the other hand, can reproduce by creeping roots and also through seed production. The creeping roots found above ground are called stolons, the below-ground roots are called rhizomes. Rhizomes are most common in perennial grasses. In broadleafs or sedges, these underground root structures are referred to as tubers.
The first step in identifying grass weeds is to become familiar with some key grass parts: the leaf blade, the collar, the ligule and the sheath. The ligule may or may not be present on some grass seedlings. The sheath is an extension of the leaf blade that terminates at the node. The areas between the nodes are referred to as internodes. Some grass plants may have extensions of the sheath called auricles. A sedge, however, generally does not have a ligule or an auricle.
The cross-sections of grasses may be either round or oval (flattened), compared to a sedge, which exhibits a triangular shape.
The sheath of a grass surrounding the stem is generally one of three types: split, overlapping, or united.
The ligule on a grass plant is a structure on the inside of the collar. It can be either membranous or hairy, or it may not be present at all.
The ligule may exhibit either a smooth margin or a toothed margin.
Auricles, appendages found at the junction of the leaf blade and leaf sheath, may or may not be present.
Pubescence or leaf hair is often a key to identifying specific grasses and weeds. Pubescence can appear in several different areas on a grass plant:
- only on the sheath margin,
- over the entire outside of the sheath surface,
- on the leaf collar margin,
- as a ring-like appearance surrounding the whole collar,
- on the leaf blade margins or leaf edges, or
- only on the blade surface, with hair absent over the other parts of the plant.
Leaves or other plant surfaces without any pubescence whatsoever are often referred to as glabrous.
A representation of a broadleaf plant with a few key descriptive parts labeled is shown above.
Broadleaf weed cotyledons are a key identification feature. However, if cotyledons have fallen off the plant or are in poor condition, they may not be helpful in broadleaf weed identification. Various cotyledon shapes are illustrated above.
Using cotyledons with true-leaf identification features can help in the overall weed identification process. True leaves generally exhibit one of several leaf shapes similar to cotyledons.
Leaf margins (leaf blade edges) also are special features for identifying specific plants. Examples of entire, serrated, pinnately lobed and palmately lobed leaf margins are shown.
Pinnately lobed leaves can be distinguished from palmately lobed leaves by examining the veins on the leaves. In a pinnately lobed leaf, the veins will arise from several points along a central axis. A palmately lobed leaf has veins that arise at the same point at the base of the leaf.
Leaf arrangement on the plant can be either alternate, opposite or whorled.
Attachment to the stem is referred to in one of three ways:
- petiolate, where a petiole exists between the leaf, blade and the stem;
- sessile, where there is essentially no petiole and the leaf base is attached directly to the stem; or
- clasping, where the base of the leaf partially surrounds the stem.
Individual leaflets may be classified as either simple or compound. Within compound leaves, there exists either pinnately compound or palmately compound leaf characterizations.
Leaf veination is different for both grasses and broadleaf plants. Veination in grass plants is a parallel type. Broadleaf plants may exhibit either a palmate veination or pinnate veination.
Some broadleaf plants may have other peculiar characteristics such as an ocrea. This is a membranous tubular sheath that surrounds the stem at the base of the petiole.
Vegetative reproductive structures also may be used as key plant identification features. Rhizomes are basically stems that develop below ground. Plant shoots can arise from underground nodes on these stems.
Tubers are thickened underground stems that develop at the end of rhizomes. Stolons are roots that grow horizontally above the ground. Roots often develop at the nodes of stolons and peg down into the soil. New shoots also develop at these sites.
These depictions of plant parts can serve as a simple guide to understanding the terminology in simple field guides or plant keys used for weed identification. The use of more complicated keys for plant identification may require a thorough understanding of plant vegetative and flowering parts.
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