To identify weeds you must first distinguish between broadleaf weeds and grass weeds. Broadleaf weeds usually have wider leaf blades than grass weeds. Each leaf typically has a main vein that divides the leaf in half with a network of smaller veins (originating from the main vein) forming a netlike pattern.
Broadleaf weeds have distinct leaf shapes and surface characteristics that can be used in identification. The arrangement of leaves on stems can also be a useful identification aid. Leaves either are alternately arranged (when a leaf grows from a node on one side of the stem and another is produced on the opposite side further up the stem) or arranged opposite one another.
Some broadleaf weeds produce leaves in a circular pattern (rosette) from a central growing point located at or beneath the soil surface. Others grow and spread by means of creeping aboveground stems called stolons or belowground stems called rhizomes. Broadleaf weeds can produce a fibrous root system or a root system dominated by a large, fleshy taproot.
Broadleaf weeds often bear colorful flowers of different sizes and shapes. At certain times of the year flowers can be very useful identification aids.
Grass weeds have long, narrow leaves with veins running parallel to each other (they do not form a netlike pattern). Grasses do not have showy or colorful flowers, and leaf shapes are similar among species. The ability to identify grasses depends on recognizing growth habits, certain vegetative features, and seedheads.
Growth habits of grasses
Growth habits of grasses can be divided into three different categories: bunch-type, rhizomatous, and stoloniferous. In plants having a bunch-type growth habit, new stems are produced by tillering. A tiller is a stem that arises from a bud in the crown and grows vertically, remaining enclosed by the leaf sheath. Although all grasses produce tillers, only those that spread by tillering alone are referred to as bunch-type grasses.
In plants with the rhizomatous growth habit, lateral growth occurs by horizontal creeping underground stems called rhizomes. Rhizomes are produced from buds in the crown that break through the outer leaf sheath. Rhizomes produce nodes that can give rise to new tillers.
In plants having the stoloniferous growth habit, lateral growth occurs by horizontal creeping aboveground stems called stolons. Stolons are produced from buds in the crown that break through the outer leaf sheath. Stolons produce nodes that can give rise to new tillers. Stolons are usually green, whereas rhizomes are usually white.
Vegetative structures of grasses
Several vegetative features can be used to identify grass weeds. The most important are leaf blade characteristics and the structures associated with the collar.
Vernation is a term used to describe the arrangement of the youngest leaf in the bud shoot. Grasses with folded vernation have leaves that are folded in a V-shape. Leaves with rolled vernation are round with no folds. To determine if the grass has folded or rolled vernation, hold the plant between your thumb and index finger and roll it. If it rolls like a straw, it has rolled vernation, if it lies flat and has edges, it is folded. You can also determine vernation by cutting a cross-section of the stem just below the leaf blade.
Leaf blades of grasses can vary in width and hairiness. Some grasses have leaf blades that are dominated by a single prominent vein in the center of the leaf. Others have equal-sized veins running lengthwise over the entire width of the blade. Leaf blade tips may terminate in a sharp point, a blunt tip, or keeled shape (boat-shaped tip).
The collar region is located between the leaf blade and leaf sheath. It may or may not contain structures called ligules and auricles. A ligule is the membranous or hairy tissue located at the junction of the leaf blade and leaf sheath. Ligules can vary considerably in size and shape and may be membranous, a fringe of hairs, or absent.
Auricles are appendages that are considered an extension of the collar. They can be long or clawlike, small or rudimentary, or absent.
Seedheads may be useful in distinguishing among grass weeds. They can appear as open-panicle types, compact spikes, and divided spikes.