Weed Seedling Identification

Most weed identification manuals feature mature weeds and use flower and fruit characteristics as an aid in identification. However, the grower of crops must control weeds when they are small, before they flower, to prevent them from seriously competing with crops for nutrients and soil moisture. Also, accurate identification of these seedling weeds often is necessary to select the best herbicide or other method of weed control.

New weed growth may originate either from seeds or from vegetative reproductive structures (rhizomes, rootstocks, stolons or runners, tubers, corms, or bulbs) of a perennial plant. True seedlings are those young plants that grow from seed and may include the annuals, which live for only one year, producing flowers and fruits that year; the biennials, which produce flowers and fruits the second year and then die; and the perennials, which usually produce flowers and fruits each year but continue to live for several years.

Weed seedlings also may be divided into grasses or grass-like plants and broadleaf plants.

GRASS WEEDS

Any or all of these vegetative characteristics may be useful to help identify a young grass weed:

• The grass weeds usually have long, narrow, alternate leaves with parallel venation (distribution or arrangement of veins), with an expanded leaf blade portion and a leaf sheath portion toward the base that encircles the stem (Figure 1).
  

  
  
  Figure 1. Vegetative grass parts



• The juncture of the leaf blade with the leaf sheath is called the collar area.



• Most grasses have a projection at the base of the leaf blade called a ligule, which may be either a membrane or a fringe of hairs or a combination of both.



• Some grasses also have claw-like or hook-like projections at the leaf collar called auricles that may partially encircle the stem.



• As grass leaves emerge from the bud shoot, they may be rolled (round) and overlapping or they may be flat and folded (V-like).



• Grasses have definite nodes (swollen ridges that encircle the stem) and internodes (portions of the stem area between nodes).



• Grass stems (culms) may be round or flattened, and leaf sheaths may be open and overlapping or they may be closed.



• Grasses may be smooth (glabrous) or hairy.



• Grasses are monocots, with one cotyledon or seed-leaf that remains in the soil after seed germination.



• Grasses are either annual, with a simple, fibrous root system, or perennial, producing rhizomes, rootstocks, or stolons.



• The seed of grasses often remains attached to the primary root after germination. If the grass seedling is carefully removed from the soil, the seed may help identify the plant.

BROADLEAF WEEDS

All of these characteristics help in identification of broadleaf weed seedlings:

• Broadleaf weed seedlings, in contrast to the grasses, usually have wider leaves with net-like venation.
  
  
• Broadleaves are dicots and have two cotyledons or seed-leaves, which usually emerge above the soil and expand to become the first visible "leaves." The true leaves then develop above the cotyledons (Figure 2). However, in some broadleaf species, the cotyledon (seed) remains in the soil and the plumule (growing point and cluster of undeveloped true leaves) emerges above the soil line.

Figure 2. Vegetative broadleaf plant parts

• The shape and size of the cotyledons and first true leaves vary considerably among species (figure 3).

Figure 3. Cotyledon and leaf shapes

• The stem below the cotyledons is called the hypocotyl and the stem above the cotyledon is the epicotyl.



• Leaves may be alternate or opposite in arrangement on the stem. In some cases the second leaf may appear so closely behind the first leaf that they appear to be opposite but later prove to be alternate.



• The true leaves of broadleaf weeds usually have a petiole (leaf stalk), but in some species the true leaves may be sessile (without a leaf petiole).



• Cotyledons are usually hairless but may be rough, while true leaves and plant stems may be hairy or smooth.



• Leaf petioles in the Buckwheat (Polygonaceae) plant family are encircled by a membranous sheath, called an ochrea.



• Broadleaf weed seedlings may have an erect stem, be viny or twining in growth habit, or may be prostrate (growing flat on the ground).

O. E. Strand, former Extension agronomist
G. R. Miller, Extension agronomist

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