Weed Seedling Identification
O. E. Strand and G. R. Miller
2002 Regents of the University of
Minnesota. All rights reserved.
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
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
- 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.
All of these characteristics help in identification of broadleaf weed
- 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).
. 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).
Some common grass weed seedlings with their identifying vegetative characteristics.
Some common broadleaf weed seedlings with their identifying vegetative characteristics.
O. E. Strand, former Extension agronomist
G. R. Miller, Extension agronomist
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