Below is a schematic line drawing of a broadleaf plant. Notice the alternate leaves on top, while the second (middle) set of leaves are opposite one another.
This is another schematic line drawing. The top set of leaves are both lobed (left) and ovate (right) as well as alternate to each other. The second set of leaves are opposite and are the seed leaves: cotyledons.
Keep in mind no real plant species would have this combination of leaf to stem arrangement, either it will be alternate or opposite or some other single arrangement.
Leaves come in many shapes, here are a few:
Cotyledon leaf shape
Cotyledons are the seed leaves. As a broadleaf weed species seed germinates, the seed leaves expand and start giving farmers problems. This is the hardest time in the life cycle (except maybe fishing seeds out of the soil seed bank) to ID a weed. Here are some generic cotyledon leaf shapes:
linear oblong lanceolate
ovate spatulate oval
round kidney butterfly
Leaf margin morphology can be helpful in ID.
Left: entire; left center: serrate; right center: dentate; right:
Left: sinuate; left center: sinuate & distantly toothed; right center: toothed; right: lobed
The parallelism of leaves can be helpful, especially with the bindweeds:
parallel leaf margins non-parallel leaf margins
Sometimes the veins in the leaf can help in weed ID. They are usually rounded and protrude above the leaf surface.
palmate venation 3 prominent veins
pinnate venation or
Several interesting and cool features about stems and branches can help you ID broadleaf weeds. The first is the arrangement of branches relative to the main axis. The branches can be opposite (below, lower set of leaves) one another or they can alternate (below, top set of leaves):
Plants in the Smartweed, Polygoneaceae, family have a distinctive membranous sheath that surrounds the branch-main axis junction, the ochrea (below).
Below are two examples of the ochrea: Pennsylvania smartweed (left, no hairs on ochrea) and ladysthumb (right, hairs on ochrea).
Pennsylvania smartweed: ladysthumb:
The root system of a weed can be very helpful in ID. Below are some examples:
fibrous root system
taproot dandelion taproot