Plant morphology
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
Leaf Shape
Leaves come in many shapes, here are a few:
heart-shaped 
lobe-shaped 
spade-shaped 
arrow-shaped 
star-shaped 
pentagon-shaped 
oval 
ovate 
elliptical 
lanceolate 
oblong 
spoon-shaped 
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 margins
Leaf margin morphology can be helpful in ID.
Left: entire; left center: serrate; right center: dentate; right:
crenate
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 
Leaf venation
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 
STEMS
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):
alternate 
opposite 
Ochrea
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: 
Root systems
The root system of a weed can be very helpful in ID. Below are some examples:
fibrous root system 
taproot 
dandelion taproot 
rhizome 
