Wild Grasses of Arizona
by Dave Thayer
If you are using a cell phone, this site shows better if you turn your cell on its
The internet is full of information about grasses. You can learn the fact that grasses feed the world (these are
all grasses: oats, barley, corn, rice, wheat), the fact that certain invasive grasses can become weeds and crowd out native plants,
that grasses don't have showy flowers because they are wind-pollinated, and much more. But how to identify them if you are a
land-owner or naturalist?
This site is designed as a simple way to identify grasses. My goal is to make it easy by focusing on how a grass looks.
Understanding most published grass descriptions requires a whole new vocabulary. I do recommend learning some new terms, but I will
try to minimize this chore by using drawings and photos and by explaining in plain English. I will show you, mainly through pictures,
how to identify many of Arizona’s approximately 460 grass species. Here is a secret: experts don't have to measure the width in
millimeters of a lemma or glume to identify a grass. They do it by looks.
All the drawings and all the photos are mine. All are free for you to use in any way you choose.
We will start with the easiest grasses and the most common. Incidentally, most of Arizona's grasses occur throughout America
and occur on many continents. Some are considered invasive pests, but frankly I am not inclined to disparage any of them...except
maybe Bufflegrass (Cenchrus ciliaris) which is taking over the Sonoran Desert.
Since the difficulty of grass identification is notorious, just recognizing 40 or so species should be sufficient
to amaze your naturalist friends. You might get hooked, as I have. In studying butterflies, I read that the small skipper
butterflies are fun to learn in part because they are such a challenge. I agree, and I hope your interest will take
you to a level that challenges you.
In the field you will need a good hand lens magnifier to start, and probably a camera, a field notebook, and
some long narrow bags for carrying grasses you collect. Subway sandwich bags or newspaper bags work well. A useful item I use
is a set of plastic ties to wrap bunches of grass and write the date and location: I use Huouo Nylon Cable Ties Self-Locking.
It will help also to have handy a tool for digging up grass plants--a rock hammer works well in the hard Arizona soils. You
should collect the roots along with the leaves, stems, and especially the seedheads. Look for stolons, rhizomes,
and other signs of perennial versus annual grass. Record this information while you are in the field--otherwise it is too late.
Later in your
studies, a binocular microscope will help immensely—prices of these have dropped greatly in recent years, probably because of high
demand. Mine weighs over 60 pounds but cost under $500: AmScope SM-4B Professional Binocular Stereo Zoom Microscope,
WH10x Eyepieces, 7X-45X Magnification, 0.7X-4.5X Zoom Objective, Double-Arm Boom Stand. I wish I had gotten a camera
mount with it.
Following are some drawings of grass parts. You must learn many of these in order to understand published and online
grass descriptions. My website is a work in progress, with new grasses added as I have time. So let’s get started. You
can go to the grasses key,
jump to the list of grasses, or start by learning a few terms below.
The Parts of a Grass Plant
- The spikelet is the unit of grass identification.
A single plant will have many spikelets attached to the seed head. Each spikelet has two tiny leaf-like
glumes at bottom and one to many
florets above the glumes.
Florets are held to
by an axis called the rachilla. Each complete floret consists of
two tiny leaf-like structures called the
lemma (lower) and
palea (upper). These enclose the reproductive parts as shown next.
Many glumes and lemmas have needle-like projections called awns.
- After the floret develops, the lemma and palea usually open a bit and the beautiful female
stigmas protrude. Pollen is ejected from the
anthers and blown in the
wind in a process called anthesis. The pollen sticks to the stigmas, fertilizes
the ovary, and a seed called a
caryopsis grows. In the
previous drawing, the florets are closed.
Be aware that all these parts can be very tiny, usually needing a hand lens to see.
- The main grass stem below the seedhead is called the
culm. Each grass leaf has two main parts, the
blade and the
The sheath is wrapped around the culm, so it looks like part of the stem, while the
grass blade extends from the culm and is the thing that makes a lawn. Each sheath extends
down to the next lower
node or stem segment. Sometimes, at the junction of blade and sheath, there
might be little ear-like structures called auricles.
ligule is often used as an identification feature. It is a thin structure
attached to the leaf at the junction of sheath and blade. The ligule may be
membranous (usually white and very thin) or
ciliate (hairy). This one is
membranous at its base (white) and ciliate at the top (little
hairs). Use a hand lens.
- In addition to its roots, a perennial grass plant may have aerial
root-like stems called
stolons which look like above-ground roots but are actually stems.
These creep along the ground, at intervals making true roots and starting a new grass plant. Similar structures called
rhizomes grow underground in shallow horizontal paths leading to new plants to
The Seedhead: Spike, Panicle, or Spike-like Panicle
At the top of a grass stem, or culm, is the
seed head or
inflorescence. All grass
inflorescences (except bamboo) are annual, growing anew each year even
in a perennial plant. When the inflorescence is narrow like those shown
here, and each spikelet grows directly on the stem, the inflorescence is called a
spike. Some grass seed heads
have branches; therefore, they are not a spike.
But when these branches grow appressed to the stem, and thus appear like a spike, they are termed condensed panicles
and are called "spike-like."
Note that all my photos with graph paper are ruled in quarter inches.
Panicles: Open Inflorescences
Seed heads like these three above take the form of spreading
panicles. Panicles take many shapes. Grasses will show panicles only if there are
branches that hold the spikelets. However, some grasses may appear
spike-like at first, when the grass is young, but they are not spikes
because they have branches. Then later, especially during anthesis when they eject pollen, these condensed
spike-like panicles open up and become more panicle-like. After anthesis, some grasses may return to a spike-like form,
with panicle branches appressed to the rachis. But even if they look like spikes they are panicles if they