How to identify grasses

There are around 12,000 species of grass, and with such similar features that they can be very difficult for a lay person to identify. I am only beginning to learn how to go about this, so I put together some of the clues I found about how to do it, and hopefully this will help other amateurs like me.

Even if you have no chance of identifying the plant yourself, it's good to know which features an expert will need to see in order to help you with an accurate ID.

First, you will need a photo of the overall form of the plant. It helps to see if it is clumping or spreading, upright or weeping.

Next, take a closeup photo of the sheath (where the blade of grass joins to the stem), and the front and back of the blade itself. The details of this part are really important, so try to get as clear a photo as you can, as close as you can. Something like this would be a good start:

Third, you will be much more likely to get an ID if you can get a picture of the flowers or seeds. Get an overview of the flower head that shows whether it is drooping or upright, and how many heads are on one stalk, etc. Then try to zoom in as close as you can on an individual seed. Get photos from all angles, and hold it against your hand if your camera is having trouble focussing.

Finally, if you are able to uncover a little bit of the root system and get a photo of that, whether there are runners or rhizomes can make a big difference to an ID.

Here are some of the vegetative features that you might need to make an ID:

And if you want to hear it from an expert, @stewartwechsler gives a thorough description of what he is looking for from photos of grasses on this thread https://forum.inaturalist.org/t/how-to-identify-grasses/8538/28, quoted below:

"Do your best to find any flower heads, new, or old, and take pictures of the whole plant in flower / seed, then the whole flower head, then detailed photos of the spikelets (the units with seeds / flowers compressed together, and, if you can you might add the detail of the individual seeds (“lemmas”) and the “glumes”, the pair of membranes under the “lemmas” (the lemmas are technically the membranous outer covering of the seeds or flowers. These may or may not have a hair tip or “awn” at the end, that we will want to see, if it has them)

"Then, especially if you can’t find any flower heads, I’d want details of the leaves, that include a sheath around the shoot, which is equivalent to the petiole of a broad-leafed plant, then any membranes or hairs where the blade comes away from that sheath. There is usually a membrane. or just some hairs, called the “ligule”, that come up from inside the junction of the sheath and the blade. There may be further finger-like membranes coming away from the base of that junction, that may wrap around the shoot to some degree, called the “auricles”.

"I would then want to see the base of the sheaths and shoots, where they come up from the ground, and how they come up from the ground. Some will come up straight from the bottom, some will start horizontal, then curve up. I would then want to be able to see the character of any hairs at the bases of the sheaths, if they are hairy, or the color and character of any grooves or stripes at the base. Some species retain old dead leaves at that base more than others. Some species will retain the veins at the bottom of the sheath on older dead leaves leaving a veiny netting at the bottom, that other species lack. I would then want to see if that sheath can be unrolled, and opened from top to bottom, without tearing the sheath. You may not need to open it, but may be able to see that there is one loose end going to the bottom, rather than a sheath that is united into a tube. Some species may then have a united tube sheath until some place before the top, after which it is open. I then want to see how far down it is open, before it becomes united.

"I then want to see the top and tip of a blade of a leaf. A view of the bottom side could help too. Some leaves are folded, and have a clear fold going down the center of the blade. These will often have an end with the two halves united at the bottom, like the front of a canoe (or “prow”), some grasses don’t have that fold, then wouldn’t have that “prow”. Some grasses have prominent veins, some don’t. I would want to see the blades well enough to see both any center vein, and the texture of the blade that may have the blade look more textured. I want to see all parts well enough to see how hairy they are, if they have any hairs, from the bases of the sheaths, to the ligules, and possible auricles, at the top of the sheath, then the top side of the blade and bottom side. Some grasses might have darker spots on the blades (I think this is a fungus), noteworthy among them is Schedonarus / Festuca arundinace-us /-a - Tall Fescue, not usually planted intentionally in lawns, but often showing up in them. If you aren’t tuned in to looking for this spots, you are likely to miss them.

"Then you may want to gently dig around the bases and see if you can find any rhizomes or stolons. Rhizomes are roots spreading horizontally under the surface from which new shoots come up periodically. Stolons are almost the same, but are runners spreading just above the surface, as strawberries spread by above ground runners."

Posted on June 14, 2020 11:20 AM by row_nature