Grasses are tough

Grasses are tough, not only do they grow just about everywhere, they can be very difficult to identify.  Unlike so many flowering forbs, grasses tend to all look about the same to the casual observer.  This situation is reinforced by our tendency to mow grasses down on a regular basis so that all we see are a collection of short green leaves.  In the US there are 169 genera and 1,398 species of grasses.  In the Northeast there are roughly 600 species growing in the wild.  Over the past couple of years I’ve making an effort to identify grasses growing on and around our property here in central Maryland.   

The first level of grass identification is to determine whether you are looking at a true grass (Poaceae) rather than a sedge (Cyperaceae) or rush (Juncacaeae).  You can make a quick distinction using the rhyme:  Sedges have edges, rushes are round, and grasses have knees (joints) and bend to the ground.  Sedges have triangular stems; when observed end-on the leaves will project out 120 deg apart.  The stems of rushes are round.  The stems of grasses have joints or knobs where each leaf blade originates.  In addition grass stems are hollow while those of sedges and rushes are solid.

While there are a few grasses that can be identified with some reliability on sight, most require close examination of the flowers and leaf surfaces with a hand lens to determine the grass down to the species.  Some easily identifiable species include the very popular little bluestem, Schizachyrium scoparium, and invasive species like Japanese stiltgrass, Microstegium vimineum, and Johnson grass, Sorghum halpense.  But for the most part you need to do a close examination to really determine a grass down to the species level.  There are a number of dichotomous keys for grass identification down to the species level.  Some rely heavily on flower (spikelet) characteristics, while others include other characteristics.  These keys ask a sequence of yes-no questions about the plants to slowly lead you to a result that matches the characteristics of the plant in question. 

Little bluestem is particularly recognizable in fall when the stems turn bronze
and the fuzzy seeds catch the light.  In summer the stems have a bluish green cast.

Johnson grass is a non-native weedy species that is often seen
 on the edges of fields and drainage areas.  It is tall 3-7'
and kind of looks like skinny corn stalks.  

Japanese stiltgrass in an invasive grass species, particularly in the
eastern United States.  It can be distinguished by the silvery midrib
on the elliptical leaves and its lax habit with many joints along the stems.

The Native Plant Trust (formerly the New England Wildflower Society) has developed an on-line key called GoBotany that allows one to enter a variety of plant characteristics simultaneously and provide a best fit.  This is particularly useful if you are trying to do an identification at a time when no blooms are available.  The plants in this database include grasses, forbs and woody species, but are limited to those found in the New England states. 

 To do a grass identification you’ll need an identification key and a hand lens (10x).  A camera is also useful to document your observations.  Since I made a commitment to ID some of my grasses I’ve been gathering a few different resources.  The Manual of Vascular Plants of Northeastern United States and Adjacent Canada, by Gleason and Conquist is a very technical key (in small print) that, while challenging to use, does provide detailed descriptions of each species. Grasses an Identification Guide by Lauren Brown is easier to use and well illustrated.  It also includes some common sedges and rushes in addition to grass species.  Its drawback is that it describes only the more common grasses.  Both of these references contain species found in the northeastern quarter of the US.

One resource I haven’t used much (yet) is Manual of the Grasses of the United States by A. S. Hitchcock.  This two volume set describes all of the grasses found growing in the US.  It does include some illustrations which makes it somewhat friendlier than Gleason and Conquist.  I’ve been intimidated by the size of this collection (especially large considering this includes only grasses, no rushes or sedges).

Here in the Mid-Atlantic region an easier to use key is the Field Guide to Grasses of the Mid-Atlantic by Sarah Chamberlain.  This guide includes botanical sketches of plants, and plant parts, to aid in the identification and includes only true grasses that are found in the Mid-Atlantic States (VA to PA).  So you don’t wade through extra species that are not likely to be present.  It includes many more grass species than Lauren Brown’s book, though it lacks the sedges and rushes. 

An additional book that I have found critically useful in all plant identifications is Plant Identification Terminology by Harris and Harris.  It provides illustrations and definitions of all the plant parts that are described in botanical Latin terms in the keys. 

Going through a dichotomous key can be time consuming, especially early on as you are wrestling with botanical Latin.  A drawback of many keys is that it is difficult to go backwards when you get something wrong, i.e., you get to the end of the key and your sample is nothing like the choices in the key.  Some of the more formal keys are easier to back up on when each step is numbered.  I’ve found with some of the book formats that they do not number the steps (choices) so that going backwards involves leafing through a lot of pages.  Perhaps putting some sort of bookmarks, like post-its, in the book would make it easier to retrace my steps. 

When I do go through a key and end up without a match I will go back to a point where I was not absolutely certain about my choice and then run down that new path.  Sometimes you just don’t have a good example of the feature that they are asking about; like size or shape of a seed, the number of veins on a lemma, or a surface is slightly or very hairy. 

Here you can see the individual florets of switchgrass.  This species has
only one floret per spikelet. Highlighted here are the glumes,
bracts that enclose the floret(s).  One of the florets is in full bloom
 with the anthers and stigma projecting out of the floret.

After getting frustrated about not getting satisfactory results I found it helpful to practice on some known species.  That way you can get some confidence with identifying botanical structures.  I had examples of switchgrass, Panicum virgatum, and purple top, Tridens flavus, that I could practice on.  After that I worked on an unknown grass that was a little past bloom, the flowers were a little tattered but mostly recognizable.  I found that one of my problems is counting florets in dried flowers.  Sometimes they just look like a bunch of chaff.  With a little practice I’m getting better at recognizing the flower parts.  I was able to ID the unknown as the relatively common, though non-native, smooth brome, Bromus intermis.

Here's the flower stalk for purple top.  The spikelets, which contain
4-9 florets, are deep purple in color.  Hence the common name.

One thing that surprised me in keying out grasses is that plants that are close together in a key may not look that much alike when viewed at a distance.  That is because many keys focus in on differences that are not easily seen rather than on obvious features like height or size of leaf blades.  While not part of some keys, knowing things like bloom season or habitat can be very useful in confirming an identification. 

In addition to running through keys, working with people who are familiar with grasses can be of great help.  For many, once you know and recognize a plant you don’t need to run through a key to identify it, they just know.  In this time of COVID-19 it is even more difficult to find and work closely with a grass expert.  I did recently find a Facebook group focused on identifying grasses called Grasses,Sedges and Rushes of the Northeastern USA.  They have been helpful in pointing me in the right direction when I have been adrift. 

The American mannagrass grows vigorously in early spring
 and is in full bloom by the end of June.

This was the case when I thought I had ID’ed a grass as drooping woodreed, Cinna latifolia.  This was thrown in doubt when I saw that the grass was going dormant in mid-summer while Cinna normally blooms in mid-summer.  Based on a lead I got from the Facebook group, I redid the ID using a different key and came up with American mannagrass, Glyceria grandis.  On reviewing the keys it appeared that I had totally messed up characterizing the flower (spikelet) parts by overlooking some of the less developed florets.  On a side note, it was interesting to see that areas that were thick with mannagrass had very little Japanese stiltgrass filling in in late summer.