ID Lab #1: Leersia virginica and Microstegium vimineum

Photo: A solitary Leersia virginica caryopsis. Note: Clicking on any photos in this blog post will take you to the flickr page on which they are hosted.

I’m often asked in the field, “How did you know what that was?” Since almost all of my (admittedly meager) botanical knowledge I’ve picked up on my own by some combination of observation, reading, and listening to those more knowledgable than myself, I often find myself unable to give a satisfying response to this simple question. Sometimes, it’s a number of small traits that, together, lead to a correct ID. For other species, noticing a single unique anatomical quirk might be all it takes.

With that in mind, I welcome you to my first ID Lab post. Every other week or so, I’m going to choose a plant or two, take some photos of it, and explain exactly how I came to an ID. I’ve set a few rules for myself. I will only use photos I’ve taken myself from samples I’ve found locally. As best I can, I will only use recent photos (no photos of summer flowers in the dead of winter!). And, since this is going to be a learning experience for me too, if I misidentify anything, I’ll do a follow up post detailing how I went wrong.

Katherine’s post about Japanese stiltgrass( Microstegium vimineum ) got me thinking. We’re so used to seeing Microstegium invasions, how many people have noticed our similar-looking native White cutgrass, Leersia virginica, often growing right up in dense stiltgrass carpets? Both are widespread, shade-tolerant, and thrive in moist forest floors, so location gives us little in the way of identification hints.

If you just want the down and dirty field ID, and don’t want to be bothered with the finer details, I’ll give it to you upfront: Microstegium vimineum has a silvery sheen along the midrib of their leaves that Leersia virginica lacks. Tilt the leaf back and forth to catch the light and check for the silver stripe and you’re all done. This doesn’t photograph well (especially since my samples dried out a bit) so you’ll just have to take my word for it.

Now that we’ve lost the philistines who don’t appreciate the subtler points of plant ID, we can take a look at some cool photos and really make some comparisons!

We’ll start from the ground up, so that means roots first. Stiltgrass earns its name with elevated rootlets between the culm (fancy word for stem) or stolon (similar to a rhizome, but above ground and primarily for vegetative growth or reproduction. Sometimes called a “runner”) and the actual roots of the plant. You can see this in the photo on the top left. The rootlets are thicker and smoother than than the roots. Compare this to the Leersia on the bottom left where the roots form immediately without an intermediate structure.

Microstegium, as an annual grass, will have a far less robust root system. This means you can pull it up with hardly any resistance. Leersia, however, is a perennial, and while it has a delicate root system compared to other perennial grasses, it is still much more firmly rooted than Microstegium.

If you try and pull up some stiltgrass, you’ll notice that a great many stems are connected by stolons that run parallel to the ground. You can see them in the photo to the right. This is the primary way that Microstegium spreads over the course of a growing season. Leersia does form rhizomes, but it generally forms smaller, more well-defined clumps than the “rolling carpet” effect of stiltgrass.

As we work our way up from the roots along the culms, we will come to nodes. Note that this is generally adjacent to where a blade attaches to the culm. We have the node from Microstegium on the top left and Leersia on the bottom left. The apparent difference in size is due to photo editing. Nor is the color particularly diagnostic. While the Microstegium appears red here, it is just as likely to be green or even yellow this late in the season. The important characteristics here are the total lack of hairs on stiltgrass (“glabrous”) whereas Leersia has hairs that point downwards. This should be visible even with the naked eye.

If you have a sample in hand, you can also gently rub your fingers along the culm. Microstegium will be smooth and waxy. Leersia will often be slightly scabrous (“sandpaper-y”), but the fine hairs that give it this texture wear away quickly, making this a less practical method of ID.

Since we’ve admired the node, we can now look at the blades. Again, the simplest and quickest field ID lies in seeing the silvery midrib on Microstegium (see first photo below), but there are other differences here too. While Leersia (second photo below) gives the impression of longer blades, as you can see below, the blades of either plant are roughly the same length. Leersia does, however, have a narrower blade (in this case, the sample is drying and the curling at the edges amplifying this impression) that tapers more towards the end.

Finally, we get to the inflorescence! In general for grass (and indeed most graminoids and forbs) this is going to be what you look at first and might be all you need to look at. But this is an exercise primarily in the close and careful observation of plants, so we have waited until the very end. On the top right is the Microstegium and on the bottom right, the Leersia. (Let’s ignore the obvious lack of seed on the Leersia. The Microstegium I collected appeared to have been mowed early in the summer, so this is later growth, hence why it looks so robust in late October.) The general impression is immediately that the stiltgrass is much more stout and robust than the more slender and delicate White cutgrass. This bears out in the field too, where Leersia often nods the entire inflorescence to the ground, hiding it completely out of view unless you pull it into sight.

Looking even closer, we can see differences in the arrangement of the caryopses (grass grains/seeds). I’m already breaking my own rules, and using a photo I took last month at Huntley Meadows in the place of the Leersia I collected. In the top left photo, notice that the seed for the stiltgrass is arranged tightly around the rachilla so tightly you can barely see it. The seeds also begin to form almost immediately upon branching off from the main stem. The Leersia (bottom left), however, holds all its spikelets in a tight plane on one side of the rachis, which is bare of seed for about the first third of its length.

Members of VNPS’ Grass Bunch would probably have a number of more technical things to add here (and, if you guys are reading this, please do in the comments below!) but, I think this about covers it for my first attempt. I hope that now you feel properly equipped to yank out Japanese Stiltgrass to your heart’s content without worrying about pulling out any Leersia virginica.

Please feel free to leave any suggestions for next week’s ID Lab in the comments or on Facebook. I’m inclined to look at some hickories, barring any other fun suggestions.