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.