Ventenata dubia and Deschampsia danthonioides
About half of our grasses are not native and some of the introduced grasses are aggressively invasive. It is sometimes important to be able to distinguish the invasive grass from a native grass before flowering. In some cases that can be difficult.
I've been told that one can separate Ventenata dubia from Deschampsia danthonioides vegetatively by the dark nodes on the Ventenata. I assumed that would work until I encountered a little annual grass in a ditch with dark nodes and upon examining the spikelets found that it was Deschampsia.
nodes of Ventenata dubia:
nodes of Deschampsia danthonioides:
So, dark nodes can occur in both species. Apparently, the Ventenata nodes are always dark and the Deschampsia nodes are not always dark. How about the ligule?
Now, this character may seem a bit subtle to you, but if I describe it, I hope you can see the difference. The ligule tip of Ventenata dubia is acute or blunt, either of which quickly become lacerate (raggedly torn, split or shredded). The ligule tip of Deschampsia danthonioides is acuminate, although it is subject to being split or lacerate as well. When the plant matures and dries out, the ligule can break off and appear lacerate, but hopefully by that time you have flower characteristics.
The leaves of Ventenata dubia average more than 1 mm wide, often 2 mm wide. The one in the photo above is 1.5 mm where I flatted it. This measurement must be taken on unrolled (flat) leaf blades. Leaf blades of Deschampsia danthonioides are 1 mm wide (or narrower). The other difference is the hairs on the upper surface of Ventenata leaves. The Deschampsia leaf blades have scabers along the edges and scabers on both surfaces, but no hairs.
So, for vegetative characters, I recommend leaf blade width, hairs vs. scabers on the top surface of the leaf, and ligule shape.
There is definitely a difference in the branching pattern of the inflorescence.
Ventenata dubia spikelet with three florets
Deschampsia danthonioides spikelet, showing the two florets and a mature floret.
Comparing the spikelets, you can see that the glume tips are often purple-tinged in Deschampsia and the glumes are nearly equal in length and have a more slender silhouette. The glumes of Ventenata dubia have a hyaline margin (translucent), differing in length (one long one short), and taper to a slender awn-like tip. Also the veins are faint on the Deschampsia glumes have fewer veins that are fainter, while Ventenata glume veins are more prominent and numerous. Glume characters are useful when looking at dry plant litter with empty glumes (lacking florets).
I spent quite a lot of time in my recent workshop looking at these characters in the field with my students. The traits are all small and it is tedious to unroll the leaves to measure them while the mosquitos are biting. Someone asked, "why take all this trouble? Just come back in two weeks and it will be flowering." Well yes, there's that.