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 lacerate (raggedly torn looking). The ligule tip of Deschampsia danthonioides is acute or acuminate. When the plant matures and dries out, the ligule can break off and appear lacerate, but by that time you have flower characteristics.
The leaves of Ventenata dubia average more than 1 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 or less wide. The other difference is the hairs on the upper surface of Ventenata leaves. The Deschampsia leaf blades have scabers along the edges and a few scabers on both surfaces, but no hairs.
So, for vegetative characters, I recommend leaf blade width, hairs or no hairs on the top surface of the leaf, and ligule tip 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 equal in length and have a more slender silhouette. The glumes of Ventenata dubia have a hyaline margin (translucent), are unequal in length, and taper to a slender awn at the tip (shown by arrows). Also the veins are faint on the Deschampsia glumes and prominent on the Ventenata glumes. Glume characters are useful when looking at dry plant litter with empty glumes (lacking florets).