California has three kinds of native needle grasses: Purple needlegrass, Foothill needlegrass, and Nodding needlegrass.
They all look pretty similar, even when fruiting, how can one tell them apart?
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The plants themselves are pretty similar and very hard to disambiguate. Foothill needlegrass tends to have thinner blades in denser bunches. Purple needlegrass tends to splay out a lot more, whereas nodding needlegrass typically stays in a bundle and droops in one direction. These are rules of thumb, however, and might not apply to all instances of a species.
The seeds can be disambiguated with more ease (click for fullscreen). Seeds should ideally be compared when ripe, that is, when you get some seeds in your hand by running your hand through the seed head. You can obtain unripe seeds by tugging on needles but the coloring and size might be off.
Foothill needlegrass seeds are the easiest to disambiguate. They're tiny and almost black. They have awns less than an inch in length.
Purple needlegrass and nodding needlegrass have very similar seeds with 4+ inch awns, but there are some key differences. Purple needlegrass seeds are slightly darker, and slightly fatter. They also have awns that are rectilinear -- they might be bent, but they're made up of straight lines.
Nodding needlegrass seeds are thinner, lighter, and have awns with curved, swooshy tips.
There's also a difference in coloring pattern where the seed connects to the awn: purple needlegrass has a greenish-white band that's quite clearly visible, whereas nodding needlegrass has a slight brownish band.