If you have any advice on these two I’d appreciate it, I’ve spent so long staring at (mostly aberrant?) Brassica nigra that I’m not clear on the proper methods of diagnosing that species. I know the seed pods make it easy, but what’s the gist outside of that?
Sinapis arvensis is crazy variable (e.g. in size: smallest plants are no bigger than a finger nail, can be up to 1.6 m tall).
My advice: look out for plants with ripe pods and go from there. In Sinapis arvensis, the pods are not adpressed to the stem, this is always a very good indication (this happens sometimes in Brassica nigra too, but not often and only partially (especially later in the season)). The pods have a much longer and usually flattened beak, flattened at least in the upper part (sometimes ‘sword’-like) in Sinapis arvensis, the body of the pods with cross section not obviously square.
Sinapis arvensis tends to be a tad more hairy than B. nigra (especially the stem) and the leaves are often much less lobed than in Brassica nigra.
My secret tip for recognising Sinapis (alba and arvensis): look at the unopened flowers. In part of them the sepals are folded as to make them look like a type of clergy hat, see https://www.vesnavestments.com/wp-content/uploads/photo-gallery/skufia_blk.jpg compare to https://inaturalist-open-data.s3.amazonaws.com/photos/141242688/original.jpg?1625512089
And of course: Sinapis arvensis - leaves (ignore sprouting plants) lack the mustard taste (wash before you taste)!
A tip I learned from Jon Rebman to distinguish H. incana from B. nigra is that the buds of H. incana are hairy and the buds of B. nigra are not.
I don’t think I have ever seen Brassica nigra in California. In the Bay Area, by far the majority of yellow-flowered mustards are the one my field guide calls “Brassica geniculata” (which, as I just saw on Wikipedia, is what you guys mean by Hirschfeldia incana). There’s a reason it’s called Shortpod Mustard.
In my part of Southern California we have both species in wild settings (urban settings tend to have Hirschfeldia only) but it is true anyone who casually thinks they know a little something about plants calls them both black mustard. I think it’s because black mustard has had a lot more media attention as being a nasty invader.
Yes in the good old days people who weren’t sure
about a plant could call it genus Brassica. Now you have to go up to tribe.
Just a couple of more things. Apparently, Shortpod mustard is the common name for Hirschfeldia incana in North America. This is highly unfortunate, since pods of H. incana are often not shorter than those of Brassica nigra. Choosing a bad name can lead to many wrong ID’s (and this can start with the Latin name, like for instance the problem we have in Europe with ‘Salix aurita’).
- If one or more leaves are clasping the stem, it’s not Brassica nigra or Hirschfeldia incana, nor Sinapis arvensis (Think Brassica rapa when encircling or Brassica napus/oleracea in the case of clasping).
- If sepals are hairy, work from the assumption that it won’t be Brassica nigra (this has been mentioned above).
- This is NOT diagnostic, but can be an indication: flower buds often clearly overtopping open flowers in Brassica nigra, flat inflorescence tops with open flowers overtopping flower buds in Hirschfeldia incana.
- Be careful with overwintering plants: Brassica nigra and Hirschfeldia incana can be easily separated by sight, Sinapis arvensis and Brassica nigra can be very difficult to separate at that stage.
- There is another species with adpressed pods and also often a bit similar to Brassica nigra in general habit and that’s Rapistrum rugosum. But apparently it’s not that common (over here in Belgium just as invasive as Hirschfeldia incana) in North America. The pods are highly characteristic from a young age onwards with globose distal segment.
I don’t think many people who know what “pods” are and where to find them are likely to also assume they are “short” enough to justify choosing that ID by name alone.