If you ever work with nomenclatural databases, you will find that a few little Latin words, usually abbreviated, become the bane of your existence: “auctores (auct.)”, “hortus (hort.)”, “non”, “sensu”, “ineditus (ined.)”. All of these are ways of saying, “Yes, I know that this field is labelled ‘scientific name’, but I’m going to put things in here that aren’t valid names and let you try to sort it out.” Something like “Festuca cinerea auct. non Vill.” is a way of saying, “Some botanists are using the name Festuca cinerea in a way that I think is inconsistent with the typification / diagnosis given by Villars”. In other words, someone misidentified some plants. However, when someone puts the name “Festuca cinerea Vill.” on an herbarium sheet, if the plant is not in fact “Festuca cinerea Vill.” then they have simply made an error. They have not created the new name “Festuca cinerea auct. non Vill.” so don’t put that in your nomenclatural database. Or, at least, put it somewhere other than a field that’s supposed to contain names, because this is not a valid name under any of the nomenclatural codes. It can be very useful to keep track of misapplied names in regionally important floras, as a way of indicating where and why past botanists are likely to apply a name to plants in a way that might be otherwise inexplicable. That’s great. Misapplication and misidentification are not nomenclature, so find a good home for it where we won’t think it’s nomenclature. Create a data structure that makes this distinction, and use it.
Much of this stems, I think, from the historic expense of paper and typesetting. As you move further back in the botanical literature, manuscripts get denser and more abbreviated. The reader is expected to infer a lot from changes in font size, indentation, italicization, and spacing. If something looks like a name, is italicized without a latinate ending, and is followed by a number or a date, that’s probably the citation of a type specimen. Of course you know what book would be abbreviated “Ber. Deutsch. Bot. Ges.” because there just aren’t that many botanical works published. Writing “auct. non L.” is a lot more compact than writing out that some authors have applied this name contrary to Linnaeus’s intent, and your reader will know that this is shorthand rather than a new botanical name. But now we have various databases that originated largely by turning this compact and abbreviated style into digital data. Something you can easily infer when reading one entry becomes a colossal mess once you put 100,000 of those entries in the “name” field of a database. The data need to be made explicit rather than left to inference. So, sure, “Festuca cinerea auct. non Vill.” is fine in a 19th century manuscript. Maybe even in a 20th or 21st century manuscript. It does not belong in a database and especially not in a “name” field.
Well, to be honest, I wouldn’t mind if we banished “auct.” entirely. “Hook. non Vill.” at least tells me something potentially useful–that I should expect Hooker’s application of a name to be different than that of Villars. “Auct. non Vill.” tells me nothing. Some unspecified author(s) somewhere misapplied the name. What am I supposed to do with that? I’ll never know when I’m encountering “Festuca cinerea auct. non Vill.” rather than just “Festuca cinerea Vill.” People don’t knowingly and intentionally misapply a name and then helpfully point this out for you. “Auct.” is someone else–unless, perhaps, someone is setting you up for a particularly obscure form of the Liar’s Paradox.
While I’m at it… “sp.” and “spp.” are not parts of scientific names. “Tamarix” is a name. “Tamarix sp.” is not. Further, these abbreviations add nothing. The name “Tamarix” already includes all of the species of the genus. In principle, the use of “sp.” vs. “spp.” serves to indicate whether you are talking about one or more than one species in the genus, but in practice people often use the two interchangeably. The idea that one can trust someone to know how many species they are looking at when they don’t know which species they are looking at is also… optimistic. Even when someone deliberately writes “sp.” to indicate a single species, the information conveyed is so likely to be incorrect that it ought to be discarded.
The use of “×” in scientific names is also an irritation. In effect, it creates an indefinite number of orthographic variants that are to be treated as nomenclaturally equivalent but which can not be enumerated or systematically identified. In strict application of ICNafp chapter H, these variants are fairly constrained. The “×” can only go before the genus, before the specific epithet, or before an infraspecific epithet. In practice, violation of the ICNafp formatting is the rule. It might be an “x” or “X” rather than a “×”. It might or might not be separated from the epithet it precedes. It might or might not be accompanied by text like “pro sp.” that people sometimes feel the need to insert into the names of hybrids.
Given that “×” is optional except in hybrid formulae, the best practice is to use it only in hybrid formulae. If you want to say, “I think this is a hybrid,” by all means, do so! However, there’s no need to mix a hypothesis about the evolutionary history of a species with the nomenclature and thus add to the pile of orthographic variants.