I have now read in two different places that you can overwinter Verbena
Rigida tubers treating them the same way as Dahlias. Is this true?

I've just treated them as a hardy perennial
that dies right back in winter and gets no protection. Winters here are
very wet but quite mild (coldest garden temp in three years was one
night at -4). I think vr must be tougher than dahlias, which don't
survive here if left in the wet soil over winter. If you have much
colder winters than that, you might get away with just mulching the soil
surface.

Thanks Janet, I know they are borderline hardy, but the comments about the
wet are encouraging. Verbena Bonariensis is also supposed to be borderline
hardy - but has done well here in North Yorkshire for the last three years.
They seed very easily, but if you have more than one variety of Verbena, you
do not know what the seedlings are. And there are too many of them to nurse
the all.