Brian writes

Mare's tail should be no problem as it's a flowering plant.
However Horse tail is very different.

So what do you mean by Mare's tail? I always thought mare's tail
referred to the Equisetums that are grown ornamentally, which are no
more a flowering plant than are the horsetails. But Fitter uses
marestail for Hippurus vulgaris, which is a flowering plant

--
Kay