Tim C. wrote:

Bear's Breech[es], used on its own rather than
alongside the Latin _Acanthus_. Does anybody know how this curious name
arose?

From the folklore of plants .... "The bear is another common prefix.
Thus there is the bear's-foot, from its digital leaf, the bear-berry,
or bear's-bilberry, from its fruit being a favourite food of bears, and
the bear's-garlick. There is the bear's-breech, from its roughness, a
name transferred by some mistake from the Acanthus to the cow-parsnip,
and the bear's-wort, which it has been suggested "is rather to be
derived from its use in uterine complaints than from the animal."

From my book from A Smith meanings and origins of names .... " In
America called 'bear's breech' from the size and appearance of the leaf
which is very big, borad and distingly hairy. The acanthus leaf was
favorite decoration in classical sculpture as in the capital of the
crintian colum. In England the bear has been dressed up and it is now
called 'bear's breeches' despite long standing authority of the
contrary."








OED is no great help, but I find that the plant is/was also
called "Bear's Foot", and in Medieval Latin _branca ursina_, "bear's
claw". The Greek word just means "spiny".

Just to confuse matters, OED says "Brank-ursine" was sometimes
erroneously applied to hogweed!

Not too erroneously, perhaps.
Bärenklau (~bear's claw) is the/a German common name for giant hogweed
(Heracleum mantegazzianum)

--
Tim C.