Andy Mabbett wrote:

I recently bought a Black Elder (Sambucus niger) for friend who's
growing a bird- friendly garden. She also has two dogs, one a puppy.

She was concerned to read on the label that the plant is "harmful if
eaten". Since both elder flowers and elderberries are a food, what's
harmful? Is it the bark or sap, or something else? Or simply
over-cautious labelling.

(From memory of some research I did whilst wondering what to do with
the 20lbs of elderberries I picked last autumn.)

AFAIR, pretty much all parts of most varieties of Elder are toxic to
some degree. Sambucus niger has berries that are toxic when unripe,
and stems, leaves, bark etc. are also toxic. Some of the other
varieties of Elder have very unpleasant tasting fruit and one (Red
Elder, I think?) has poisonous fruit.

Anyhow, I've made some elderberry wine and some elderberry ice-cream
and it doesn't seem to have done me any ha