Mike Roscoe wrote:
[...]
I've always wanted to grow S. nigra but haven't found one in
any of my local garden centres.
[...]

Lor' bless yer, guv'nor! Not the kind of thing GCs are much interested
in. Just whip a cutting or two from the nearest roadside -- you might
even succeed if you do it now. Or dig up a seedling from the same place.
Or grow it from seed: I've done that, and it's very easy if you protect
the babies from slugs.

I once rooted an elder cutting by accident in, I think, March: I'd
dropped some paving slabs and didn't move them till May. Underneath, I
found a broken-off bit of elder which had produced a quite convincing
root, which could have been grown on if I'd wanted it. But it would be
worth waiting till leaf-fall: that way you can scout out the local
elders and tie a marker onto the one whose scent or berries you like
best, as the wild forms differ a lot.

--
Mike.