In message , Mike Lyle
writes
Stewart Robert Hinsley wrote:
This was seen growing, apparently spontaneously, at the edge of a
garden access road. My best guess is sneezewort (Achillea ptarmica),
but it doesn't seem quite right - the habit is too compact, and the
leaves too short. Any other offers.

http://www.stewart.hinsley.me.uk/Images/Compos06.jpg

The other leaves in the photograph are Solanum nigrum.

Is your specimen aromatic?

Unfortunately, I didn't think to check that feature.

We had something in West Wales which I was
/sure/ was sneezewort, but it was lightly and pleasantly aromatic; but
since the Collins Field Guide explicitly said it was "not aromatic", in
Italics, I assume this is a diagnostic character, which implies a
similar species I don't know about. But I'm wondering if there may be
more varieties or forms of A. ptarmica than are recognised (at least by
the Guide: I haven't anything more advanced).


But you've jogged my memory. There's a plant known as Sweet Nancy or
English Mace (Achillea ageratum syn decolorans syn serrata), not
mentioned in the 1st edition of Stace, but with 3 records from Great
Britain, which from a photograph I took on a visit to Cumbria has
similar foliage. (It wasn't flowering at the time.)

Digging around the web I find some confusion - the name Achillea
ageratum is applied both to a plant with yellow discoid capitula, and
white radiate capitula. Achillea decolorans and Achillea serrata appear
to only be associated with the latter.

There are other white-flowered Achilleas, but I'm not finding a match.

For all I know it may be Achillea ptarmica.
--
Stewart Robert Hinsley