In message , Kay
writes
In article , Stewart Robert Hinsley
writes
In message , Cat
writes
Some weeks back, I said I thought a mysterious plant in my garden,
growing alongside my dahlias, looked like them to such an extent - and
yet not like them - that I thought it might just be a virused dahlia.
Now, having observed the plant closely since, I have come to the
conclusion that my theory is pretty daft: the plant looks increasingly
less like a diseased dahlia, and more and more like an obscenely
healthy something else.
Have I been lovingly nurturing a common garden weed?
Can anyone identify the beast?

Mugwort, _Artemisia vulgaris_

I'm hesitate to question your identification since you have much greater
wildflower knowledge than I .... but the leaves are not as grey and
nowhere near as serrated as I know for Mugwort. Can it take this form?

My first reaction was some sort of chenopodium but if so it's huge!

You could well be right. Artemisia and Chenopodium are sufficiently
superficially similar that it took me some years to learn to distinguish
them in the flesh. (Which means I can still be fooled by photographs,
which give less context.) Looking again at the flowers, it does look
like a Chenopodium. Checking Keble-Martin, Red Goosefoot, Chenopodium
rubrum, looks the best match. (The only goosefoot that I see regularly
is Fat Hen, Chenopodium album.)

My experience is that Mugwort, as opposed to the other Artemisias, is
not particularly grey, or dissected-leaved. I'd say that Fat Hen is
greyer than Mugwort. (If you don't look at the underside of the leaves.)

According to Stace, Red Goosefoot can reach 1m. Fat Hen can reach 1.5m.
It's not the height, but the width, that seems wrong. Goosefoots are
mostly annual, and I'd expect them to be single stemmed, even if
extensively branched; the photograph looks like a multiply stemmed
plant. Good King Henry, Chenopodium bonus-henricus, is perennial, but
only reaches (according to Stace) 50cm.
--
Stewart Robert Hinsley