Could it be Eupatorium capillifolium?… Wait, is this in the SW UK?

Thanks, mac

Yup! The other side of the pond from you

Don't think so based on E. capillifolium doesn't appear to have red stems and it's not in any of my books. :-))))

Fof

Compare with Matricaria discoidea, Pineapple Mayweed.

I did suspect a member of the Asteraceae, but that is as far as I got. Not Chamomile and I dismissed Pineappleweed as I have seen it many times, growing through cracks in the pavement and small and I don't recall them having red stems.

I've just checked through my books and none of them mention it having a red stem, which is such an obvious marker. All the images I see on Google have green stems.

Hmmmmm..... Now to wait for flowers

I'm taking a risk here, but could it be a floppy Cosmos bipinnatus?

This is a photo of Tripleurospermum maritimum with red stems.

Tripleurospermum maritimum (Sea Mayweed) is far less common than Pineapple Mayweed and quite specialised as to habitat. Given where OP lives it is a possibility but Pineapple is much more likely. It’s quite variable depending on growing conditions. There’s also Scarlet Pimpernel, Sow Thistle and a thing whose name escapes me for the minute in the pictures. All common and typical arable weeds here, especially of paths and trodden places.

Not a Cosmos Christopher. We don't have the heat for Cosmos to germinate outdoors until quite late in the year and it wouldn't be that big yet.

eta Swine Cress!

I did dismiss T. maritimum, while going through my books, because of the plants' prefered habitat. However, I now suspect that that Jay is correct in the ID.

This, and a second specimen, are growing in the soil the waller imported, but I have no idea as to its provenance.

Christopher. Thanks for your I/P but I have to agree with floral. Not Cosmos.

It is wise to take Floral and Jay's ID over my it kind of looks like cosmos.

We await the flowers .....

Then, of course there’s T inodorum which is also far more likely than T maritimum. The latter really is predominantly a beach plant and generally grows above the high tide line.

We do indeed. :-)))

Not T inodorum, as have lots of them in the wild flower area. Foliage and stems definately different.

Colin French's, "A Flora of Cornwall", does list a few record of T maritimum more inland than its usual habitat. Also, as I said, I have no idea where in Cornwall the soil came from, apart from, probably, somewhere west of Newquay.

Well, both specimens have flowered. They have to be T maritimum.

There are only 2 in the garden, both in the imported soil, 2m apart. One with red stems, and the other, green

Checking one of my go-to sites, Tripleurospermum maritimum, and the images are a match.

Red stemmed

Green stemmed

and a trailing Lobelia has popped up through the Swine-cress (Lepidium coronopus)

Thanks, gurus, one and all.

Fof

A very Cornish ’weed’. Nice. I wonder where the soil came from.

That does look like a fun weed to have in the garden. Is it a perennial? The flower looks like my Ox-Eye Daisy, Chrysanthemum leucanthemum - which the botanists have changed by now I would imagine.

Chris. It appears to be a biennial or short lived perennial.

I like them, but they are both in totally the wrong places.

I'm going to try and gather seed then terminate them. Sorry plants. :-(((

I wonder if there is any way to propagate them vegetatively - cuttings or division?

Any gurus, with any ideas, out there??