yep it is a weed, looks like what I know as mugwort.

It looks a bit like the native Artemesia (Lad's Love), or are the leaves red? In which case it could be Red Orache? A bit hard to see the colour in the pic. I would still pull it out as the middle of the lavender is not the right place anyway! If it's Red Orache you could save some seed from it to sow elsewhere, if Artemesia, just get rid of it before it spreads everywhere!!

Do you know any Druids? Try selling it to them, it is sacred to them because it is a mild hallucinogen. Not a lot of people (want to) know that.

Not ladslove, the leaves are wrong. Are you going to try eating a bit and tell us what happens?

No. It is definitely Artemesia, also called Mugwort. And I've retired so don't send for me if you overdose. Actually, it is very mild, so try it on a very small Druid.

Not arguing with artemesia, its just not the abrotanum aka southernwood, the one we've always called ladslove!

lads love is the southwood to me too. thats why botanical names are important I guess.
I always forget mugwort is Artemesia.