Horsetail

https://en.wikipedia.org/wiki/Equisetum

I thought it was simple, but now I see it's not.  

What is straightforward is that it's a big pain to get rid of.  At least it's mainly in the slabs.  The old allotment had a lot of it (or one of the types), and that part of my undoing.  The new allotment doesn't seem to.  

We thought we had managed to kill it off last year.  Turns out it was just autumn.