This is a...

(...PITA!)

I've looked in the 'Upright stems' section of Pictures of Common Weed Leaves but can't see it there.

What is it?

I've got quite a few of them in my garden and they keep sprouting up. Roundup Gel doesn't seem to have much effect, and is difficult to apply on those spindly 'branches' anyway.

 

It's Marestail! Also called Horsetail
Had that in my previous garden and never was able to eradicate it.

 

You need to crush the plant to break the waxy coating before putting the gel on

 

Gad, it sounds awful. Roots to 2m down?! Sheesh...

 

Don't even waste your energy trying to dig it out.
I was very confident I could do it on our light soil.
Hopeless.It is said to go down to the coal measures.

On the Radio 4 "Gardens Question Time" many moons ago, this question of getting rid of it was put to the experts.
The reply came...The only way to get rid of it is to move house!
I agree. You just have to learn to live with it!

When the dinasours walked the earth, the "trees" they would have seen were a tree form of horsetail..known as Calamites.
They formed our present day coal seams.

Calamites - Wikipedia

 

I agree, @Silver surfer.

However, you can subdue it, and certainly stop the spread, if you pull out the young shoots each time they appear. They come out like asparagus shoots, if you pull gently at the base of the plant. A bit of a pain each spring, but it can be slowly pushed back, if you work at it methodically...

 

Moving house did the trick for me

 

Oh believe me Sian in Belgium, we tried that as well.
Maybe you had a different species.
Afterwards it was even worse .... instead of one upright stalk..... from the same stalk a forest would regrow.
There was just such a big area covered by it... it was hopeless.We never did beat it.

https://fthmb.tqn.com/OZ9HIP6v69Mnf...etum-arvense-big-56a588da5f9b58b7d0dd463c.jpg

Moving worked for us.
Thankfully none here in this garden.

 

Well, I found that over a number of years, yes, pull out one, 5 come back, pull out 5, they come back weaker, then next year, repeat...

Moving house also worked wonders for us too, and I checked the garden thoroughly before deciding on our current place. So I escaped the marestail, but ended up with pure sand instead of soil. Next place, we dig holes before we sign!!