OMG, I'll cut it off tomorrow first thing in the morning, thank you!

Pull it out by the roots. It is perennial so if you just cut it, it will grow back.

Thanks, but it will be a big operation pulling the roots out of the retaining wall. I did cut already and will play the vigilante :-)

Here are 3 more volunteers, growing in bark. What are they? Thanks!

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Volunteer #2

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Volunteer #3 growing above volunteer #4

2. is a grape. 3. is a Circaea species, Enchanter’s Nightshade. It's a real menace weed with a running root.

#3 has been pulled out, thanks!

#2 is a nice surprise, double thanks! But how do I cultivate a grape plant into a nice garden feature?

I wouldn't bother with a random seedling. If you want a grape vine buy a named cultivar.

Given the nature of Circeae, I doubt you’ve seen the last of it.