You might remember from my explorations of the former death strip how the Berlin 'wall' was in fact quite a sophisticated double-layered border fortification. In some places the border exclusion zone extended even beyond the wall itself, into what they designated the 'Grenzgebiet' (frontier area).
These existed in areas where the border was more vulnerable for some reason, like when it was harder to watch over it, when the walls of buildings formed part of it or where the wall was vulnerable in some other way. The zone was then declared part of the Grenzgebiet and access heavily restricted.
Only people considered not at risk of trying to escape were allowed to live in any housing that remained within these areas and they were only allowed guests with the permission of the authorities. Similarly, where the Grenzgebiet included cemeteries, those visiting family graves were only allowed in if they had a 'Grabkarte' (grave card) which allowed them permission to do so.
This rail on Schönholzer Straße might at first seem like something purely to chain bikes to, or to stop pedestrians from wandering into the middle of the road, but it's actually connected with the Grenzgebiet here.
The 'Hintere Sicherungsmauer' or 'Hinterlandmauer' (the eastern side of the border fortification) ran along the northern side of the pavement here, where the grass is in the first picture. The poles served both to highlight to pedestrians that the area behind them was out of bounds and to remove any chances of cars straying off the road and hitting the wall. Signs reminded citizens that anyone entering the area could only do so with the correct permission (and, of course, papers to prove it).
Where there seemed to be more danger that a vehicle could be driven head-on into the wall, they often put out things like these:
While I can see how the plants might once have made these mildly more attractive, they don't go a long way towards disguising the fact that they're fairly brutal chunks of concrete. Certainly in their current overgrown state, I can't imagine many people being convinced of their decorative value.
Making them hollow would have made them lighter (and therefore easier to transport and manoeuvre) though, as well as cheaper - they could then just be filled with soil to bulk up their weight once they were in place. They're sometimes known as Blumenschalensperren (planter barriers). These particular ones are on the corner of Esplanade and Dolomitenstraße in Pankow.
The picture above is taken looking away from where the wall would have been. You can just see the road Esplanade leading into the distance to the left of the picture - the fact that both Esplanade and Dolomitenstraße led towards the wall here would have been the reason for these being put where they are. There's a third one, which you can see in the picture below, and I imagine there would have been a fourth one where there's now a gap for a cycle path.
This view - looking towards the former death strip - also gives you an idea of how thick the concrete in these planters is. The one in the foreground has a circular hole in it for the plants, so the corners are rather thicker than some of the others, which have square holes.
Either way, the concrete's a good 10cm thick even at its thinnest point. It would certainly take rather more than a daredevil driver in a speeding Trabant to break through these. The fact that your death as a result of that would have been reported as the result of criminal actions intended to harm the stability of the GDR rather than just that you rammed into an overgrown flowerpot would probably have been little consolation.