Very nice! I went to the Middle East Magazine and looked at the article. The wall was very well done. That’s interesting that you used a stucco net. Is that working well for you?
I think choosing what to do with your plants over winter is a matter of taste. Some will go into hibernation and change color only to come back in the spring. Others may die, but look better in the warmer months.. so that part likely depends on your plant selection and your weather there in Istanbul.
However, I have heard of people putting up a bit of a mesh or cloth covering over the wall temporarily to shield it from the cold/frost. When the weather is particularly bad.. or even just at nights, the mesh could be draped over top of the wall to help protect it.
Your other option, of course, it to keep changing out the plants. I think your final solution would be up to you and your client in terms of the look they want in the winter as well as the costs of changing the plant material.
