I don't have an answer but may I add something to the question please?
Looking at the photos I see that some of the planters are hanging from metal railings and are next to/near a metal table and what looks to be a metal floor. Other planters are next to a brick wall. Could the effect of the metal and brick be causing increased heat and decreasing the humidity in the immediate area? I think the word may be 'transpiration"...perhaps the plants are losing too much moisture through their leaves due to the arid local micro-climate?
