Thank you for your question, Alan. I cannot tell from the photos what type of light they receive. These prefer the cooler North or East sides of houses, and the heat absorption of dark surfaces will only compound any heat issue. Your companion plants appear to be shade-loving as well. So, most leaf tip browning is a water-related issue. In the absence of exceptionally non-draining soil, under-watering is far more likely than over-watering, especially in our recent drought conditions. You might find some further thoughts in the following Extension article: http://www.hort.uconn.edu/plants/detail.php?pid=209 Good luck!