excerpts from the facebook post: https://www.facebook.com/SteveMartino.LandscapeArchitect/posts/866162183415784
My first cactus roof is 6 years old……i have posted this roof in the past and i have seen many internet comments ranging from it being really brilliant to being really stupid, the majority of comments are wondering when the roof will collapse from it’s increasing weight as it grows.
It has grown as i predicted, it stopped growing at 2 years and has maintained that size for the last 4 years. i have observed that when i go to transplant old over-grown succulents out of a pot there would sometimes be no soil left in the pot, it had disintegrated over time and the pot would just be a solid mass of roots without any soil. the plants would be doing fine so i figured that the prickly- pear cactus could grow without soil if it had some gravel to support it while it sent out roots.i think it reached the growing capacity of the gravel base,a horticulturalist may have a better explanation.
i have also observed that pads can fall off and land on pavement and continue to grow, the new growth will turn up vertically, they can even develop new pads and flower before they use up the moisture in their leaves.
the cactus pads are planted in 2 inches of 1/4” minus crushed granite, we added micro sprays for irrigation and that’s it.