The last green roof that I wrote about was planned, although not in a typical manner. https://tonytomeo.com/2017/11/25/green-roof/ It is still my favorite green roof. Otherwise, I am none too keen on the fad. Very few buildings benefit from green roofs, and green roofs really do take more work than conventional landscapes in the ground.
The sort of green roof pictured here was most certainly not planned. It could have been the result of a an uncleaned gutter. All sorts of weeds can grow in the damp debris that can wash off of roofs, particularly in damp and foggy coastal climates where moisture so often drips from the edges of roofs. This gutter is just a short distance from the beach in Santa Cruz. The willows in the San Lorenzo River are next door.
Knowing what I know about this particular type of willow, I would guess that the cleanliness of the gutter, or lack thereof, was not really the problem. These aggressive willows can germinate in the slightest bit of debris, even under a single leaf that did not get rinsed or blown away fast enough. Once germinated, their finely textured roots are experts at clinging to anything that might otherwise get rinsed away. If there is not enough debris and dirt for them to grow in, they simply collect their own. Now that it has started the process, it will continue to collect debris and expand its root system until it gets removed or ruins the gutter. It seems to have already collected enough debris to share with a few grassy weeds nearby. It is mostly dormant now, and might have defoliated in the rain since this pictures was taken a few days ago. However, if it stays, it will resume aggressive growth as winter ends.