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.
Plants are quite ingenious with their technology of exploitation of animals and people. Many get insects, birds, bats, spiders and anyone who is animated within their environment to disperse the plants’ pollen for them. Plants who prefer to not rely on wind, water or gravity to disperse their seed exploit a different range of animals to do so. They know how to compensate for their immobility.
This sort of exploitation is generally not as bad as it sounds. Many pollinators are rewarded for their service with nectar or surplus pollen. Dispersion of many types of seed is likewise rewarded with the fruit that surrounds the seed. Many types of nuts produce significant surpluses of seed to reward squirrels for burying them, and leaving just a few to germinate and grow into new plants.
However, there are many types of seed that are not so gracious, and several that are potentially dangerous because of the tactics they use to exploit those who disperse them. Mistletoe is an odd parasitic plant that makes very sticky berries. Those that do not get eaten by birds (for later ‘deposit’) can stick to the feet or feathers of unconsenting birds in order to catch a ride to other trees.
It is sneaky but effective. Most other plants that use this technique are small annual plants that rely on mammals instead of birds. Instead of sticking to feathers, their seed are designed to stick to fur. Such seed are not often a problem for wild animals who have short fur that the seed can stick to only for short distances before slipping out and onto the ground where seed really wants to be.
Domestic animals are not so fortunate. They have longer, shaggier and maybe curlier fur that weed seeds such as foxtail and burclover can get very entangled in. Because foxtail is designed to go into fur but not come out, it is seriously dangerous if it gets into the eyes, ears or noses of domestic animals. Because dogs and cats go wherever they want to, it is very important to eliminate such weeds from gardens where dogs and cats live, and to hopefully do so before they go to seed.