I wonder if you can tell me what this gelatinous greenish substance is that is spreading all over the gravel driveway at our house. It looks kind of like seaweed. We tried spraying pesticide on it, but it came back with a vengeance. I’m worried about slipping on it, and would like to know how to get rid of it.
What you are describing sounds a lot like Nostoc, also known as witches’ butter, which is a type of cyanobacterium (blue-green algae). It is often found on surfaces which are made of some form of calcium carbonate rock (such as gravel, steps, and sidewalks). According to The Seaweed Site, nostoc is “common on limestone gravel near path edgings […], particularly in autumn. It can suddenly appear after rain, seemingly from nowhere, on paths, on roofs and sometimes in poorly-growing lawns. This gave rise to an early belief that it was material from shooting stars that had fallen to earth, hence some of the English common names Fallen Star, Star Jelly and Witches’ Butter.”
It apparently causes trouble in plant nurseries, too, as this document entitled “Nasty Nostoc” describes. (Source: Heather Stoven and Jennifer Parke in Digger magazine, June 2014, Oregon State University.) Many of the treatments mentioned in the article are not appropriate for use on gravel, and with any treatment, care must be taken not to contribute to toxic runoff into stormdrains. Avoid using phosphorus fertilizer, as that can encourage development of nostoc. Try to keep water from pooling on the gravel (hard to do in a rainy Seattle fall season). If the algae is only on a small area, you may be able to shovel it up and dispose of it. If you can solarize the area during the hottest weeks of the summer, that may also be helpful (covering with clear plastic for two to four weeks).