Here is a question from Sara in Evanston. I am grateful to her because it allows me to hold forth about one of my favorite garden organisms:

   "Do you have any idea what the substance is that we've found in piles on the mulch in kids' parks in Evanston and Rogers Park? It is orangey-yellow, and foamy, and is wet inside. It looks like vomit, but it is exactly the same in different parks not close together, so that seems unlikely. It appears in several different piles at once, but always on the wood-chip mulch in kids' parks."

I believe what you are finding, Sara, is most likely Fuligo septica, the aforementioned dog vomit slime mold. To answer everyone's most urgent question first, it's utterly harmless to people, pets and plants. In fact, Fuligo septica is edible. Native people in some parts of Mexico gather it and scramble it like eggs. I hear they call this dish "caca de luna," which I will let you translate for yourself, and which is an even more entertaining name than dog vomit slime mold. 

Slime molds are misnamed. They are not molds (a kind of fungi); they are not plants, animals or bacteria, either. A slime mold is a completely different kind of critter. We see it in just part of its life cycle, as a plasmodium, which essentially is one giant cell with millions of nucleii. It is formed when two spores come together in something a little bit like sex and begin dividing into a large creeping blob of protoplasm surrounded by a single membrane. The plasmodium moves by slowly flowing or streaming, gradually engulfing and consuming fungi and bacteria that are present on decaying plant matter. Many a horror movie has owed its inspiration to plasmodiums.

Slime molds often are found on mulch, especially in places like gardens and parks that are regularly watered. But they evolved in forests. I never will forget a hike I took through the temperate rain forest of the Olympic National Park in Washington, where within a couple of hours we came across at least a dozen kinds of slime molds in different forms and practically glowing colors, some of which appeared to be visibly moving. Not that slime molds move very fast -- 1 milimeter an hour is a pretty good clip for a plasmodium.

There are many kinds of slime molds. Physarum polycephalum ("many-headed slime") is another one common in dark, cool, damp, woody places. Fuligo septica, the kind we most often find in Midwestern gardens, appears on wood mulch or trees or leaves after rains or watering as a bright yellow mass that makes people think their dogs are sick. It is said to sometimes reach the size of a pizza. Over the next couple of days it ages to pinkish tan and hardens as it forms new spores. Eventually it dries up and disappears, leaving the many tiny spores behind to wait until the conditions are right for another plasmodium.

Slime molds have been doing this with great success for millions of years. They are one of those ancient organisms that has not changed much over time because what they were doing was working just fine. Fascinated scientists are sequencing their genome.

The appearance of these yellow blobs seems to alarm some people; in 1973 in Dallas, a slime mold was taken for an alien invasion. In fact, Fuligo septica is native to our woods. There's no reason to worry about it. It is not a disease. It's a natural phenomenon that doesn't hurt anything. Slime molds help break down plant matter, which aids the microorganisms essential to the healthy growth of plants. Like so many other things that creep people out, they are actually good for the garden. 

There is no way to prevent slime molds; the spores are all over the place, just waiting for the right conditions to become active. If the appearance of a slime mold in your garden offends you, break it up by raking out the mulch or blasting it with the hose.   

But if I were a kid, or had kids, I would immediately propose the study of slime molds as a summer science project, if not a culinary adventure.

Check this blog, Cheryl's Garden Party, for the prettiest picture you will ever see of a slime mold. Here's one where Wayne Armstrong gets seriously into the science of slime mold reproduction, with pictures of the progression of a dog vomit slime mold. Here's a Web page that describes scientists' theories about how slime molds move. And best of all, here's a site that tells how to culture a slime mold with banana peels.