Q. I have a large rock/stone in my yard and it is turning black. What could be causing this? Also, what is the difference between a rock and a stone anyway?
Roy from Warren
A. Thanks for your question. Without a picture or something else to work with it is difficult to say for sure what is causing your rock to turn black. However, this time of the year, one thing it could be is lichen. Lichen, unlike most things, grows during cool to cold weather and stops growing when it warms up. Lichens grow slowly, so you may have just noticed them.
Most of us are used to the gray- to green-colored lichen on tree bark. But not all of them are these colors. Many are bright orange-red and even black, yellow and white.
Lichen are a unique type of life called a symbiotic organism. This is where two (sometimes more) different organisms come together to form a close relationship, frequently for their mutual benefit. For lichen, the two organisms are various forms of algae and various forms of fungi. The algae provides the food via photosynthesis and the fungi provide structure and protection for the algae.
Lichens are harmless. They do not damage plants or other structures. They live on the surface of trees, rocks, fences and many other surfaces. There are at least 13,000 species of lichen found all over the world: from deserts, to the tropics, to the arctic. About 8 percent of the solid surface of the earth has some type of lichen growing on it. Lichens are considered “pioneer” life forms because they are the first living beings in inhospitable surroundings where, over time, they make conditions favorable for other life forms to be established.
When you have a chance to take a look at your rock, if the surface of it looks flakey or crust-like, it’s lichen. If it does not look this way, take a picture of it or, if possible, take a sample and send or bring it to the clinic and we will examine it. It may be that it is mold growing on a sugar substance. Either way, it’s interesting stuff.
Oh, and the rock question. In language, the words rock or stone are sometimes used interchangeably, but in general, a rock means something larger and more immobile than a stone. In geology, the word stone is not used, except in combinations such as limestone or sandstone. Everything else is a rock.
For information about lichens see: http://go.osu.edu/lichen
Today’s answer is from David Sprague, OSU Extension master gardener volunteer. Winter hours for the Plant and Pest Diagnostic Clinic vary. Submit questions to the clinic at 330-533-5538 or drop samples off at the OSU Extension Office in Canfield.