You walk down a forested path. Moisture hangs in the cool air, and sunlight struggles to pierce the thick canopy above. The ground is so dense with underbrush that it's hard to tell where one fern ends and another begins. Ahead, water spills from a fuzzy green ridge; it washes over a fallen tree trunk and laps at the cobblestones beneath your feet. No, you haven't stepped onto the set of "Avatar"; you've simply found an ideal habitat for moss.
If you live somewhere lush and shady, like coastal Washington State, moss needs no introduction; it's about as ubiquitous as the Starbucks Coffee Company. But folks in more arid climates simply know it as that carpetlike plant that occasionally grows on stuff -- the ground, rocks, trees, sidewalks, walls, roofs. While most of the approximately 15,000 species of moss are some shade of green, others can appear rose, yellow, black and even silver. Moss ranges in size from Ephemerum, which grows just 0.02 inches (0.5 mm) tall, to Dawsonia, which may reach a height of 27.5 inches (70 cm) [source: Richardson].
Mosses differentiate themselves from flowering plants and trees by not doing certain things: They don't anchor themselves with roots; they don't germinate from seeds; and, they don't move water and nutrients through a system of internal vessels.
So, then, how exactly does moss work?
Mosses have been around for a really, really long time. In fact, these plants -- which scientists believe evolved from green algae -- probably originated 299 to 251 million years ago during the Permian period, though some evidence suggests that they may have appeared as early as 350 million years ago during the Devonian period [sources: Britannica, Richardson]. Today, mosses thrive in moist temperate forests, but can live just about anywhere, from the equator to the Arctic Circle. They're known to survive in all kinds of harsh environments: on rocks and trees, along seashores, on sand dunes, around hot springs and even at the bottom of lakes!
Mosses, along with their close relatives hornworts and liverworts, are members of the phylum Bryophyta. They're further classified into three groups: granite mosses, peat mosses and true mosses. There are only about 100 species of granite mosses, which are generally blackish in color, small in stature, and grow in mountainous or arctic regions. Bright green or reddish peat mosses typically grow in soft, muddy bogs and include around 350 species. True mosses are the most common type of moss by far, boasting approximately 14,000 species. This group includes the green carpetlike plant that most people picture when they think of moss.
What could all these different species possibly have in common? For one thing, they share the same basic structure. All mosses anchor themselves with small tufts of rootlike structures known as rhizoids. A simple stem supports the leaves, which are spirally arranged like the stripes on a candy cane. They're usually lance-shaped with a pointed tip, and are seldom more than one cell thick. The stem and leaves, collectively known as the gametophyte, house the reproductive parts of both male and female plants. Sometimes emerging from the top of female gametophytes is a sporophyte. This structure consists of a thin stalk called a seta that supports a little brown ball known as the capsule. Topping the whole thing off is the operculum, which looks like a hat sitting atop the capsule.
Right now all of these parts may seem pretty strange to you, but on the next page you'll find out how they help the moss feed itself and reproduce.
The Moss Life Cycle
So, if mosses lack the roots, internal vessels and seeds common in flowering plants and trees, how exactly do they survive?
Mosses absorb water and nutrients in a couple of ways. Some have highly absorbent surfaces that allow them to draw in moisture and minerals from the water that flows over the outside of the plant. Others are able to pull these materials up the external surface of the rhizoids and to the stem thanks to a principle known as capillary action. This phenomenon occurs when the adhesive force of the water molecules to the rhizoids is stronger than the cohesive force between the water molecules. You've probably seen this same principle in action in your kitchen; it's what draws water up the fibers of a paper towel.
Once the water and minerals move into the leaves and stem, they're either transported through cells or between cells, depending on the type of moss. The water finds its way to the leaves, where photosynthesis occurs. Here's a little primer on photosynthesis: It's kind of like baking cookies (yum), only instead of flour, eggs, and chocolate chips as ingredients, the plant uses carbon dioxide and water. Both processes need an energy source to begin: For cookies, it's heat from an oven; for moss, it's sunlight. When the kitchen timer goes off, you have warm cookies and a great smelling house; when photosynthesis is complete, the product is sugar and oxygen. The moss releases the oxygen into the air, but the sugar combines with the minerals to form substances that help the plant grow and reproduce.
And speaking of reproduction, moss has a pretty handy way of accomplishing this feat as well. Remember how the stem and leaves of the plant are known as a gametophyte? Well, before sexual reproduction can begin, vase-shaped archegonia, which produce eggs, develop at the tips of female gametophytes, while antheridia, which produce sperm, develop at the ends of male gametophytes. Sperm from the antheridia swims through water to fertilize eggs in the archegonia (this is a big reason why moss prefers moist climates).
Once the egg is fertilized, a sporophyte soon develops in the female plant. If you remember from the previous page, the sporophyte is the structure with a tall, thin stalk supporting a little brown capsule with a hatlike operculum. The capsule produces spores, which are essentially the equivalent of the seeds found in flowering plants and trees. As the capsule dries out and matures, the operculum pops open and releases spores that are carried away by the wind. If the spore comes to rest in a nice moist spot, it germinates and grows into a new gametophyte.
Sometimes mosses reproduce asexually, as well, meaning they skip the whole process described above. With the right amount of moisture, pieces of moss can break off, move by wind or water, and, amazingly, grow into new plants.
But why just sit around here and talk about how moss grows when you can conduct your own moss-growing experiment in your backyard?
How to Grow Moss
When choosing plants for your landscaping, you probably reach first for grass, flowering plants, bushes and trees. But why not grab some moss? Moss can serve as an appealing green carpet in soil that's too acidic and shady to support grass. A garden is also a great place for moss, where it can act as a visual anchor beneath plants, stones or statues. Some moss lovers even claim that its verdant green color can actually promote relaxation and reduce stress. It's long been popular practice in Japan to make moss a part of landscaping.
If you already have moss in your yard, or know where to collect some (legally), you can try transplanting it where you want it to grow. Like grass, some mosses can simply be scraped up and replanted in a new location. You can either completely cover the desired area with the plant or space it apart and let it grow together. Another method is to shred a handful of green moss with one cup of buttermilk in a blender, then "paint" it on the soil, rocks or other surface where you hope it will spread. Just remember to clean the blender really well before you use it for smoothies again!
You can also buy moss, but there's a good chance they won't sell it at your local nursery. If you're having trouble finding it, try an online retailer like Moss Acres, which sells numerous varieties for lawns, gardens, roofs, walls and more.
Once you get some moss to grow, you have to care for it. Fallen leaves and twigs can stifle its growth, so use a leaf blower or shop vacuum to keep the surface clear. Avoid the urge to yank weeds or tree seedlings that emerge; you'll only hurt the moss. Instead, snip these unwanted intruders with a pair of garden shears. Moss should also be fed at least once a year -- in mid-spring -- with a mixture of 1 quart buttermilk to 2 gallons of water. And don't forget that moss always needs water. Misting the plant frequently, especially in warm weather, will not only keep it from turning brown, but will also encourage growth.
Now for the tricky part of this article: While we think moss makes a perfectly lovely green carpet for a yard, not everyone likes having moss around. In fact, some people actively try to kill it. More about that on the next page.
How to Kill Moss
Moss lovers, cover your ears. It's time to talk about some of the problems moss can pose and what can be done to, well, kill it.
There are many reasons, both practical and aesthetic, why people choose to get rid of moss. If it's growing in your lawn, eradication isn't a necessity; moss won't overtake established grass and generally grows in shady, acidic soil that isn't good for turf anyway. Those who prefer fence-to-fence sod, however, may see it as an eyesore and choose to kill it. Moss isn't likely to cause much damage to brick or concrete, either, but moss on a sidewalk can be slippery and may need to be removed for safety reasons.
Rooftop moss can be a bit more problematic. Sure, moss on your roof will make your house look like something out of "The Lord of the Rings," but roofing professionals insist that the plant can be a real cause for concern. One issue is that moss's tiny rootlike rhizoids can grow into small cracks in the roofing material, causing the roof to deteriorate. Moss may also grow under shingles, lifting and loosening them from the surface. Finally, moss can hold moisture on your roof, creating a habitat for decay-causing organisms like fungi. All of these issues have the potential to cause leaks and send water into your otherwise dry home.
So how do you get rid of moss? For lawns, pesticides can be an effective way of killing the plant, though they may be toxic and should be used sparingly. A better way to attack the moss is to treat the root of the problem. Make sure the area drains well, cut back overhanging branches and decrease the soil acidity with lime to discourage future growth. Moss on sidewalks and bricks can simply be scraped off with a shovel or hoe, but if it grows back it may have to be treated chemically. Before using any moss-killing product, test it in an inconspicuous place first to ensure it won't stain the surface you're treating.
And then there's your roof. Just like the lawn, your roof is best treated by addressing the underlying conditions that cause moss growth. Any plant matter resting on its surface can collect moisture and allow moss to take hold. Trim back overhanging tree branches and clear off leaves, pine needles and twigs. If this doesn't completely eliminate the problem, chemicals similar to those used in your yard are available for the treatment of roofs. Zinc is also a very effective deterrent to moss growth; strips made of this metal can be installed along the roof peak or beneath the shingles.
I hadn't given moss much thought before writing this article. It's not easy to find in the semi-arid Utah valley where I live. Then one evening after I had written the section on the moss life cycle, my wife and I took a walk along a river in a nearby canyon. And then I saw it -- moss! I launched excitedly into a description of the plant's structure, pointing out each part to her. I'm sure she was thrilled!
What, then, did I learn from writing this article other than the origin of moss, its structure, its reproductive strategy, its methods for acquiring nutrients, and how to grow and kill it? The world is a pretty interesting place if you just take the time to learn a little about it.
More Great Links
- "AP Biology: Learn the Moss Life Cycle." YouTube. Feb. 7, 2010. (March 22, 2012) http://www.youtube.com/watch?v=7lr4CjPMO80
- Australian National Botanic Gardens. "What Is a Moss?" April 15, 2008. (March 23, 2012) http://www.anbg.gov.au/bryophyte/what-is-moss.html
- Conrad, Henry S. and Paul L. Redfearn, Jr. "How to Know the Mosses and Liverworts." Dubuque, Iowa: Wm. C. Brown Company Publishers. 1979.
- Cullina, William. "Gardening with Moss." Horticulture Magazine. June 28, 2011. (March 22, 2012) http://www.hortmag.com/plants/plant-profiles/gardening-with-moss
- Encyclopedia Britannica. "Moss." 2012. (March 22, 2012) http://www.britannica.com/EBchecked/topic/393741/moss
- Glime, Janice M. "Bryophyte Ecology." 2009. (March 22, 2012) http://www.bryoecol.mtu.edu/
- Marshall, Jessica. "Swimming Pools Kept Clean by Going Green." Discovery News. Oct. 28, 2009. (March 22, 2012) http://news.discovery.com/earth/swimming-pools-moss-green.html
- Martha Stewart Living Television. "Moss Basics." MarthaStewart.com. 2012. (March 22, 2012) http://www.marthastewart.com/265643/moss-basics
- Mergel, Maria and Philip Dickey. "Tales from the North Side: Problems with Moss." April 2007. (March 24, 2012) http://watoxics.org/files/moss.pdf/view
- Oregon State University. "Living with Mosses." Spring 2000. (March 22, 2012) http://bryophytes.science.oregonstate.edu/mosses.htm
- Pierce Conservation District. "Pasture Management: How Plants Grow." 2012. (March 23, 2012) http://www.piercecountycd.org/tip_plantsgrow_p.html
- Richardson, D.H.S. "The Biology of Mosses." New York: John Wiley & Sons. 1981.
- Schenk, George. "Moss Gardening: Including Lichens, Liverworts, and Other Miniatures." Portland, Ore.: Timber Press. 1997.
- United States Geological Survey. "Capillary Action." March 9, 2012. (March 23, 2012) http://ga.water.usgs.gov/edu/capillaryaction.html
- Whitman, Ann. "More Moss." National Gardening Association. 2012. (March 22, 2012) http://www.garden.org/articles/articles.php?q=show&id=1135