Why Is There Moss on River Rocks?

Moss grows on moist, shaded surfaces.

Mosses are small, simple plants that grow in cool, moist places, including by rivers. Moss is usually seen as patches of green carpet made up of lots of tiny plants, each with a stem about the same thickness as a hair. While different types of mosses might look similar, there are actually many different species. About one hundred different species grow in an average U.S. county. Many moss species grow by freshwater rivers and ponds, often on rocks that are partially submerged.

Growth Near Water

Moss is one of the few organisms that first evolved to live on land. Algae that had adapted to live on land evolved into simple plants 540 million years ago. These early mosses and mushrooms were on land before any animal made it out of the water. Because they are closely related to water-living algae, most mosses still live near bodies of water or other wet areas. Moss survives dry spells by drying out or by storing water in dead cells.

Growth on Rocks

When moss moved on to land, it started growing on rocks because there was not yet much soil. The moss's shallow roots help it attach to a rock and eventually break the rock into dirt. Due to its shallow roots, moss gets more nutrients from water washing over it than through its roots. Cracks on rocks are especially good places for moss to grow because they trap soil and water, which provide essential nutrients.

Reproduction

Moss needs water to reproduce. The sperm of the moss plant travels through water by swimming or being splashed onto the female reproductive organ. The river provides a good medium for moss reproduction, which explains their abundance on river rocks.

Types

Different moss species have different nutrient requirements and grow on different types of surfaces. Areas near rivers provide a habitat for many types of mosses to thrive. In the Cat River areas in Ontario alone, there are over 100 different species of mosses, including black rock moss, apple moss, fine-split-lobe liverwort, fine wet moss, marsh mnium and fern moss. In Antarctica, where only 2 percent of the land is ice free and suitable for plant growth, mosses and liverworts flourish compared to other plants. Even so, Antarctic mosses only grow around wet melt lakes and streams, as well as on rocks with wet cracks. The mosses that grow in Antarctica include Grimmia antarctici, Bryum pseudotriquetrum and Ceratodon purpureus.

Cultivation

Mosses can be cultivated to decorate gardens. They provide an alternative to grass lawns for ground cover, and unlike most other plants, mosses can grow on rocks and stones. To grow moss on rocks, you first need to spread mud on the rock surface. Take some healthy moss, mix it with buttermilk and water, then blend it to get a milkshake-like consistency. Paint this mixture onto rocks in a shaded area and wait for your moss to take root and flourish.

References

• Wild Lands: Moss
• Horticulture and Crop Science: Bryophyta
• Kidipede: Moss
• Backyard Nature: Mosses
• The Garden Helper: Growing Decorative Mosses in Your Garden

Resources

• Antarctic Diary: Casey