Moss on green roofs

Green roofs are growing up! They have an official organization and accreditation program because there’s more to a green roof than meets the eye. The “green” part, the vegetation layer on top — for which moss is a highly desirable candidate in shady areas — is only the icing on the cake. The “roof” part, or “black arts” as it is called in the trade, comprises several layers of substrate and a growing medium, and requires expertise to specify, install and monitor.

Green roofs are gaining tremendous popularity in the US, which takes its lead from Europe and Canada. Green Roofs for Healthy Cities, the organization that sponsors the training and accreditation program, lists on its website several new laws and government incentives intended to entice or otherwise drag building owners and city planners into the 21st century. (greenroofs.org)

Moss is an especially desirable plant component and has many advantages for green roofs because it:
  • Is lightweight
  • Retains ten times its weight in water
  • Thrives in shade produced by building shadows
  • Is drought tolerant once established
  • Grows rapidly — three to six months for full coverage

Eco-lawn grass alternative now available

Too much sun for an entire moss lawn? In our quest to make lawn care as maintenance-free and chemical-free as possible, Moss Acres announces Eco-Lawn seed, a solution for larger sunny or shady lawn areas that just aren't practical for moss. Eco-Lawn grass requires minimal mowing, only once every four to five weeks, and much less water, if any (it is drought tolerant). It is also quick to germinate, establishes deep roots, and is less vulnerable to grubs (because it requires less watering). Find out more at Wildflower Farm_Eco Lawn.

Mosses and lichens

Cladonia lichen and Polytrichum piliferum, Bristly Haircap moss  Photo courtesy Sonia Allen
If it can be said that mosses are the carpet and upholstery of nature, then perhaps we can extend the metaphor to say that lichens are nature’s paint, wallpaper, fringe and frills.  Dr. Trevor Goward, a lichenologist at the University of British Columbia, goes further to describe lichens as fungal greenhouses, algal farmsteads, ecosystems, organisms, and emergent property.

Peltigera aphthosa
Lichens, of which there are more than 13,000 species (3,600 in the US and Canada), are found in every climate but can grow at much higher altitudes and under harsher conditions than mosses. According to Dr. Irwin Brodo, author of Lichens of North America, “Lichens grow in the leftover spots of the natural world that are too harsh or limited for most other organisms. They are pioneers on bare rock, desert sand, cleared soil , dead wood, animal bones, rusty metal, and living bark. Able to shut down metabolically during periods of unfavorable conditions, they can survive extremes of heat, cold, and drought.” Like mosses, which respond to stressful conditions by going dormant (also recall poikilohydry!), lichens are highly adapted to survival.

Caloplaca ignea
Symbiotic in more ways than one
The world of lichens offers wonderfully technical mouthfuls of terms such as squamulose, crustose, foliose and fruticose, which describe growth forms that the novice observer might simply call scaly, crusty, leafy or shrubby. Whether a frilly-edged fan adorning a log, or a powdery etching coating the face of a rock, lichens are a beautifully symbiotic coupling, or composite, of fungus and a photosynthetic partner — either algae or cyanobacteria. The fungus contributes carbon to the relationship and the photobiont contributes moisture.

In her 1907 book, Mosses and Lichens, Nina L. Marshall describes part of the relationship between these companion plants. Of lichens she notes initially, “...these little plants...lie upon the rocks, secrete an acid which dissolves the hard minerals...They have the power to condense moisture from the air...for they must have water as...food and as a medium by which mineral-salts dissolved from the rocks may enter the interior of the plant and may pass from cell to cell to those parts where they are to be worked up into plant food.”

Cladonia cristatella
Human use of lichens
Like mosses, lichens are used by birds to line or form their nests. Humans, too, have found myriad ways — some traditional, some modern — to use lichen: food and feed, brewing, medicine, dye, perfume, cosmetics, herbal tinctures, antibiotics, salves, deodorant, poison, decoration, fiber and tanning (hides).  Thanks to Sylvia Sharnoff, a list of uses is compiled on lichen.com.

Teloschistes exilis
Lichens as bioindicators

Lichens and mosses are sponges and can take in particulates and pollution, just as they do moisture and other nutrients.  Lichens are particularly vulnerable to air pollution, and are often used as bioindicators of increasing levels of pollution.  “When lichens disappear, they give early warning of harmful conditions,”  according to Dr. Brodo.  He explains that “the association between high diversity of lichens and pristine habitats is so clear that scientists use lichens as indicators of ecosystem continuity — to help them identify areas that should be protected.”


Ulongissimalg
Also, lichens, mosses, and cyanobacteria together form a microbiotic crust, which stabilizes soil and is a major source of nitrogen in certain ecosystems, all of which are threatened by overgrazing and development. Tread lightly!

To learn more and see gorgeous photos of lichens (and some mosses...), visit:


All images courtesy Sylvia and Stephen Sharnoff unless indicated otherwise.

Inviting moss diversity on green roofs

Moss diversity occurs naturally on shade roofs

When the moss roof on the springhouse shown below and to the right was planted two years ago, a mixture of 75% Hypnum imponens and 25% Dicranum scoparium fragments was used, inviting friendly competition between the two species. However, this moss-friendly environment of moisture-holding aggregate and water-retention gel created a landing zone for spores of many other moss species. Now, there are at least 12 species of mosses on the roof, of which Thuidium delicatulum (Fern Moss) is predominant.

According to Dr. Robin Kimmerer, there are millions of microscopic moss spores floating through the air at any time.  All they need is a moist place to land and start growing.  The micro-crevices of roofing media create the perfect, moist spot where spores can germinate happily.

A German green roof specialist who has seen green roofs in Europe mature over the last 20 years recently opined that eventually all green roofs will turn to moss. We’re not so sure about this, because mosses, which are tiny and non-vascular, cannot outcompete the larger, vascular plants.  However, for a shade roof this may be true.  So few plants survive in shade, whereas mosses are ideally suited for it.  Perhaps this is the environment where mosses can eventually outcompete the vascular plants.

Thuidium delicatulum, Hypnum imponens, Bryum cf. capillare, Plagiomnium cuspidatum, Dicranum scoparium and Ceratodon purpureus are some of the mosses that have settled in to populate the roof of this springhouse. Image by Keith Bowman.
Moss is an especially desirable plant component and has many advantages for green roofs because it:
  • Is lightweight
  • Retains ten times its weight in water
  • Thrives in shade produced by building shadows
  • Is drought tolerant once established
  • Grows rapidly — three to six months for full coverage
And, since moss has no roots, constructing a green roof using moss also requires less growing medium.

Moss Myths

I regret having to break it to those who are navigationally challenged, but moss does not grow only on the north side of a tree.  It is found there predominantly because that side is generally more shady (in the northern hemisphere, that is).

And, despite having names that include the word “moss,” plants such as Spanish Moss — an epiphyte, Reindeer Moss — a lichen, Club Moss — a lycophyte (seedless, vascular plant), Irish Moss — a perennial, and Sea Moss — an algae, are not mosses at all. Mildew, unlike moss, is parasitic and requires a host.

Five reasons to keep moss in your yard

Do you know or live with someone intent on eradicating moss from the lawn, garden or patio? Here are five good reasons to let it be:
  1. Moss does not compete with grass or other plants: it grows where most other plants will not.
  2. Moss is not mildew, mold or algae, nor is it related to them: it merely likes similar conditions.
  3. Moss does not invade bricks, shingles, stucco or concrete. It has no roots, only hairlike rhizomes that tether it to a substrate.
  4. Moss is a valuable home gardening asset. Live moss plants sell for $5 - $18 per square foot.
  5. Save money and time on lawn care.  Moss requires no mowing (or gas), water, fertilizer, lime, herbicide or pesticide.
If you can think of more reasons, please post them!