The Importance of Moss

Here's a fun word I stumbled across in this week's research: poikilohydric.  Essentially, something exhibits poikilohydric properties if it doesn't have it's own root system to keep it hydrated and must rely on outside sources for water.  Cacti are poikilohydric.  Mosses are poikilohydric, too, an interesting similarity when we think of the environments in which they typically grow: one in a desert environment, and one in the shade.  This similarity is important when you think about survival of both species.  Cactus plants are able to thrive on little water, and mosses can also live in a dessicated state without damage to their life cycles, making them more efficient in droughts than vascular plants with a root system (Anderson 320-321).

Now, because mosses aren't able to get their own water from a root system, they are dead handy at retaining large amounts of water.  Studies show that in rainfall of about an inch, mosses will retain up to 16% of the water in their capillaries (Anderson 321) to store for later use.  This heartiness ensures that after rainfall, moss has the ability to reproduce in staggering quantities.

 
This moss-covered filed can be found in Iceland.

Why are talking about moss in this blog?  I think it's more important to ask why we haven't talked about moss yet.  Let's look at some of the characteristics of moss that make it an awesome concept in green roofs and rooftop gardens:

• Water Retention Rockstar
• Lightweight and Handy
• Able to Survive (even thrive) for Long Periods Without Water (making them more efficient than grass)

We've discussed that it's important to always have a roof analyzed for its load-bearing qualities, particularly if you're planning to dump a water barrier, dirt, sedum, and plants on top of it.  Is moss a viable alternative to the heavy layers common in many manufactured green roofs?

In 2008, a study was conducted at the Oregon State University, Corvallis, campus that aimed to find out.  The authors of the study, Malcolm Anderson, John Lambrinos, and Erin Schroll note that there is little to no data existing to quantitatively compare moss to vascular plants in their functional performance on green roofs (Anderson 321).  Their findings are interesting, but they caution that further studies over a wider variety of moss would be essential to form a fully organized compendium of the benefits.  

Three moss candidates were chosen for the study based on the likelihood of their ability to thrive on roof tops based on a survey from the University's bryology class: Dicranoweisia cirrata, Racomitrium canescens, and Antitrichia californica (Anderson 322).  The study's chart of the three mosses chosen is pictured below:

The moss was collected from local areas, cleaned of any foreign material and readied to transplant to a green roof medium called Pro-Grow Extensive mix originating from Sherwood, Oregon (Anderson 323).  Moss transplant is easy, given their lack of root systems.

After the specimens had sat for two weeks in a greenhouse, a layer of each moss was put onto three separate green roof mediums, while a fourth was kept in it's original state.  It was now time for the experiments to begin.

The first scenario experimented with stormwater runoff.  For a brief explanation of stormwater runoff and issues, see here; we will re-visit this concept in future posts.  For today, let me say that with a freak rain shower this afternoon, the alley that feeds into the parking lot at my day job became a river; this is one of the negative associations of stormwater runoff, particularly in the urban concrete jungle.

To test the mosses, researchers simulated rain from a recent observed storm in Portland, Oregon onto an established green roof.  The rain was added to each moss and the natural medium swatch in five minute intervals over a 30-minute period, with runoff volume recorded every five minutes following each deluge before the next started.  After the mats were thoroughly saturated, plugs from each were taken.  The measurement of water retention to the total original weight of the moss was established by weighing and drying the moss in intervals until it reached equilibrium with it's greenhouse measurements.  The medium was handled in the same fashion (Anderson 323).

The results showed the natural medium at a 32.6% retention.  A. californica was only slightly higher (33.8%) and D. cirrata took an intermediate retention level at 37.3%.  R. canescens had the highest level of retention at 45.3% (Anderson 326).  Imagine how handy it would have been to have that variety on my employer's roof this afternoon; I might not have had to kick off my open-toed shoes and fjord the alleyriver to clock back in!

  Following the lab trials, two separate outside roofs were planted edge to edge with R. canescens, the top performer.  It's stormwater performance would be compared to a set of established green roofs that were either planted with vascular plants, or containing only medium, or containing only a liner.  Runoff was then measured during a natural rainfall and two simulated rain events conducted between November 2008 and February 2009, the active growing season for the moss.  Simulated trials were held December 1, 2008 when the material was dry and again December 17 when the moss was still in a saturated state.  The moss was exposed to natural rain January 6th and 7th, 2009 (Anderson 324).

The results showed that the moss roofs held their own and surpassed both the vascular plant roofs and natural medium roofs in their water retention abilities.  In every trial (see specific data in table below) the moss outperformed the factory medium and the vascular green roofs (Anderson 327).  Bless the liner's heart, it does look like it tried, don't forget to give it a round of applause!

So, what does all this mean for green roofs and rooftop gardens?  Moss is a lightweight medium that reduces total load capacity on roofs.  It is able to survive and reproduce with little to no water and it creates a natural barrier to water runoff following storms.  The Oregon tests indicate that a much thinner layer of moss in comparison to the natural medium can hold comparable amounts of water (Anderson 329).  Moss transplants easier than root system plants and after establishing itself in the environment, moss can go long periods in a psuedo-dormant state of dessication without dying or killing plants around it.

There are many more moss species to study in-depth before a 100% guarantee that they are more effective than sedum in green roofs can be established, and with any living substance, there are a great many variables to consider.  Among these is the knowledge that many forest mosses that are used to growing on already living trees may perform poorly in urban conditions.  It is interesting to note that the Oregon study mentions bryophytes (a fancy name for mosses and other non-vascular grow-y things) can be cultured with success on perlite substrate, a major ingredient in green roof medium (Anderson 332).  

Extensive research will need to be conducted to prove moss as a viable light-weight medium for green roofs.  Until that research has been more extensively conducted, I'll leave you with a last image, from the Asheville, North Carolina Arboretum, this breathtaking moss roof was installed in June of 2012 by Mountain Moss Installations:

Unlinked Source: 
Anderson, Malcolm; John Lambrinos and Erin Schroll.  "The potential value of mosses for
     stormwater management in urban environments."  Urban Ecosystems 13 (2010):
     319-332.   Springer via ProQuest.  Accessed April 18, 2014.