Saturday, August 21, 2010
Plants for Green Roofs - The new MetroVerde Carbon Pirate Variety
First off, even though we are a registered nursery in the State of Florida we do not generally sell green roof plants. The plants here and the way they were raised are for our own green roof projects. We are sharing the information of both our successes and failures in hopes we will someday have a greener world. Our commitment is to restore volumetric green to the urban core.
We've mentioned before in the blog the practice of hardening off drought tolerant and succulent plants before installing them on green roofs, but today MetroVerde officially rolls out its Carbon Pirate (trademarked) line of green roof plants.
What is so special about the MetroVerde Carbon Pirate plant line is the time and preparation put into the plants in anticipation of roof duty.
Realize we focus on the Southeast US and especially Florida, so our approach may not be the best approach for other regional climates. But here in Florida - green roofs face the big 5 H's, Heat, High Humidity, Hard Freezes, Hard Dessicating Winds, Hurricanes, Heavy Downpours, Hot Periods of Long Drought (Looks like 7 and I could add a Host more) - the point is that once you step up onto a Florida roof, no matter what time of the year, your breath can be taken away. And you understand why you cannot just plant a nursery grown specimen on the roof. May look good for a few months but after that - well just look at most of the previous attempts here across the state (except for a select few).
Drought tolerant plants possess specialized physiological processes that respond to changes in nature such as rainfall patterns and temperature. They have developed these functions over the eons and rely on these adaptations for survival in areas where water supply is unpredictable. Many have very shallow radial root architectural structure with diameters that exceed many times the plants actual form. These roots want to grow laterally rather than downward in response to the infrequent rainfalls typical of their native habitat. During a light rainfall event the stormwater tends to barely penetrate soil surface (depending on soil types).
As the rain ends and the water either evaporates or infiltrates, the drought tolerant plant broad surface area of roots responds quickly, storing the liquid for later use.
Additionally the drought tolerant have developed an array of mechanisms for dealing with drought such as CAM - or Crassulacean Acid Metabolism, and we will discuss these in upcoming posts, but lets get back to root structure.
For example, a two foot tall cholla plant often has a thirty foot architectural root diameter. The fundamental distinction of any drought tolerant plant is the broad lateral root architecture. Note the difference between some plants that do well in dry areas yet long, vertical systems that tap into the ground water supply. Examples of these would include; Acacia, Parsley, Burdock, and Mesquite (whose roots have been know to grow down vertically over 200 feet!).
In choosing a green roof plant you first want to select one with roots that want to grow radially, not vertically. But there are logistical limitations with plants that have radial root systems - they do not like pots or containers. They need room for their roots to grow.
Recall the times you have gone to a nursery and purchased a plant and returned home to find the plant's roots had circled and circled the inside of the pot - we call this being 'root bound'. Not good for the plant.
However, practically speaking nurseries are not going to grow one plant in a large tray because of the economics of space, real estate and cost-effective efficiency (I couldn't afford to either).
Hence the dilemma. A drought tolerant plant grown in a two inch deep container that is three inches by three inches wide will not survive over term on a green roof. The available functioning root area will only be approximately four and one half cubic inches. Now as long as the plant is in the green house with irrigation applied (strange to think of applying irrigation to a drought tolerant plants) the plant will probably survive. However, if raised in a small pot and irrigated, the green roof plant when planted on a roof most likely won't make it.
Drought tolerant plants for the most part have water intake processes that work gradually. They are used to taking in required water volumes at a lower rate per root surface area using a large root diameter rather than absorbing large volumes of water with a limited root surface area.
The change from greenhouse container with irrigation to the roof is usually just too much and the plants will not function as expected without intense maintenance, watering and fertilization.
Though a topic for another blog session, my firm belief is because here in Florida we have a severe water crisis and nutrient pollution crisis, designing a green roof with irrigation is a fundamentally not environmentally sound. I know, the cistern argument - hold the rainwater and irrigate with rainwater, but anyone with a cistern in Florida will tell you it is empty most of the time and then you have to use potable makeup water..... I'll reiterate - my firm belief is because here in Florida we have a severe water crisis and nutrient pollution crisis, designing a green roof with irrigation is a fundamentally not environmentally sound.
Working with some of our green roof species we've 'trained' (this almost sounds cruel) certain plants to bridge the gap between being raised in containers and roof plantings. Using drought tolerant plants (we will discuss in future blogs) - and let me note here there is a difference between drought tolerant plants and the succulents and cacti - we are not referring here now to succulents or cacti - using drought tolerant plants grown in standard nursery containers, we apply restricted watering schedules over a long period of time - usually over a year, to harden off the plants and prepare them for roof service.
These plants are grown in hot greenhouses with background humidity levels and irrigation amounts of less than 1/2 inch per month.
Once installed on a green roof with a mat based foundation, their roots quickly grow out laterally and with cumulative input from humidity and natural rainfall, they survive and begin to fill in the roof area. In contrast to the irrigated, happy plants that are fatally shocked once installed on a roof, these plants are relieved to find their home on a roof. We call these plants MetroVerde Carbon Pirates (trademarked). They sequester C through respiration, produce oxygen, clean stormwater, provide habitat and create a much need sense of place.
There is so much to talk about here - tray systems versus mat based systems, whether or not to irrigate, cost, weight, fire hazards, maintenance, volunteer species, disease - but we have plenty of more blog discussions coming! In the meantime happy green roofing and as always you can e-mail Kevin with your questions - firstname.lastname@example.org.