Thursday's post, "Planting Green Roofs, Seeds, Plugs or Plants" brought in several comments about another Green Roof planting option - pre-grown mats ( usually Sedum ).
Sedum has long been associated with Green Roofs throughout the centuries and across the world.
A quick Google image search of the phrase "Sedum Green Living Roofs" turns up beautiful and stunning photos, most grass-like or lawn-like, very neat, quite contained and appearing to be lightweight and thin.
There are also many Green Roof companies across the globe. Twitter is another good place to keep up with the trends of Green Roofs, plants and technology. 'Sedumdak' is one such Twitter user I follow and Michel Heus' website is located here.
Sempergreen is another Sedum-based green roof mat company found on Twitter and the web, producing 'Instant Green' and their photos truly are beautiful.
The advantage of Sedum mats, or any other mature plant mat is the 'Instant Green' effect a roof receives upon install. Typically, the plant mats are brought in on rolls or on pallets, cut and stacked much like sod.
Moreover, Sedum requires little in the way of green roof soil media depth producing a beautiful effect with a very lightweight. The weight reduction translates into cost savings due to the roof structure design for a lighter load.
Sedum mats are great for stormwater treatment. They absorb rainfall thereby reducing runoff quantities and help keep our waterways clean (as do all green roofs).
Sedum is extremely drought tolerant. I like Sedum because the plant has evolved photosynthetic processes allowing for both fast growth (C3 and primarily C4) under higher CO2 levels, and also CAM - Crassulacean Acid Mechanism processes where plant leaf stomata remain closed during bright sunny periods to conserve water, opening at night to take in CO2. The CO2 is then converted into Carbon malate compounds, stored in vacuoles or around the RuBisCo compounds so photosynthesis can take place internal to the leaf the next day when the sun is shining and stomata are closed.
Sedum, once established needs very little additional water.
Sedum mats are one of the most popular green roof system components around the world and have been for years.
However there are issues with sedum mats, as with any Green Roof system, and while these issues may not be impossible to overcome, they are worth reviewing.
Sedum mats are primarily grown from one Genus of plants, Sedum. The 10-20-30 rule of biodiversity we recommend for 'Green Roof' plantings suggests a green roof use no more than 10% of the plantings from any one plant species, no more than 20% of the plantings from any one Genus and no more than 30% of the Green Roof plants from any one plant Family. There are several blog notes about the 10-20-30 rule here.
This is not to say Sedum mats do not support biodiversity, they do in fact and there are some great studies to show how Sedum is important to wildlife. In fact, some native Sedum are protected by various local and state regulations and a green roof would be a perfect place to assist in a species recovery program.
However in nature you find usually a wide array of plant types in an ecosystem. Monocultures of exotic plants pepper the Florida landscape, a result of our overuse of just one plant Genus. The 10-20-30 rule we feel is important for Green Roofs. Many Sedum mat companies also offer native wildflowers as a supplement to the Sedum mats. Check with Sedumdak and Sempergreen for more information on the value of Sedum mats for biodiversity support.
Many years ago, when I first became interested in Green Roofs some of the first plants I purchased from Lowes included Sedum acre and other Sedum. They were succulents, drought tolerant and beautiful. They did great during the cooler weather but as soon as the afternoon temperatures soared to 140F plus on the roof and the humidity reached the high levels it does during Florida mid-days, the Sedum would develop a fungal infection and turn to mush.
This occurred even in the most well-drained, sandy green roof soils I tried. In fact Sedum and Green Roofs here in Florida ended up not making good companions.
I am aware of university studies going on across the Southeast and am sure a horticultural variety will be engineered to work on Green Roofs in Florida someday.
So while Sedum mats are fantastic for more northerly climates, here in Florida I have a difficult time with them. I also hear the same from green roof professionals in other tropical areas.
However, in addition to seeds, plugs and plants, the option exists for 'Instand Green' in Sedum mats.
As I mentioned before, these mats have been used across the globe for centuries. They provide a great way to treat stormwater and are typically very lightweight and cost-effective.
Drought tolerance is another significant factor, especially since we as a world are facing a water crisis and green roof irrigation with potable water is rapidly falling out of sustainability favor.
Seeds, plugs, plants and mats. Right plant, right place. Know your green roof plant options.