Choosing Green Roof Plants - Why the Equation ""#S/LAmm2lt200"" is so important.

Today we discuss again the importance of understanding leaf structure when choosing the appropriate green roof plant, and specifically the part of the plant most commonly called the stomata.  If you recall from biology class, stomata are the openings in the leaf that allow for CO2 to be taken in and O2 to be released.  The stomata are operated and regulated so to speak by the stomata guard cells.  OK but what does that have to do with Green Roof Plants?  Its important!  Lets see how...

All plants need water in their leaves for photosynthesis to occur.  If the plants dehydrate and have no water in their leaves, they quickly die.

Now envision two plastic bags, one 100 small holes and the other with 10 small holes.  Fill the bags up with water and what bag holds the water the longest?  Same concept applies to stomata and leaves.

Looking at the equation above - "#S/LAmm2lt200" translated says based on our studies - the ideal green roof plant for Florida and the Southeastern US is a plant where the number of stomata (#S) per leaf area in millimeters square (/LAmm2) should be less than 200.  Sorry for the way the equation looked but Blogger kept thinking I was writing HTML code and refused to let me save it any other way - and since I am a plant person (and a lawyer by education too) I didn't want to take the time to solve this issue.

OK but what plants have a stomata ration of less than 200 per mm2?  First of all lets look again at leaf structure.  Waxy plants tend to have lower leaf stomata per leaf surface area ratio.   What are waxy leaved plants?  Some examples are cactus, agave and others. Other plants, typically with a soft, non-waxy leaf have much more stomata per mm2 than the cactus and agaves.

Plants that probably will do well on a low-maintenance, non-irrigated green roof can generally be chosen successfully from the cactus and agave families (and others - we will get to those in another discussion).

Plants such as perennial peanut and other commonly thought of as drought tolerant plants have little or no wax and 4-40 times as many stomata, meaning they loose their leaf water much quicker.

Now the above discussion is really a big over-simplification of the entire process.  You see perennial peanut as much more aggressive operational controls over their stomata - slowing down the opening during hot periods or periods of drought to conserve water.  But the lethal interior leaf temperature on most plants is in the area of 110 degrees F (45 C)and above.

As I've mentioned before in the blog notes I record leaf surface temperatures of 140 degrees F ( 60 C) and under these conditions the plants most all first suggest just do not survive long term on a roof.

So know your green roof stomata equation and enjoy a low maintenance green roof.

Does this mean only cactus?  Absolutely not. 

Waxy leaf plants may be a plant to use.  Those species with tougher, leather-like leaves can also do well.  Then there are those more herbaceous plants that just do well in the sun.

Native species of ferns sometimes have a low stomata to leaf surface area ratio.

A green roof can look beautiful!

Call us and we will help you with your upcoming project.  As always, feel free to email us with your questions.

Check back soon for more species or call Kevin 904-294-2656.