The discussion of the different types of plants focuses on varying patterns of photosynthesis metabolism processes. All of this may sound complicated but it is really quite simple and the reason we are distinguishing between plants is so we can predict behavior and select the best plant for each individual and unique green roof!
Having the ability to predict how plants will survive, thrive and grow on a green roof is vitally important to the green roof profession.
Moreover, understanding how plants will behave allows for the project's economic costs and impacts to be analyzed. Of course, there are many variables to consider when choosing what blend of C3, C4 and CAM plants to use, factors such as;
- Wind direction and speed
- Sunlight exposure
- Temperature ranges
- Average humidity per month
- Rainfall amounts per month
- Existing local seed source
- Nearby allopathic plants
- Smog and air pollution
- and other factors
The "assumption of relative consistency" is a phrase that means - we know what to expect.
So for green roofs the assumption of relative consistency means we know there will be times when the roof will be very hot (mid day summer) and times when the roof may be very cold (early winter mornings). There will be times when the green roof will have plenty of sunshine for photosynthesis (when the sun is high in the sky) and times when the days are short and the sun is low on the horizon (winter solstice).
The times when the days are short is when every bit of sunlight is important. During the shorter days there is an assumption of relative consistency that deciduous trees will loose there leaves and sunlight can filter through the tree to the roof.
This assumption of relative consistency about winter sunlight where summer leaves had provided shade is a factor green roof design incorporates.
Yet plants, like children - and many adults - have a mind of their own also.
|Deciduous sweet gum left, wont loose its leaves|
There have been record cold temperatures and the solstice has come and gone so the environmental triggers for leaf drop have been present.
But the plant has a mind of its own.
And the leaves just will not fall off the tree - no matter how long I stare at this tree!
And the greenhouse and green roof are not getting the sun exposure I expected this year.
So the theory of assumption of relative consistency of plant behavior may not always hold true and definitely will have exceptions.
Certainly the exceptions to the rule do not justify discarding plant behavior through modeling.
But what is important is to realize that plants do have a mind of their own and green roof design, though highly technical is also a function of art. The green roof artist has an understanding of how plants may behave on the green roof based not only on assumption of relative consistency data, but also on an inner intuition only a plant person can understand.
Happy Green Roofing!