|National Weather Service's Historical Rainfall Maps|
Green roof plant modeling process considers light and wind to be the two primary design variables for factors affecting green roof plants. Sunlight relevance to green roof plants and for that matter even ground level landscape designs is usually referred to in terms of 'Photosynthetically Active (and Reactive) Radiation, or PAR. Too much PAR and the plants can burn, desiccate and wither. Too little PAR and the plants fail to grow.
Along with PAR is the photosynthetic pathway of the green roof plant and a host of other survival mechanisms such as photoperiodism, phototaxicity and phototropism.
Wind impact too is a design variable that must be accounted for during green roof plant layout. Strong desiccating winds can harm green roof plants with as much severity as PAR overload. Wind can pull so much water out of a leaf that the plant's vascular system will be overwhelmed and interestingly, no matter how much green roof irrigation is added to the planting bed, the plants still die.
Micro-irrigation usually alleviates the stresses of long droughts and so on those green roofs, available rainfall impacts may not be as much a controlling design variable (though still extremely important) as wind and light.
Still I find it very interesting to study rainfall patterns across the U.S. and across the world. Nature has laid out and sorted the different types of vegetation across our continent in a manner relating to wind light and also according to rainfall amounts.
Yes, it is a simple and very broad generalization to say that following Mother Nature's lead supports project landscape or green roof plant potential success.
When I look at the above map depicting historical rainfall amounts published by the National Weather Service, I see three main, broad patterns. The Northwest and the East (red and orange areas) receive most of the rainfall across the U.S. Broad leaf dicots and C3 monocots fill these regions. Florida and the Central Plains (green areas) receive less than average precipitation and are vegetated with great stretches of grasslands. Here in Florida the pine flat woods which make up much of the state are filled with C4 ground cover grasses such as the Andropogons and Sporobolis species (if you live in Pensacola though you may want to choose wetland plants for your green roof due to all the rain they have been receiving lately). Finally, the areas depicted by the least rainfall amounts (less than 20 inches per year - light blue geographic regions) are inhabited by cacti and other succulents.
So if I were designing a green roof for an area outside of Florida I'd think of this map first.
I may or may not end up following Mother Nature's lead after examine a complicated host of other factors, including client intent and if I choose not to follow then my green roof planting design better be spot on in producing the ecology my selected green roof plants will require.
The roof is a seriously harsh place to grow plants. Use of Mother Nature's millennia of trial and error as guidelines for selecting green roof plants is smart green roofing. It is hard to beat local native plants on the roof or across the ground.