Watch out for patterns on a green roof. The randomness of natural plant habitat is far more interesting to the subconscious than pretty and unnatural geometric patterns.
Having walked and waded through Florida's swamps most of my life, I've had to always watch for alligators, poisonous snakes and biting insect nests. Swamp trekking involves slow tedious movements, especially if the water you are walking through is dark and you cannot see what you are stepping on. Logs, holes and other obstacles often times lie just under the surface making a fast get-away nearly improbable.
Importantly most of the potentially dangerous animals in a swamp are non-aggressive unless bothered. An alligator typically (I say typically because there have been known aggressive and fatal attacks) won't bother you unless it feels threatened. Same way with rattlesnakes and stinging insects.
A water moccasin is a mean creature and just as often as not they will chase you. Pushing my way through three meter tall saw palmetto along the edge of a creek one day I had a large water moccasin, Agkistrodon piscivorus, jump out of a saw palmetto, Serenoa repens, and begin striking at my legs. In my attempt to retreat I tripped and fell into the water where the snake continued to pursue after me and left with the choice of dispatch with my machete or have the moccasin bite me between the legs I cut the reptiles head off swiftly. Unfortunately, the snake's aggressiveness is embedded rather strongly within its DNA and the severed head continued to jump and strike for most of a minute, quite frighteningly.
I say all of this to make a point about biodiversity and green roofs.
One of the many things I've learned from the swamps is a subconscious awareness of patterns. When I am in the field where dangerous animals are located I look for patterns. In the plant community there are few large geometric community patterns other than the flower. But on the ground plant habitat is always random in visual nature. However, when my eye catches a glimpse of a solid geometric pattern in length, width or height I immediately look to see what type of animal I am approaching.
A Eastern Diamondback Rattlesnake, for example can kill an adult quite quickly with its potent venom, yet this snake just wants to be left alone. So if I am in a thicket of plants and look ahead to see the distinct pattern as portrayed here in Wikipedia, I change my path so as not to interfere with the creature.
Plant communities will always exhibit more randomness in their visual patterns than animals. This is true even when walking through a seemingly uniform pine flatwoods area covered in meadowbeauty or wire-grass. The habitat may seem uniform yet there is the randomness of spacing between the pines and between the plants.
Recognizing patterns has saved me from stepping on a still lying 'gator' many times.
And the random beauty of plant communities, the lines, curves, colors and hues is nature's way unique way of fingerprinting - everything is unique. There are no exact and identical plant formations in nature.
Complicated uniqueness is the focal point of bio-diversity. Each plant system or wildlife community is diverse in biological constituency, hence the tern 'bio' - 'diversity'.
A nature-focused, native species green roof should be designed accordingly.
The subtle inundation's and changing ground elevation lines should be reflected on the roof. Green roof soil media gradations of 10 mm to 50 mm across a relatively flat stretch of roof are important. Elevation changes in living roof soil media provide much more than visual interest - they serve as wind breaks, assist in the collection, transport and storage of rainwater, dew and fog and provide shelter for visiting wildlife.
Too many times we may simply rake a green roof sol media flat. Allowing slight fluctuations in the soil media mimics nature and ultimately may allow for more chances at green roof community planting successes.
The same holds true for species mixes on green roofs. The general rule for a nature-based biodiverse planting is the 10-20-30 rule. The 10-20-30 rule guides us to plant no more than 10% of the green roof with any one Species, 20% of the roof with one Genera and 30% of the roof with one Family.
Monoculture plantings on green roofs are confusing to wildlife and provide sub-standard communal and foraging habitat.
And while it may be easy to call out in design the one or two proven plants for a green roof project, the long term habitat value will be diminished with a green roof that only contains one type of plant. Remember, even if the design intent is to have a solid, one color and one texture appearance across the roof, this can be accomplished with using a mix of similar appearing species. One does not have to create monocultures on green roofs to achieve design intent!
Learn to recognize patterns, and shy away from them.
A good, biodiverse green roof planting design should afford stunning random beauty. The key to infinite design beauty is total plant randomness.