Green roofs are popular now and a huge question I have is with respect to plants developing root binding due to the modular tray walls. I am sure there are many satisfactory answers out there however I am yet to be convinced!
With trays and modular systems - not only will the planting media 'float' out of the tray in a heavy storm event, but the tray walls restrict horizontal growth - and all plants tend to grow outward from their original diameter. Once roots start circling they strangle each other, competing for space.
Mats allow for the roots to grow as far as necessary in any direction. If there is another plant in the way, then competition kicks in and the most aggressive species wins over - however the end result is that the green roof system is always vegetated.
Florida is a tricky climate to use any green roof system in - let alone modular systems or trays. Our climate is totally different than the climate up north and though trays and modular systems may work up north - they are much more difficult to make work in Florida.
Of course - if you want to spend $$$$$$$ on maintenance and design then trays and modular systems can be designed to work acceptably, but why spend the extra dollars when the mat systems will provide the same amount of 'green', put less of a structural load on your roof, and do not require irrigation!
Check out MetroVerde's website - they offer a mat system that has proven itself in Florida - heat, humidity and tropical storms (see the posted video of tropical Storm Fay on Green Roofs)
The two photos here are of sedum center dieback from the plant being root bound, and the actual root circling action. Modular trays are not good for the long tern health of the plants on Florida Green Roofs.
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