I saw a planting in Mexico of bougainvillea where the soil depth was about six inches. The planters were six or eight feet long. It looked fine but as J. Musser says if you don't irrigate then the soil dries out pretty fast.
If you are using a soil less mix and shallow planting depth the big issue is some plants are going to die out. Edges and corners are going to be hotter and drier. I sure hope that the planting area is accessible for maintenance and watering or it will look poorly after a while.
Green roofs normally use plants that spread by roots or rhizome so if one or two die the rest fill in. Bougainvillea with it's thorns and scrambling habit should be planted on a wider spacing but if one dies then there is big hole for weeds to start in.
I see a direct conflict here between the practice of a green roof: self sufficient, not requiring irrigation, low weeding requirements and using a sprawling vine which will need water and weeding.
The more I read about using this plant the less it seems appropriate for a green roof. From here
- Unless they are pruned regularly, bougainvillea grows into a tangled
mass of old and new growth, and overcrowding often leads to pests and diseases
- It should not be planted in extremely windy, unprotected
areas Bougainvillea is very susceptible to girdling during a
storm. The bark will rub off at ground level when stems
whip in high-speed winds.
- needs regular fertilizing with formulations having NPK ratios of 1:1:1 or 2:1:2. Applications of soluble minor elements help prevent leaf chlorosis
- Bougainvillea tolerates drying and if possible irrigation should be adjusted to be a little on the dry side. They are sensitive to over watering but should not be allowed to completely dry out.
So your client wants to put a plant on a roof where:
- the recommended planting distance is six to nine feet spacing
- it needs regular pruning
- should not be exposed to high winds
- fertilized regularly with all the issues that entails with salt buildup in the irrigation system and soil and ensuring consistent application levels
- should not be left to dry out totally yet should not be over watered
How about you suggest an initial planting of ice plants to fill in the spaces until the bougainvillea grows in? That way there is something to stabilize the soil that looks good right away and if the bougainvillea does great it can shade out and out grow the ice plants in time?