Plants like humans respirate (take in and expel air). Plants do this through their leaves, as well as through their root system, absorbing oxygen needed to break down glucose, creating energy for the plant to develop.
This process of converting glucose releases some Carbon Dioxide (CO²) which is also expelled through the leaves and roots.
When a plant stands with its roots in water, and has no adjusted root system like aquatic plants, the required oxygen/air commonly available in the soil is displaced (blocked) by water, resulting in the plant not being able to breath/respirate (aerobic respiration).
Due to the lack of oxygen, the plant respirates anaerobically, producing alcohol, that eventually kills the plants root (root rot).
FLL Guidelines are widely accepted as the leading authority for the planning, execution and upkeep of Green Roofs.
It could be argued that FLL Guidelines would not apply, yet from the other hand, what the standard describes has some food for thought.
Here in coastal BC, we face prolonged periods of rain of up to 6 months (fall and winter) and not all options for watering are equal. In some instance, plants are standing in a water-logged medium, drowning rather than thriving.