Green roofs are systems that can be controlled and maintained at different levels. Typically, an intensive system is highly controlled, with a variety of specific plants that are often carefully trimmed and tended. Public access is often available, and there may be additional hardscaping such as paths, benches, or even playground equipment. An extensive system is designed to be more self-sustaining, and typically features a small variety of perennial or evergreen plants chosen for their drought-tolerance, low profile growth, and hardy natures.
While brown roofs are strictly, speaking, a subset of green roofs, the philosophy and goals behind brown roofs are different. Primarily, brown roofs are intended to increase biodiversity, provide a habitat for native plants, and support native insects that may be suffering in an urban environment. Brown roofs are often seen as more rugged and more natural.
Green roofs, whether intensive or extensive, feature carefully selected plants that may or may not be native or, in the case of intensive roofs, even well-suited to the environment, while brown roofs are not planted at all. Rather than choosing specific plants, the growing medium is left to self-vegetate from seeds blown in on the wind or carried in by birds. In fact, you could call brown roofs “extreme” extensive green roofs, if that weren’t such a mouthful. They certainly feature a minimalist approach.
The concept is that the natural sowing of seeds will bring in species that are native or well established in the local environment. With little support in the way of irrigation fertilization offered, vegetation on a brown roof will compete naturally and over time the hardiest species will prevail. Some practitioners will start a brown roof off with growing medium and even plant cover crops for the first year or so in order to keep it from blowing away, but as seeds arrive naturally, they will germinate and take over. It will typically take longer for a brown roof to become fully established, and it may not always be aesthetically pleasing, but it is virtually no maintenance and provides important habitat for local birds and insects.
It is important to remember that even brown roofs need similar basic construction (a reliable waterproof layer and consideration for the roof load) as a green roof. It’s definitely not the same thing as letting trees grow in neglected roof gutters.
