By: Jörg Breuning & Samantha Yurek
"A weed is a plant that has mastered every survival skill except for learning how to grow in rows." - Doug Larson
Photo Jörg Breuning, Green Roof Service LLC: Hieracium aurantiacum, Orange Hawkweed. Haunted by environmentalists and often branded as obnoxious, farmers and nurseries stamp Hieracium as invasive.
It is all about perspectives, experience and evolution. Weeds are typically plants growing where they are in competition with cultivated plants or simply unwanted by humans. However, seeing it from a philosophical point of view any organism has a right to exist. It is the intention of each organism to multiply, spread and adjust to the surrounding environmental conditions. This adamant approach has also helped humans to develop and thrive over ten thousands of years. The word native has become a stereotype of certain plants that should be growing in a particular region. But with human perspectives being far from uniform, the term 'native' has been skewed over time. The term is irrelevant when looking at the larger picture of evolution.
An unwanted plant on intensive green roofs could be ideal ground cover for un-irrigated extensive green roofs or vice versa. Being an outside element, green roofs will undergo natural succession and evolution as described above. This succession can effect the function of a green roof in the long run for the better or worse. Ideally, maintaining a green roof helps to guide the plants in a stable coexistence with minimal succession, creating little maintenance. In most cases it doesn't matter whether the green roof plants were planted intentionally or not.
Un-irrigated extensive green roof designs tend to have extreme conditions; therefore the plant pallet is rather narrow. It is very difficult for most plants to sustain themselves over decades. Introducing many of these so-called 'weeds' on to a rooftop environment may prove to be more efficient because of the vigorous growing capabilities. Yet many factors such as unique weather events during the establishing phase (3-8 years), artificial irrigation, pre-grown nursery trays, excessive fertilization, can be counterproductive in modern green roof technology.