There is one question that needs answering before any green roof maintenance plan is designed or implemented. And that is – how is it supposed to perform? Understanding what a green roof was put there for will help you to maintain it successfully.
There are some basic principles that apply to any green roof, like making sure the plant-free areas stay plant-free and all drainage outlets are kept clear. Watering during the first 9 weeks establishment phase is also fairly standard.
Aesthetic performance is becoming ever more important on overlooked roofs so an understanding of what the green roof should look like is important. Is it understood as a wild and natural space or viewed as a ‘flowery’ landscape? How neat should it look? This will determine how many trips need to be made to remove spent flowers and dead plant material.
Ecological performance is quite often specified as a requirement, and the ability to understand which wildflower species are important and which can be invasive weeds is very useful. For example nitrogen-fixing plants such as clovers can be fabulous for supporting pollinators, but can take over from their neighbours if left uncontrolled. This reduces the plant diversity, over-all flowering period and attractiveness to other insects.
As a rule of thumb, I would recommend that any plant starting to increase to over 20% coverage year on year on a biodiverse roof should be held at bay to avoid stifling other species. Green roofers should beware of condemning a species as invasive if it happens to be thriving more than its neighbours in a particular season or in a challenging year, so for this reason continuity of maintenance is important.
We’ve not even touched on grasses – needless to say an understanding of whether they are designed to be part of the planting scheme or not is very helpful! There are many suitable grasses like briza media which can make a lovely addition to a biodiverse or aesthetic green roof, but the 20% rule of thumb can be applied to ensure that more aggressive species like couch grass don’t take hold.