Maintenance on a green roof varies depending on each project's specific variables such as sun and wind exposure, seasonal rainfall, type of vegetation, soil depth and more. You can design a system that is low maintenance but do not make the mistake of thinking that a vegetated roof is 'no maintenance'. Certainly intensive systems (deeper soil and wider variety of plant species) undoubtedly need seasonal maintenance. Apex Green Roofs includes 1-2 years of seasonal maintenance visits as part of the install quote to ensure that vegetation thrives during the establishment period. Occasionally facility managers decide to pass on continued maintenance services in subsequent growing seasons because the vegetated roof looks great. Clients who drop maintenance always come back to us within a year or two asking for us to fix up the green roof because it no longer looks good. What happens is the initial cost of maintenance care shoots up for that first season due to increased time and materials needed to remove invasive species and nurse back healthy growth.
Here is a great article about a 13,000 sf green roof installed in 2011 at the Burlington International Airport that gives a very accurate overview of the general challenge green roof advocates, growers and installers experience in trying to convince green roof owners not to skip seasonal maintenance care. It doesn't have to be a lot. A little goes a long way.