Living roof maintenance and development
A living roof is a form of garden - it requires maintenance and will reward you with seasonal changes.
I'll say it now: if you want a maintenance-free, cheap, green roof, get green tiles. What is popularly known as 'green roof' is known professionally as 'extensive roof garden' which is more exact. First of all it's a garden, a collection of various living and developing plants, housed in a custom-made environment, which happens to be a roof, and that complexity is reflected in the cost of design and materials.The other interesting word is 'extensive' which really only makes sense if compared to 'intensive green roof' which in fact is a whole roof garden with deep soil or containers, hence heavier, costlier and with greater need of water. An extensive green roof covers your roof more or less completely, with plants growing in shallow soil, therefore weighs less, and is not meant to be walked upon except for occasional weeding - the intensive ones are really roof gardens in which you should be able to hold parties with your friends and dance all night. You can't do that on extensive green roofs because they can't hold much weight, and after you've finished weeding them your legs won't either.
All reference material does mention that occasional weeding is necessary until well established. What does 'occasional' mean? Here I'm sharing the truth about my green roof to show why good planting and design matter, and why periodic maintence should be taken into account for the first couple years of a green roof's life, and should be included in a contract for a green roof as part of maintenance budget.
One month after planting the roof looked great. I spotted a few weeds I meant to pull up, idly thought it looked like a familiar edible plant, then went away for a month. I came back to a roof that was covered with lush green growth - a jungle of Redroot Pigweed (Amaranthus retroflexus). Yes it is edible, but it also eats up whole roofs. While superficially it looked very succesful, I had to get rid of them because their rank growth would starve or shade out the plants I'd chosen to cover the roof. There is a saying: One year's seed, seven year's weed. And this year I'd have to stay on top of them. But where where these weeds coming from, since the mulch should have prevented them from getting blown in by the wind? It was clear that the great majority had actualy piggybacked into my garden on the back of my chosen plants, because they were deeply rooted into them. In particular in the Sempervivum, which has a very fibrous tough rootball, meaning you have to literally rip the whole plant out of the root, clean it, stick it back in, and cover it again with gravel.
Three day of work gave me ample time to mentally redesign the roof to accomodate some kneeling space for weeding. This fall I've weeded again and added some plants, replacing the ones that were only marginally drought tolerant and could not survive the desert conditions of our summers.
Now, as winter creeps in, is the time to appreciate the changing colours of the Sedums, turning red with the frost. The newly introduced Stipa tenuissima waves gently in the wind, and the rigid shapes of succulent plants, which will carry their dried flowerheads long into winter, capture the raking light.