I posted about my green roof in April last year, and thought it might be helpful for anyone considering doing a similar scheme to post an update. Here in Gloucestershire, this year has been so much wetter than the previous two, and it has made a big difference to the plants on this unirrigated roof. I've decided that I'm not impressed with the system I used to construct the roof: I don't believe (though admittedly I haven't excavated to find out for sure) that the plant roots are able to penetrate the filter fabric to get through to the water reservoirs in the honeycomb structure below, and the substrate mix dries out in a flash, as well as compacting into a surface which appears pretty hostile to plant life. Having built-up mounds as I have constructed looks nice but must make the growing conditions more difficult for the plants, as water runs off and the substrate, being raised, dries out fast. In dry conditions, many of the plants that aren't succulents, tend to wilt quite fast.
Saying that, it's actually looking really good at the moment, and is a delight to look out at from the upstairs windows, and even from below, it's a surprise to see flowerheads waving in the breeze above the roof fascia. When I think of how sterile it would look if the roof was just flat and black, I can't believe anyone doesn't plant their flat roof! The planting is an experimental mix of succulents, mostly grown from cuttings (very easy) and wildflowers, mostly grown from seed, with the more successful self-seeding prolifically. There are also a few annuals like Mesembryanthemum, for a bit of unsubtle razzle dazzle in the (occasional) sunshine! I've included some rockery plants like Helianthemeum, Alpine Phlox, prostrate Thyme and carpeting Campanula, as well as Chives and Alpine strawberries, which are thriving and spreading. Of the wildflowers, Viola tricolor (Field Pansy), Bladder Campion and Toadflax are doing best. Some grass has found its way in, and I mostly tolerate it, though some gets removed on rare maintenance visits. There are quite a few weeds, and it can look quite messy at times through a critical designer's eye, but the weeds don't automatically get removed, as this planting is for wildlife as well as for aesthetics, and I've enjoyed watching Chaffinches picking up weed seeds, and Blackbirds taking bits of dead stalk for nesting.
There are still some bare patches, particularly on the shallower edges of the mounds, but one of the Sedums is steadily colonising any available space and I shall let it have its way on these bits which nothing else wants to grow on, and weed it out where I have more interesting alternatives.
My advice to anyone planning similar would be, first of all - go for it, it can be beautiful and diverse and good for wildlife. On the practical side, research the substrates and build up, and if the roof is not to be irrigated, ensure that there will be enough goodness and body in the soil to hold moisture and support plant life. Bear in mind that mounds will dry out even faster, and if in sunshine, you may need to stick to succulents unless you are prepared to experiment and replace failures. There may be ways to re-use 'grey' bathwater by siphoning out to the roof if you have a bathroom high enough above, which would help the plants greatly, particularly in the early years. Finally - enjoy!
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