Talkback: Plants for green roofs with soil depth 100mm

I don't have a green roof, but what I do have is a back garden that my partner had previously "landscaped" with nothing but gravel and paving stones, that I use the same way. By raking back the gravel and adding a small amount of potting compost mixed with gravel and sand, then re-covering with the clean gravel, I have created an extremely shallow bed. The gravel holds heat and a tiny bit of moisture, and although I can't plant it too thickly, we have had a great display of Livingstone Daisy (reared in the greenhouse and planted out as seedlings), and both Californian Poppies (Eschscholzia californica) and normal field poppies scatter sown. Some self sown pansies and violas are also creeping in, and this year the real success story has been Lysimachia monelli/Anagallis monelli/Blue Pimpernel, also transplanted. You suggest that the Cal. Poppies flower May-June, but if you either leave some seedpods on your earliest plants to self sow, or scatter new seeds in July, they will keep going until it frosts. It is September and not only are some of mine sown in May still going after being cut back, but, but seeds self sown by the ones I didn't cut back have germinated and are starting to flower. The gravel keeps them warmer at night so I get a longer season, and they seem to come back very reliably, even after a hard winter.



I would offer 2 tips: (1) Don't assume "green" means to cover every scrap of space with a plant, and (2) scatter seeds and see what comes up. Your conditions are going to make more difference that what it says on the packet.