As the summer months approach, keep a look out to see how well any green roofs around you cope with less water and more heat. Here is my 4 point guide to dealing with summer conditions by clever use of plants and substrates.
1. Pick the right plants!
Moisture retentive succulents are great especially in the drier southern and eastern parts of the UK, and it’s worth mixing at least a few succulent species through your roof planting. Some of the hardiest, most drought resistant wildflowers like Centaurea scabiosa and Dianthus deltoides will be the best choices, alongside oft-ignored alpines like Petrorhagia saxifraga.
2. Decide how nice they need to look.
There’s a substantial difference between plants surviving drought and plants looking good through a drought. To achieve a reasonably healthy-looking roof with minimal irrigation the plant choice in point 1 will be the key. However a broader range of planting can be successful if you can either make provision for additional watering when needed, or simply accept that during a drought they may not look so fantastic.
3. Depth is king.
Whichever plants are chosen for the roof, they are rather dependent on the substrate they are placed into. Any plant in too shallow a substrate will struggle for water – see our depth diagram on the guides and tools page. All too often substrate levels are cut to reduce costs, and this will have a negative effect on the drought resilience of the green roof.
4. Consider local rainfall in your substrate design.
Looking at the local conditions your roof will face is really helpful because you can choose the best substrate to suit the expected rainfall on your roof. In dry areas it can also make a big difference to the roof aesthetic – green roofs with a high visual impact should really have a different substrate from survival or bio-diverse roofs, as the plants’ requirements will be slightly different.
All these points are closely linked and should be considered at the design stage of a green roof that is to thrive in our changing yearly weather and climate. If you do see any green roofs that are suffering in the sunshine this summer, see if you can figure out why!