Plants and Planting: What's Possible?
Type of plants and planting vary according to the depth and type of substrate, the location, elevation, exposure and slope. Most green roofs have a elatively thin layer for plants to grow in compared to soil depths on the ground because of the limitations of weight and the amount of materials required.
Thin growing media and good drainage mean that many green roofs tend to dry out, so plant species which can withstand dry conditions and/or recover well after drought are specified. Sedums are widely used, but coastal plants like Thrift or Kidney Vetch and other species of grasses, perennial wildfl owers and annuals also make good green roof plants. Plants with large tap roots do not grow so well and can damage the roof structure. Woody shrubs like Heather are best avoided as they can build up dry material which is a fire risk.
There is significant opportunity for creative design of plant communities with locally tailored mixes or planting to encourage wildlife. In many cases the precise conditions on a roof are not known in advance so a range of species are often used, with some thriving and others not; the mix of plants in time ‘evolve’ to the conditions.
In extreme drought, plants may die off and regenerate from seed set in previous years so the appearance of a roof may vary a lot, which should be regarded as one of the attractions. Plants can be established as seed sown directly onto a roof or added as plugs, turf or ‘blankets’ of pre-grown plants.