Maggie Fennell, of Boningale Ltd, gives her thoughts on this misplaced belief regarding planting on extensive green roofs, and outlines some of the different species which can be used, as well as the maintenance considerations which must be taken into account…
Extensive green roofs are traditionally dominated by sedums for their extreme drought tolerance, or wildflowers chosen for biodiversity support and preference for low nutrient substrate. These survival skills make them natural candidates to cope with extreme weather conditions and the harsh growing environment of a rooftop.
Low-growing sedums are often chosen to survive on shallow substrate depths and can provide fairly uniform winter cover. Wildflowers are mostly deciduous and create big bursts of flowers and a longer display of colour, with greater provision of nectar and some reaching a height of half a metre or more. This variety of plant forms and flower types is great for wildlife, and even once they have finished flowering and died back they provide valuable resource to insects and the local ecosystem. The removal of dead material should be considered at an early stage of development to ensure there is adequate access to bring excess biomass down from the roof, and scheduled in accordance with biodiversity objectives – refer to section 5 of the GRO code for more details.
However, dispelling the myth mentioned earlier, there are many more plant communities that can thrive in these conditions, particularly when you have the full 150mm substrate depth to allow for extra root growth and storage of water that will be accessible to the plants. These may be particularly useful to consider for extensive roofs that are overlooked, or those with challenging limitations for removing excess plant material but requiring a varied plant range for biodiversity and aesthetic performance.
Coastal, alpine and steppe/prairie ranges that can offer a variety of low-growing, evergreen or semi-evergreen plants with a wide variety of forms and flowering seasons would fit the bill – as long as they are hardy, drought-tolerant and happy with low nutrient availability. Look for plants with silvery, hairy or waxy leaves, dwarf cultivars and a long flowering season. Low-growing varieties might provide great pollinator support with less requirement for material removal and maintenance, and plants such as small ferns could be helpful in shady areas. The RHS gives further guidance on optimising biodiversity support when using non-native plants, such as avoiding double flower forms. Bulbs can also be a great, economical way to extend the flowering season.
When putting together a plant mix, think about how the community might develop over time. A range of different plants will react differently to extreme weather conditions providing greater resilience. It is also possible for some species to gradually disappear over time or be crowded out by vigorous neighbours, which may only become apparent over several years.
The maintenance planning should consider whether the emerging plant community is still robust enough to withstand next year’s unknown weather cocktail, or if it has become dominated by one or two species that could become vulnerable and die back – as can be the case when invasive grasses take over.
There are further important practical considerations for your plant palette including the lead times, growing season and commercial availability of material. A large range of perennials may be commercially available as 9cm pots rather than plugs – although plugs can also be available and are advantageous for quick root establishment. If 9cm (P9) plants are used there may be more displacement of substrate depending on planting density.
Plants should be grown in a peat-free medium which supports the root integration into the substrate, as well as the wider responsibly-sourced growing media agenda.
Many species can be requested to be grown to order in the required size in season from March – July.
My ‘top tip’ for successful planting is to be clear on what performance you want and design accordingly. Readily available wildflowers and sedums could actually provide a more resilient community when combined rather than used separately. As we have ever greater need for clever optimisation of space and thrifty use of resources, we can employ the wide diversity of beautiful, multi-functional plant life to create the best nature-based solutions to support wildlife, people, and the planet.