Green roofs can extend the life of a roof membrane, but how easy is it to ensure the vegetation will still look great in 15 or more years’ time? Firstly you have to get to the bottom of exactly what performance your client expects, which includes how they expect it to look over the years and changing seasons. I often talk about this in relation to the initial design, but it then has to be applied to the maintenance programme as well.
Ironically green roofs are one of the most stressful environments in which to establish plants, and yet they are also expected to look great with hardly any maintenance. While this is not impossible, it does take a bit of thought and considerable horticultural expertise.
There are four main ways to reduce the maintenance burden, and they can be applied as appropriate to achieve the client’s desired outcomes.
1) Reduction of plant growth.
By making the plants grow more slowly you are reducing the need to cut back and remove excess growth which can look untidy. This can be achieved by using ultra low nutrition superstrate and/or choosing slower growing plants. This naturally will have an impact on how quickly full plant coverage is achieved, and how lush and flowery the planting will look particularly in the short term.
2) Reduction of weed establishment.
The low nutrition superstrate is the best way to avoid weed ingress. However if lush plant growth and fast coverage are required then denser plug planting can be used to ensure that there is less space for incoming weeds to establish. Sowing annual seed to fill spaces while the perennials establish is also a good trick.
3) Acceptance of changing plantscape.
Letting plants evolve into a managed ecosystem involves less maintenance than a designed plan in which plants are expected to stay where they are put! However this is a significant departure from the organised swathes and arrangements common to traditional landscapes.
4) Randomised planting.
The mixing up of species is another departure from the ‘normal’ landscape planting, which allows a better resilience to changing seasons and weather conditions. Encouraging plants form a community means that should one species suffer from harsh weather or over-enthusiastic wildlife, its neighbours are less likely to be affected.
Unsurprisingly both the substrate choice and planting design are the keys to controlling maintenance, and must be related at each stage to the client’s visual expectations!