Should one plant a green roof in July and August? It is advised that spring an autumn are the best times to install roof plants to avoid extreme weather – an untimely heatwave can scorch plants leaving them brown and unsightly.
In built up areas the aesthetics of green roofs are under ever increasing scrutiny. Clients can have varied expectations concerning the acceptable level of ‘wild’ appearance, with ubiquitous sedum blankets trying to offer a uniform look and biodiverse wildflower roofs at the other extreme.
Both of these planting types can look less than green in extremely hot weather. Sedums are inclined to turn red under stress, and less drought-tolerant wildflowers can look brown and unhealthy.
In today’s more sophisticated market it is possible to be more discerning with plant choice, with a range of different alpines and wildflowers available to suit different UK climate zones. It can also be beneficial to mix up alpines and wildflowers to take advantage of the varied performance of both.
After choosing a good mix of drought tolerant plants, the best way to avoid scorched plants is to allow as generous a depth of substrate as possible, and ensure a watering facility that can be applied when necessary.
When planting it is important to keep plants thoroughly watered, make sure that the roots are well buried and not standing proud of the substrate even after settlement, and that the plants are grown in an appropriate medium to ensure the roots can integrate easily into the substrate.
Avoid planting in a heat wave if you can, but planning around unpredictable summer weather simply means tuning the watering regime and installation to the current local conditions. A robustly designed green roof with a diverse planting scheme and generous substrate depth will always help.
Of course, making green roofs look ‘green’ in winter is an entirely different challenge, so another post on this subject will follow in a few months’ time.