‘We need to plant in December, with good coverage and species diversity, and it needs to look green straight away!’ This brief is already beyond the performance capacity of any standard sedum or wildflower mix. Although it is challenging, it is certainly not impossible.
As a slight change from my standard ‘last species flowering before Christmas’ blog (it was Helianthemum nummularium and Bellis perennis this year), I will look at winter plant performance in general. The challenge of keeping extensive green roofs looking truly ‘green’ throughout winter relies on a little more horticultural knowledge and plant diversity than most ‘off the shelf’ solutions can provide.
To start with the easy answers, there are some sedums that stay looking fairly green over winter. It is reasonably common for a lot of standard sedum species to drop a lot of their leaves and turn at least partially red, especially in a harsh winter. This is natural in autumn and commonly accepted with deciduous trees and shrubs. However if green roofs are expected to literally look ‘green’ all the time it can be seen as poor performance, even if the plants are perfectly healthy. Some of the most commonly specified native sedums are good at keeping green foliage, particularly Sedum acre although Sedum album often stays fairly green as well when healthy.
Wildflower roofs are generally accepted as a summer flowering deciduous option, and can be cut back to minimise untidiness in winter. However there are a few species that provide surprisingly good winter coverage. My favourite is the under-appreciated Bellis perennis, not only for its long flowering period but the evergreen foliage that stays looking bright green and robust throughout the winter.
A judicious combination of certain wildflowers and sedums can therefore provide a much better visual performance over winter than a standard random mix. However, there are plenty of other green roof plants that are tried and tested, and commonly neglected. Of these Petrorhagia saxifraga is good at providing a green base, spent grey flower heads can be trimmed back if desired but do not grow too large. Campanula portenschlagiana is my top tip for evergreen performance in a genus that is otherwise very deciduous.
A growing range of other alpines is becoming available to the discerning green roof installer, and we are always happy to advise on plant selection for particular situations.