green roof at sandringham flower showGreen roof design: choosing the right plants

Any experienced gardener will tell you that if you want to grow plants cheaply and easily without making yourself a whole lot of work, make sure you match the plants’ needs to the conditions you are planting it in.

A deep rooted plant won’t survive in shallow soils without irrigation and supplementary feeding; a sun loving shrub will languish and die in the shade, a tender exotic will struggle to survive a winter similar to the one the UK has just had.

When designing a green roof, the ‘right plant right place’ rule takes on a new level of importance.

In broad terms there are four types of plant that are candidates for establishing on to green roofs.  Trees and shrubs are totally unsuitable for all but the most complicated living roofs – so they’re not being considered in this blogpost.

Annual Plants

An annual plant germinates from seed, grows to maturity, flowers and sets seed all in one year.  Good examples of UK native annuals are cornflowers, field poppies, chickweed and corn marigold.  As colourful and pollinator friendly as these plants are, they’re not ideal as the dominant choice of vegetation on and extensive green roof.  But by all means sow some seeds every spring to add extra colour and interest to the roof but consider them as a supplement to the evergreen planting that will keep the roof functioning through the winter.

Annual plants are not often seen at all between November and april and they either seed themselves a bit too vigorously – so they produce a thick, disease prone covering in year 2 – or the seeds fail to germinate and the roof is vulnerable to invasion by less desirable plant species.

Herbaceous Perennials

Herbaceous perennials are typically found in formal planting schemes or in cottage gardens.  This is my favourite group of plants but I would be very wary indeed of using them in a green roof planting scheme.  In general, these plants like humous rich soils and access to plenty of water and nutrients.  Most have luxurious, sappy growth that would suffer terrible damage in the extreme temperatures and windy conditions on a roof.  Another disadvantage is that they tend not to be evergreen and so would contribute very little towards insulating the building in the winter time.

Herbs

Next we have the herbs.  Some herbs such as thyme, oregano, sage and chives will thrive very nicely under green roof conditions and would offer a great source of pollen, nectar and shelter for minibeasts.  There is a chance that over time the plants will become overgrown and woody, in which case they will need pruning or replacing from time to time.  If you include these plants in your green roof design, be sure to factor in some extra money for ongoing maintenance.

Hardy Succulents

Hardy succulents are the UK’s number one choice for green roofs and with good reason. Sempervivums and Sedums are well suited to rooftop microclimates.  They only need a shallow layer of growing medium and therefore impose less loading on the building.  Their needs are simple; a light feed in spring, plenty of sunshine and a very – and I mean VERY – occasional watering should the UK experience a lengthy drought.  Perfect.

sedum roof on shedSedum Matting

Sedum matting is the green roofer’s easiest option.   The plants are already growing and give good coverage from the outset.  Species have been chosen to give a long flowering period, good frost hardiness and all year round foliage.   Plus:  sedum matting is a doddle to install on to a green roof.

 

More about sedum matting

How to put a green roof on your shed using sedum matting