When a professional building design team chooses green roof plants for a commercial green roof, are they thinking of the attractiveness and biodiversity of the roof they will create?
Most likely not.
The two leading players in a building design team are the Architect and the Quantity Surveyor. The Architect is likely to have many other considerations on his mind than just the roof and the Quantity Surveyor will want the least cost solution
The factors that will most likely be uppermost in the design team's brief from their large corporate client will be that the building comes in under budget, on time, to a high build quality and lastly that they can tick an eco-box that the building has a green roof (which may well give the developer some tax/financial incentive.)
To satisfy the above requirements, the plant of choice for extensive green roofs has become the sedum mat. There are companies that now produce 'rolls' of sedum plants that can be unrolled, like a carpet on to a new roof, with a proprietary mix of sedum plants. The advantages of this system are that it is light in weight, sedum is very drought resistant (so requires no irrigation) and the system is very quick to install. The end result does not look too bad either.
However have you seen the cost of those sedum rolls? A basic sedum turf mat with between 8 and 15 different varieties of sedum starts at £23/m2 and goes up to £33/m2 for a fully established system.
Costs like this can be absorbed in a large project. Particularly given the speed of installation and establishment.
However do you want to pay that amount for your green roof plants? And wouldn't you like a wider choice of plants to give greater biodiversity, year round plant interest and at a lower cost?
The first option could be to grow your own sedums. Sedum grow very well from cuttings and so the purchase of a few 'mother' plants and then preparing lots of cuttings will help you to quickly establish a large stock of young plants.
Don't worry that initially there will be large areas of bare earth. The general recommendations is that you aim for ground coverage of about 60% after 1 year.
The advantage of establishing your green roof over a period of time is that you will quickly find out which species thrive in your local environment and which don't. Species of sedum that are widely used on green roofs include Sedum album, Sedum acre and Sedum rupestre.
The main requirements for green roof plants are that they should be drought and wind resistant. The type of plants that you should consider will originally have come from environments with thin shallow soil and the plants will have evolved to cope with exposure to extremes of sun, wind and variations in moisture.
There are four groups of green roof plants that I can suggest you consider alongside the already popular sedum. These are Bulbs, meadow flowers, ornamental grasses and Mediterranean plants.
Bulbs create a marvelous splash of colour in the spring.
Bulbs originate in desert environments and store moisture in their bulb below ground until the spring when they flower, reproduce and then store more food in the bulbs below ground again until the next spring.
To grow bulbs you may need to create local thickenings in the soil of the green roof to provide the bulbs with their required growing conditions. These local thickenings can also give your roof some additional interest over the traditional flatness of those corporate roofs!
Bulbs to try as green roof plants include grape hyacinths and dwarf narcissus.
Meadow flowers grow in a variety of environments from the chalky meadows of England to high level Alpine meadows. Many of these plants are annual so establish a spectacular display of meadow flowers you will need to buy a packet of specialised seeds. The sowing requirements for these are minimal. Just sprinkle on the surface of the soil, mix in with the surface of the ground and give it a gentle water and then wait.
It normally takes a couple of months for the seeds to germinate and many seed packs are specially selected varieties to give ongoing displays for several months. When one species finishes flowering another is timed to start in many cases from May through to November.
Growing your own meadow flowers is an option however it is possible to find meadow flower seed mixes that are ready prepared. There are a variety of mixes selected for colour themes and also to provide a succession of colour from May through to November. A great source of seeds and inspiration for meadow flowers is Pictorial Meadows, based in Sheffield.
Well selected Ornamental grasses will give a sense of height and movement to areas of your roof. These grasses, known as fescues, do not spread like weeds. They grow in clumps as they would in their traditional environment. They grow to varying heights (don't choose the very tall growing ones!) and the seed heads, movement of the leaves and stems in the wind and changing colours from green to brown with the season add variety to the roof.
Varieties of ornamental grass to consider as green roof plants include koeleria glauca and Koeleria macrantha and the Festuca species.
Mediterranean plants by the nature of their original home are used to hot dry climates such as on an exposed shed roof. The most popular selection with this would be the low growing plants such as varieties of Thyme, Basil, Chives, Oregano, Lavender and Rosemary. Be careful with plants such as rosemary that can grow taller as they can get caught by the wind and pulled out by their shallow roots.
Many people are so impressed by their first view of a green roof and the concept that plants can be grown where they had only ever seen tiles that they think sedum is the only option. Don't fall into this trap!
You can design a low maintenance drought and wind resistant garden for your shed roof. The instant plant cover which is a project requirement for commercial projects does not apply to your roof. At a recent talk that I attended by Nigel Dunnet at RHS Wisley it was recommended that achieving a coverage of green roof plants of 60% after one year as a reasonable target.
Leave the Sedum monoculture to the professionals. You can create something a lot more interesting!
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