plant choices

A green roof is a marriage between engineering and ecology.  These are two disciplines that don't often join forces in any other situation and so green roofing is a bit of a learning curve for both builders and plantsmen.

If conditions are wrong for a particular plant species, it will be very difficult to keep those plants in good health. 

When choosing plants please consider the following:



Sedum plants are happy in a shallow layer of growing medium and
can be selected to give a long flowering period



Depth of growing medium: this will affect loadings on the building, but it is also vital that the chosen plants have enough growing medium to support them.  Sedums can survive in just 20mm of growing medium; the majority of wild flowers need at least 150mm.  Bear in mind too, the method of establishment…sedum blankets need no extra growing medium, sedum plug plants will need to be planted into at least 100mm depth of substrate.  Plus, the deeper the substrate…the more likely it is to support unwanted plant species…so more weeding for the maintenance team.

Aspect: some plants detest shade, others embrace it.  If you have an area that’s likely to be in permanent shade – eg behind PV panels, either make sure there is enough growing medium to support shade loving plants OR plan to have a plant-free area here.  In a sunny spot, make sure you choose plants that can tolerate intense summer heat as well as winter cold.  Never forget that the temperature on a roof is usually and exaggerated version of the temperature on the ground.  Hotter in summer, colder in winter.

Surrounding area: If your green roof is in close proximity to deciduous trees, you will need to remove fallen leaves in the autumn time.  Please budget for this annual task and ensure debris can be removed from the roof safely.

Height of the building: which will have a bearing on wind speeds and ambient temperature; remember that even a slight breeze can out plants in a remarkably short time. Look for drought tolerant plants that are not likely to be buffeted about or scorched by strong winds.

 Access to water:  Green roof plants will need to be watered in the first few weeks of establishment, and may also need irrigation during extreme drought and immediately after feeding.  Choosing drought tolerant plants and the correct depth of growing medium will considerably reduce the need for irrigation once the roof is established but please include a functioning water tap somewhere in your design.

Mode of establishment: green roofs that are seeded or plug planted are likely to need more maintenance in the first 2-3 years than green roofs created with vegetation mats..this is because gaps between the plants are a great place for weeds to establish.  Where there is complete plant coverage it is more difficult for unwanted plant species to invade.