Guide: Sedum Green Roof Plant Selection
by Dr. Anna Zakrisson on Tuesday, January 15, 2019
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Green roof sedums
Sedum plants are thick-leaved, drought-resistant plants of the family Crassulaceae and are sometimes referred to as stonecrops. There are about 400 species of sedums with a wonderful variety of colors, shapes, growth rates, flowering season, and evapotranspiration rates.
Why use a sedum green roof?
There are both evergreen and deciduous sedums, but they have one thing in common: they are generally rather difficult to kill. This is also why sedums are so suitable as green roof vegetation, especially on extensive living roofs.
Extensive living roofs have a very thin soil layer, ranging in depth from 2-13cm (0.8-5in). The shallow soil layer is sensitive to the external environment such as temperature fluctuations and drought.
There are many different strategies for success in the plant world. Some species grow incredibly fast to reach high into the sunlight before other species can get there. Others aim for fast horizontal growth and act as excellent “suffocators.” Others even give out compounds that poison the soil for other plants in order to have space for themselves.
The plant world is vicious!
Sedums are probably best described as ‘tolerators’ because they can handle stress and abuse from so many different angles such as heat, trampling, water shortage, lack of nutrients, 30 days of rain, and the list goes on and on. They have found their niche where others can’t grow.
A sedum green roof – what to do first?
The first step is to collect data on precipitation where you want to build the roof. It is important to match the precipitation levels with the plant evapotranspiration (ET) rates. Evapotranspiration is the combined loss of water from evaporation from the plants and soil transpiration.
You can read more about evapotranspiration in our article: What is Evapotranspiration?
Precipitation rates tell you how much water is added to the roof and the evapotranspiration rates tell you how much water will leave the roof as vapor.
In most areas of the US, there is between 75-100 cm (30-40 inches) of rainfall, which is enough to sustain a crop of Sedums, but not enough to sustain a crop of regular perennials. Once you establish your desired plant type and confirm their ET rates, you know if, and how much you need to irrigate. Sedums are likely to survive in most areas without irrigation, but different sedums have different requirements, so take care to do this first step carefully. You’ll be stuck with your choice for many years to come.
Sedums and inhospitable green roofs
A green roof is an inhospitable environment, particularly extensive green roofs with their thin and well-drained soil media profile, which is prone to drying out. Sedums are expert survivors for these types of conditions.
Sedums can be spectacular survivors. Even when they look totally destroyed, with some TLC Sedums display a remarkable ability to bounce back from a catastrophe. Their survival strategy is to drop leaves, shrink stems, draw nutrients into their core and shrink into a tiny bit of what it once was. Then, when good times return, they have this magical ability to regrow to their prior status and beyond.
Albeit, there are conditions where sedums are at a disadvantage, and that is during continuous waterlogged conditions. These plants do not like to get their roots too wet for too long, and the roots might start to rot if they do.
For their roots to be soaking wet for weeks is fine, they may get some temporary root damage, but the plant will replace these roots within days if not weeks. However, being soaking wet for most of the year will cause the Sedum crops to wither away and die.
The sensitivity of sedums to soggy soils can cause issues in climate regions with severe rainfall. In these cases, it’s extremely important to ensure that the plant choice is carefully matched with the soil and the roof drainage layers.
There are still a lot of improvements ahead of us regarding green roof soil media. We are happy to see research into this field is slowly taking off indicating a promising future for the development of cost-effective, resilient roofs.
Good soils matched with the right plants is a winning combination!
Sedum plant growth types
There is a wide variety of sedums. Sedums used on green roofs can broadly be separated into four different classes:
- Ruderal Creepers – like Sedum album, acre, and sexangulare these are fast to establish themselves on the roof. They crawl over the soil media layer and have very shallow roots. The creeper-types have super-fast growth rates, sometimes expanding up to 100 % in one week. They also die back quickly if water is in short supply.
What sometimes appears to be death is often summer dormancy.
These creepers have shallow roots (2-5cm/1-2” deep), and when planted in typical green roof soils with relatively low water content (based on FLL standards), the root zone gets too hot and dry, so these Sedums easily go dormant on the roof.
This class also generally display low evapotranspiration rates. These Sedums are critical though, as they protect the soil from heating up in summer and protects the soils from blowing away during winter as most other deciduous Sedums go dormant and leave the soils
- Needle-type sedums – such as Sedum rupestre Angelina and Sedum reflexum Blue spruce.
These sedums often have one tap root ranging further into the soil media than the creepers. Their needles are loosely packed around the stem.
These sedums often display very low biomass on the roofs and quickly disappears when other sedums take over. These are highly ornamental and not a reliable long-term Sedum and often disappear after 2-3 years.
- Ornamental sedums – such as Sedum spurium Fuldaglut or Red Carpet. These guys are pretty with their bronze or reddish foliage but have poor functionality with regards to evapotranspiration.
This class often have a very weak root system with little water being pumped adding little to the stormwater management of the roof. They also go summer dormant should the roof get hot.
- Perennial type with deciduous behavior – such as Sedum spurium John Creech, Immergrünchen, kamtschaticum, floriferum, and Phedimus takesimensis.
These are deep-rooted plants with high evapotranspiration rates. Members of this group often out-compete the other types as they can reach the water table even when it’s low. They are also able to remove a lot of water via transpiration adding great value to the stormwater management qualities of the roof.
Their deciduous quality supports soil life as the Sedum crop returns a large volume of biomass litter each season when they drop their leaves.
Which sedum species should I plant on my green roof?
Looking at this list, it might be easy to fall into the trap thinking that one should only plant the perennial type and avoid the other types. That would be a mistake.
Diversity means stability and resilience. A resilient roof means less maintenance and lower risk of the vegetation crashing. A vegetation crash could lead to a full vegetation-replacement, which is extremely costly.
The different sedum classes have distinct functions on the roof. It is nice to have all four groups, but it is critical to include deciduous and ruderal creepers.
The creepers ensure fast establishment ensuring that the soil media is covered fast and protected from wind and the elements. This cover enables the deep-rooted perennials to develop their root systems slowly.
Creepers act like a skin that protects the soil from heating up which will benefit soil system stability and functionality.
The deciduous Sedums are needed to provide summer resilience as they can handle heat the best. They have high evapotranspiration rates and create, and support, the soil ecology through an active and deep root system. This group also return the most litter to the soil in late fall.
This litter acts like a blanket that protects the soil and plants from desiccation, and the soil biota uses it as feeding stock. Thus, nutrient cycling is improved which improves plant health and reduces nutrient leaching.
Also, should the situation occur that the deep-rooted deciduous perennials would die off due to disease, percolation, or waterlogging, the creepers can yet again expand for soil cover! Even though they may at times have very low abundance, they almost always remain in small numbers, ready to quickly expand should the conditions be favorable.
Bees and butterflies and sedum green roofs
Sedum plants not only look beautiful, and to your surprise perhaps their flowers also provide nectar for insects and can act as a refuge for many organisms.
Nevertheless, this beauty shouldn’t keep you too distracted when you select plants for your roof. A roof that is selected based only on visual attributes will at best become extremely high maintenance and at worst fail.
Yes, an all-silver-gray roof might look amazing, but if the silver-gray plant you have selected is a purely ornamental sedum, and your maintenance budget is not on par with that choice, you may well find yourself without any living vegetation after a few years.
However, if your green roof is being installed for visual purposes only, silver gray is indeed very beautiful, and I would completely understand the choice. Just make sure that you have a sufficient maintenance budget allocated for the installation.
Apart from the economic incentive for selecting a diverse pallet of sedums for the roof, it also has ecological effects as a resilient roof will require less fertilizer applied as well as fewer pesticides. This might also be good to know should you be required to comply with particularly strict local environmental regulations.
Purchasing sedums for green roofs
Most serious suppliers of sedums for green roofs can give detailed information about their plants. Also, a great method is to purchase mats of plants that contain about 15 different species from different functional groups. That way, the roof vegetation will go through a natural cycle of succession and the plants most suited for your local climate will become the most dominant ones.
Every roof is different; every green roof profile can be different in height and makeup, wind tunnels or wind vortexes created by buildings can have unforeseen consequences as they dry out certain sections faster than others. Neighboring buildings can reflect heat and light onto your roof that was unexpected.
All these physical elements have an impact on which plant assortment that will thrive.
Next, add the environmental impact of a very wet year, or the impact of a really hot summer, or a really, really, cold winter… Each year has a slightly different outcome as each Sedum has a preferred set of conditions under which it will thrive faster than the others. This will re-set itself each year.
Still, most of the other plants of the palette will still be present, though in low numbers, ready to expand should the conditions be favorable.
Five different roof locations planted with identical plants will look completely different five years later.
Thus, start with a big and diverse group of plants and let Darwinian battles take place. Thus, in turn, gives the building owners the most satisfaction in a green roof that is resilient and strong, and easy to maintain.
Aim for a resilient and high-quality green roof. The good news is that this high quality doesn’t come at extra cost. Just let nature do its job: natural succession.
Feel free to contact any of our Purple-Roof experts for more information on green roofs!
Planting in a Post-Wild World by Thomas Rainer and Claudia West.