Plant Selection for a High-Retention Green Roof
by Dr. Anna Zakrisson on Monday, March 29, 2021 updated Tuesday, March 30, 2021
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If evapotranspiration is the objective, what plants are the best choice?
Evapotranspiration is the amount of water the soil and plants can release in vapor form. Which plants are the best choice for a high-retention green roof? This is a complicated question and warrants a complex answer. Not only do your specific local climatic conditions play a role, but the plants also must match the green roof type. For example, if you have installed a 5cm extensive profile with a highly draining substrate, you would need to go for a different plant palette than if you have installed an intensive Blue-Green roof system. To optimize evapotranspiration, you need to match water availability, local climate, plant resilience, and crop coefficients.
What is a crop coefficient?
A crop coefficient can briefly be explained as a descriptor of the evapotranspiration of a specific crop. In its most basic form, the crop coefficient (Kc) is described as a corrector of the reference evapotranspiration value (maximum evapotranspiration possible). The crop coefficient considers many crop factors such as leaf area, growth stage, crop development, planting date, canopy cover, canopy resistance, and averaged evaporation rates from the soil.
The Reference ET (ET0) is determined using weather data (weather stations). You can also use an atmometer (ET-gage) to determine evaporation rates. This corresponds to the maximum theoretical ET rates of that system.
Video about crop coefficients from the University of Lincoln:
The crop evapotranspiration (ETc), in its simplest form, is then calculated as ETc = Kc x ET0
It is essential to understand that the crop coefficient changes over a growing season as leaf areas expand, the root system develops, and the plant matures. Therefore, it can be misleading to look at bundled crop coefficients to make grand assumptions about a specific plant species' suitability for a green roof, especially since (extensive) green roofs are such extreme environments with highly fluctuating temperatures and often low water availability (if not irrigated).
Green roof profile and evapotranspiration
To reduce irrigation needs, it is vital to not only look at the plant palette, but also the green roof profile. Adding mineral wool to the profile improves the retention capacity, but only to a certain point. In the end, it is the rate of plant evapotranspiration that sets a limit to when adding more mineral wool/substrate will no longer improve the roof's retention capacity.
Selecting plants for the green roof
It is tricky to give good global advice, mainly because of substantial climate differences, and local regulations. Some regions have frequent precipitation, with small showers every day (e.g., Seattle, USA); other regions have a propensity for more intense rainfalls and higher temperatures (e.g., Munich, Germany).
This is the first step: identify your climate and base your green roof design on this. This will ensure that you have the best setup already from the start. For this, you can use our RETENTION MODELER.
Green roof example
I will use a high-retention extensive roof (Sponge-Roof type) in a central European climate to illustrate how the green roof profile & plant selection process might proceed.
Firstly, I took a look at the Retention modeler and selected an extensive profile that will reduce plant stress by as much as I can, while at the same time allowing me to stay within the load requirements given by the architect and the engineers. I ended up with the following profile:
In this example, I have added Sedum plants to the profile. The main reason for this is their hardiness and ability to survive extreme conditions. However, please note that other plant types might be a good fit for your local climate, especially if you live in a cold region with a reasonable amount of precipitation, e.g., Stockholm, Sweden. Plants can include grasses, herbs, or even mosses.
Below, you will find a description of Sedums as green roof plants. Sedums are commonly used on extensive green roof systems, and their crop coefficients have been well established. Nothing is preventing you from interplanting other species to achieve higher biodiversity. Just get creative within the frames of making sure they all survive on as little irrigation as possible.
Read more about Sedum plant selection in this article: Guide: Sedum Green Roof Plant Selection
What are Sedum plants?
Sedums have a considerable capacity to rebound from heat, cold, too much or too little water, and in the worst case, what appears to be a dead plant can merely be a dormant summer stage, and the plant can bounce back fully within weeks once the weather cools down.
In winter, Sedums tend to shrink towards the crown but still have enough coverage on the green roof to be 90% covered, albeit in a reddish maroon color. Protecting the roof from wind erosion is the most critical wintertime task. Having this semi-dormant dense winter coverage naturally prevents the opportunity for in-blown weed seeds to germinate, sprout and thrive. Sedums are often called 'adaptive' plants as they can so easily survive where others cannot.
It is essential to know that there are 600 different Sedums. In Northern Europe, creeping Sedums are often used. These usually have tiny needle-like leaves, often with relatively low evapotranspiration rates partially due to the low leaf area. In many other parts of the world, these needle-type Sedums such as S. album, S. sexangulare, and S. acre are not doing too well during the hot summers as they go dormant. There are also thick, deep-rooted, broadleaf Sedums such as Sedum kamtschaticum, Phedimus takisimense, and Sedum spurium with higher survivability in hot climates.
Evapotranspiration and Sedums
When water is not available, many Sedums have a special adaptation that allows them to preserve their thirst for water: so-called facultative CAM plants, meaning they have an interesting photosynthetic modification that allows for gaseous exchange to occur during the cooler nighttime. This means that their stomata, the holes used for gaseous exchange, are closed during the day leading to limited water loss.
Many non-succulents are not adapted in the same way and are stuck with traditional C3 photosynthesis. This means that the stomatal gas-exchange pores are open during the day, allowing for high potential water loss, leading to maximum ETc that generally are higher than that of Sedums. However, it also means that these C3 plants will need irrigation in hotter climates, or they might shrivel up and die.
Nevertheless, again, it is critical to underline the climate-dependence of the argument above. Further, methods such as rainwater harvesting can create a sustainable and well-maintained green roof despite a lower stress resilience of non-succulent plants. Nonetheless, we should keep in mind that many extensive green roofs (globally) are built as workhorses to reduce annual runoff volumes.
To achieve an effective system with optimal evapotranspiration and significantly reduced annual runoff, it is not enough to look at crop coefficients and weather. We must match the local climate, building load restrictions, economics (installation & maintenance), irrigation possibility (and source), crop coefficients, and plant stress tolerance. Even if we build an incredibly effective system on paper, but it requires a financial input that is not realistic, or the annual system evaporation has been calculated based on maximum plant ET, the system could potentially fail.
It is critical to have an overview of all these factors before deciding on profile and plants.
Sedums can be a safe option due to their resilience, but this is far from the only solution, and there are beautiful herbs and other plants out there that do well in many climates. Just make sure you have the data before you make a decision and ask an expert! Also, I know I have mentioned this before: it is possible to use a sedum mat as a base with interplanted biodiverse vegetation to ensure fast soil coverage and prevent roof failure during hot summers if irrigation is not available.
Please note: If the aim of your roof is to cool a building, you must maintain ET rates through summer, and drip irrigation might be needed even for Sedums.
Please, contact us if you have any questions!
Purple-Roof is a non-proprietary concept that is sold as various products all over the world! Data is provided by the hydrology laboratory at Green Roof Diagnostics.