Is Green Roof Biodiversity Important?

by Oscar Warmerdam on Friday, August 21, 2020 updated Thursday, April 28, 2022

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Green Roof Biodiversity - Challenges and Another Perspective

The biodiversity of nature is essential for human existence on this planet. Biodiversity means ecosystem stability.

The biodiverse nature provides us with essential items ranging from medicines to food, and materials. Apart from being necessary for our survival, this also provides business worth €40 trillion, representing half of the global Gross Domestic Product (European Commission, 2020). Biodiversity is also intricately linked to climate change, and these topics cannot be viewed in separation from each other.

In short: biodiversity is extremely important and must be taken seriously.

Because of this, it is easy to understand the strong focus on biodiversity on green roofs. However, I would argue that a strict view on what encompasses a biodiverse green roof may hinder living roof acceptance and proliferation.

Viewing Green Roof Biodiversity in a Context

Biodiversity means the variety of life found in a particular place. Hence, if a green roof is planted with 30 species instead of 8 species, the green roof has more biodiversity. However, the “place” when considering biodiversity is often larger than a roof and might include a site, a neighborhood, or a city. In those instances, we might look at the total diversity of plants (and microbes, and insects, and animals) within that area. If the area has very few Sedum green roofs, then adding a Sedum green roof increases biodiversity, just as if very few birch trees can be found in a city, then adding birch trees increases biodiversity.

Further, if adding a particular plant attracts a type of bee, or butterfly, or bird that is in need of additional habitat in an area, biodiversity increases as well. Hence, biodiversity is all relative to what is already present in a location.

Green roof biodiversity and Sedum roofs

I am a Sedum grower, so of course, I can be accused of being biased. Still, I would like to share my experience with extensive green roof biodiversity (extensive vegetated roofs generally have less than 10 cm green roof soil media). In the end, what I am most interested in is that the roofs are successful and provide promised functionality.

I am often being asked if green roofs plant assortments are diverse enough, and here is my short and long answer: The short one says no, they are not diverse enough. The long answer is that an extensive green roof is a harsh environment, and we are starting with a narrow assortment that is proven to establish a green roof rapidly. This assortment almost certainly changes within a few years.

Design vs. natural selection

We can easily discuss biodiversity in the context of what the designer chooses to plant on a project. Still, I think the dialogue is much more constructive if we focus on the long-term implications, as initial planting is only the beginning.

I have seen so many well-intended designers try to forge nature into color patterns or plant palettes for green roofs. They look beautiful for a while, but after about 1-2 years, most of the design is permanently gone from the green roof because it could not survive.

Succession is the natural process describing how ecological communities change over time. On a green roof, eventually, only the plants most suitable for the conditions will remain. This is natural selection in action. It is also too often forgotten that the roof has no commonality with the conditions at grade.

Of course, you can choose to create specific designs, but be aware that maintaining geometric designs requires continuous maintenance efforts, an uphill battle against nature. These can be very labor-intensive and costly and may not be a very “green” or sustainable strategy.

Sempergreen USA maintains 100,000m2 (1,000,000 sqft) of green roofs. For R&D purposes, we try to see what, where, and why certain non-succulent plants survive on a 10cm (4”) green roof. Thus far, we have not been able to identify any specific patterns to predict whether non-succulent plants or which non-succulent plants will remain and thrive after several years.

Can we add more soil to support better green roof biodiversity?

Of course, with a deeper soil layer, a more diverse plant palette can be maintained. However, there are many challenges to this. Most buildings have a limited roof loading capacity. Increasing this is always possible, but it adds cost, and any kind of construction cost increase tends to decrease the likelihood of green infrastructure being implemented.

Is that really what we want?

In the end, a sedum extensive green roof is much more diverse and brings so many ecological benefits ranging from stormwater to cooling, to pollution capture compared with a black or white roof. Sure, it might not be perfect, but the comparison should not be to that of natural forests, but to a black, virtually sterile roof. If our target is to green our cities quickly, we need to be less stuck in dichotomous thinking and go for the methods that are tried and tested and that we know work. Sometimes this means installing a super diverse intensive green roof, and sometimes, this is a thin extensive sedum roof. Both increase urban biodiversity.

Further, we can always add plant species later on in life if we find plants that can cope as well as succulents.
Besides, as we are trying to green the roofs without the need for water, the plant palette that is available to deal with extended periods of drought shrinks by 99 %. If you want more diversity, you need to irrigate. Without irrigation, few plants will survive. LEED does not like you to irrigate (and I agree).

Drought tolerant green roof plants – why we can’t just add any old plant to the roof

Most plants, if they experience severe drought stress shed leaves or portions of the plant. At a certain point after extended periods of drought, most annuals, perennials, or shrubs throw in the towel and die, with no chance of survival or rebound. Succulents are different and can tolerate these harsh conditions. They can shed leaves, stems, and even parts of the crown and go dormant or semi-dormant while waiting for better times, and when rain and cooler weather return, they bounce back and regrow to their former state.

I have been in the perennial plant business all my life, and it is easy to recognize drought-tolerant perennials. The University of Colorado had selected a whole line of drought-tolerant species, but often they have taproots. These taproots allow the plant to draw water from deep reservoirs. But within shallow green roof profiles, we need drought-tolerant, shallow-rooting plants. Some very interesting grass species can tolerate these conditions, but they produce dormant foliage in the winter that is considered a fire hazard on some buildings.

I am all for finding more diversity on the green roof, but I have tried it on many roofs that we maintain. We often observe that when we start with lots of diversity, the roof assortment narrows down to just a handful of plants, and native perennials selected for drought tolerance (at grade) are the first to perish in green roofs because they cannot develop deep roots.

Even when we start with 15 of the hardiest Sedums, after 2-3 years, we see 1/3rd of the species disappear, after 5-7 years; another 1/3rd of species disappears, with the remaining 1/3rd of species not only surviving but colonizing the green roof. Interestingly, 1/3rd of the Sedums is unpredictable and a surprise every time, this depends on the neighboring shade, wind, reflection of the glass, presence of exhaust vents, time of the year it was installed, and the type of weather we had during the establishment period after planting.

So, why not use topsoil on the roof?

You do not want to use topsoil on a green roof. Firstly, topsoil often comes with a remarkably high load of weed seeds (and not the funny kind). It should also be added that the engineered green roof soil is intended to be adverse for 99.9% of common weeds.

These engineered soils are nutritionally lean and coarse so that hand weeding, or worse: chemicals, are not needed while trying to get rid of weeds. Nature does it, for free.
It should not be forgotten that highway-weeds grow in clay or loamy soil that is superbly effective in holding onto water, and this soil can ‘pull’ water upwards through a powerful capillary action, meaning the soil by itself can draw water upwards from the deep reservoirs. These plants often have roots 1-3 m (4-10 feet) deep.

None of this can happen on a green roof due to the lack of soil depth, and the coarseness (low capillary capacity) of the engineered soil, and low organic soil content. Further, using topsoil on a green roof means that any type of vegetation now has a chance to survive, such as unwanted weeds with aggressive root systems that penetrate the insulation and waterproofing.

Also, such a roof will amass substantial biomass throughout the year, which will also have to be managed.

Are grass lawns not a better strict target for increasing urban biodiversity?

In one of our meetings with Montgomery County, Maryland, they said that a full 28% of that county was covered in some form of mowed/maintained form of grass (this included any type of non-agricultural grass). This means there are 370 km2 (142 square miles) of grass or lawn in the 1300 km2 (507 square miles) of the county.

As a comparison, there are about 0.18 km2 (2,000,000 sqft) of green roofs in that county, which is about 0.01%. I cannot help but point this out every time and suggest that instead of focusing on the biodiversity of green roofs, our main focus should instead be on these lawns and to create biodiverse urban meadows instead of irrigated, fertilized lawns. This is particularly true for the US.

Granted, if you do not address this green roof lack of diversity problem right now, it will become a 28% problem one day. However, if the biodiversity issue becomes a block for the adaptation of green roof strategies, we need to re-evaluate our stance. Again, even the least biodiverse Sedum roof creates more biodiversity than a black or white roof and adds many other critical ecosystem functions.

Let’s get the ball rolling, and as we collect more data, we can add biodiversity to the already existing roofs. Let’s not aim for 100% perfect but at a “minimum viable product.” We all know well from the business world that this generally is the fastest adaptation strategy and might be the best way forward to ensure that urban biodiversity improves fast and reliably.

  • European Commission. (2020). EU Biodiversity Strategy for 2030 | European Commission.

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