Green Roof Plants - How do They Survive?

by Dr. Anna Zakrisson on Tuesday, January 15, 2019

sedum green roof plants

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Green roof plants – what’s their secret?

Do green roof plants matter?

“Do green roof plants matter?” Yes, absolutely! The vegetation on a green roof is a central part of the engine that drives evapotranspiration. Evapotranspiration helps to ensure efficient green roof stormwater management. Also, if the roof isn’t covered by a protective layer of vegetation, the soil might simply blow away or even flush away during a rainfall.

Selecting green roof plants

The choice of green roof plants is important and should be carefully considered. The layer of vegetation should be selected based on climate region and be well-tailored to the soil.

Vegetation and soil are closely linked, and healthy soil biology is essential for happy plants.

If the vegetation layer crashes, the functionality of the roof is destroyed.

water drop on plant video
Vegetation protects the soil by slowing down the water flow

What are sedum plants and what makes them so tough to kill?

So, which plants should you use on your green roof? Often, sedum plants are a popular choice on extensive green roofs. Why is that? Well, sedums are the camels of the plant world, incredibly hardy and tolerant to drought.

Usually, a mixture of different sedum types is used, including fast-growing ruderal species as well as slower-growing, more deeply-rooted deciduous varieties.

The fast-growing ruderal species can colonialize a roof at astonishing rates. This fast colonialization creates a protective blanket over the soil and gives the slower growing species time to spread. The ruderal sedums are more sensitive to drought than the deep-rooted species and will die back with time, particularly during hot and dry summers.

Sedum plants are particularly common on extensive roofs. These roofs have very shallow soil layers, so the plant roots experience extreme temperature fluctuations and long periods of drought.

The secret of sedums

What is the secret to the hardiness of sedums? Sedum plants make use of a neat little variation of photosynthesis called crassulacean acid metabolism (CAM). This modification makes it possible for the plants to close their gas exchange openings (stomata/guard cells) during the hot daytime and open them during the colder night when less water is evaporated.

Some sedums even switch between CAM and “normal” photosynthesis (C3) depending on environmental conditions. These plants are referred to as facultative CAM plants.

Broadleaf and ruderal sedums are used on green roofs for different purposes. Deciduous broadleaf sedums add to the natural cycle of life more so then the ruderal species by adding biological material to the soil. However, the fast growth of the ruderal species also has its benefits, particularly when it comes to quick plant establishment.

green roof plants: drops on sedum
Droplets on a sedum leaf

What about using local vegetation on green roofs?

A lot of research is currently being done on the use of more local or native vegetation. Local vegetation has evolved in the relevant local weather conditions, which could, in theory, make the roofs more resilient, with lower maintenance requirements and lower plant stress. However, carelessly using unresearched plants can end in tears.

Even though the local plants have evolved for your specific climate, they haven’t evolved to the extreme conditions on a green roof, they are not used to living in 4” (~10cm) of soil, as their tap-roots prefer 8ft (~1.5m) deep profiles, and they are not used to the FLL-growing media as they are used to growing in a loamy soil. Thus, they may require a lot of irrigation or other types of maintenance.

Care should also be taken when using plants that create a lot of dead biomass that can dry out and become a fire hazard.

On intensive green roofs, much larger vegetation with deeper roots is often used. These roofs can be beautiful urban gardens. However, unlike many extensive green roofs, intensive roofs are rarely built for the sole purpose of stormwater management. Thus, the demands on the vegetation are different.

As both storms and droughts become more frequent and intense, and climate change continues to push temperatures to the extremes, it will be even more challenging to navigate through the list of plant species that can handle these difficult/trying conditions

green roof plants local vegetation
Local vegetation on Norwegian green roofs

How do I pick plants for my location?

First, one must consider the evapotranspiration (ET) rates of the plants, and see how this matches up with the local rainfall patterns and rain volume. This will tell you if you need irrigation or not.

If you are designing the green roof mostly for aesthetics, be willing to accept more plant replacement risk. We always remind people to look at the forest. It’s mostly green trees out there with very little color coordination taking place.

Plant replacement is costly for the building owners and choosing plants that are not optimal for the local climate might be devastating. It is also not uncommon that aesthetic traits like flower color or leaf shape take priority over function. If an architect wants a roof with only yellow plants to achieve a particular landscape design, such a roof will suffer extremely low diversity and will not be resilient. It might look pretty but will be very maintenance-intensive and costly.

Focusing solely on aesthetics might be an option if the purpose of the roof is to be visually pleasing. If the green roof is not visible and is mostly functional, be sure to go with the strongest, simplest and most foolproof solutions and chose plants that can handle drought, are low maintenance and have low nutrient needs.

One common approach in green roof construction involves planting pre-vegetated mats containing up to 15 different species, all with varying growth strategies and climatic adaptations. The ones most suited for the local climate will eventually take over and thrive on the roof.

green roof plants stomata
Stomata or guard cells are responsible for the gas exchange in plants. This is also where most water is lost from the plants

Plants and Retention and Detention

Green roof vegetation isn’t there just to look pretty. It has a job to do, and that is to transpire. When plants transpire, water is lost from the soil, preparing it to absorb the next storm. The combined water loss from the soil and the vegetation is called evapotranspiration, and this is the primary process by which green roofs retain stormwater.

Annual retention is 20-40% better with full vegetation compared with open soils, with the foliage canopy alone adding an approximately 10% retention bonus to the cake.

Healthy foliage is critical for detention through vertical and lateral resistance to flow. When it rains, water drops come down at an average speed of around 30 km/h (approximately 20 mph). The canopy of leaves dissipates that energy as it bounces on the flexible leaves, allowing water to trickle along the stems or gently drop onto the floor below the vegetative canopy.

As it gathers volume on the soil floor, water seeks a path of least resistance, and lateral friction created by dense vegetation keeps it in place longer, allowing it to “stand” and giving it time to percolate.

For these reasons, year-round foliage coverage is crucial to effective stormwater management on green roofs. Full winter coverage due to the use of ruderal species is critical to prevent winter soil and plant desiccation, and year-round plant coverage is needed to prevent the wind from blowing soil and much needed organic particles of the roof.

Foliage is the skin that protects the content.

Plants and nutrients

Plants require three macronutrients for growth: nitrogen (N), phosphorus (P) and potassium (K).

Micronutrients needed for plant and microbial growth include iron (Fe), boron (B), chlorine (Cl), manganese (Mn), zinc (Zn), copper (Cu), molybdenum (Mo), nickel (Ni).

green roof plants nutrient requirement
Plant nutrient requirements

Nitrogen and green roof plants

Nitrogen comes in many forms such as nitrate, ammonium, and amino acids.

Nitrate, in comparison to ammonium, moves easily through the soil and can thus be leached and lost from the roofs. However, nitrate makes up the main proportion of plant-available nitrogen, making nitrate a double-edged sword for green roofs, as we need to strike a careful balance to ensure there is enough of this essential nutrient available to plants without having so much that excess is leached in the runoff.

Too much nitrate fertilizer applied can end in very little benefit on the roof and very much harm downstream.

Plant nitrogen availability is determined by the net rate at which inorganic nitrogen (nitrate is the most relevant form in our case) is replenished. That means the soil could theoretically have a high total nitrogen content, but if it is the wrong form of nitrogen, the plants can still suffer from nitrogen starvation.

Nitrogen fertilization can also have profound effects on root and shoot morphology and ratios, with higher nitrogen inputs leading to a significant increase in root growth. This has often been interpreted as “more is more.” However, research now shows that too much nitrogen can have negative effects on plant growth. It is thus vital to find the best balance between the nitrogen concentrations needed for optimal growth, while at the same time ensuring that nitrogen is not lost in the runoff.

The best way is to achieve this is through the establishment of soil biology that is healthy and stable in order to minimize the amount of fertilization needed.

We don’t want a high maintenance roof.

We want a roof that takes care of itself – for free.

The key to this is soil biology.

green roof plants nitrogen cycle
Nitrogen-cycle in natural systems

Phosphorus and green roof plants

Phosphorus is needed for many fundamental biological processes, for example, as part of the backbone of our genetic material. Phosphorus is also part of the energetic currency (adenosine-tri-phosphate: ATP) of biological systems.

Phosphorus is mainly available to plants as phosphate, and this nutrient has been the most difficult one to retain on the roof. Many studies show green roofs as phosphorus sources as most soil media contain phosphorus-rich compost as a component. The lost phosphate is a significant environmental problem as this nutrient is one of the main causes of algal blooms and eutrophication in lakes, streams, and estuaries.

Luckily, there are many ongoing studies currently tackling this issue. Once again, solutions will involve the biology of the soil combined with some nifty engineering approaches.

Often, the most novel approaches appear on the borders between subjects.

green roof plants - phosphorus cycle in natural systems
The phosphorus cycle in natural systems

Potassium and green roof plants

Potassium (K) is a different type of macronutrient compared to nitrogen or phosphorus. It doesn’t become incorporated into the plant tissues the same way but is involved in many critical enzymatic processes. Potassium is also needed for guard cell (stomata) regulation.

Thus, plants with potassium deficiency often become slow in their responses to changing hydrological conditions, with the sluggish closing the plant’s “water-loss valves” upon drought resulting in heightened drought sensitivity.

The importance of nutrient ratios

Plants need sufficiently high concentrations of these macronutrients to grow at maximum capacity, but they also need these nutrients in optimal ratios.

Ecological nutrient ratios are determined through stoichiometry, defined as: “the balance of multiple chemical substances in ecological interactions and processes, or the study of this balance.”

Nutrient ratios vary based on an organism’s specific nutritional needs. Nonetheless, within ecological compartments, these ratios are reasonably constant. This consistency enables us to make predictions of when a nutrient becomes limiting in a system.

A limiting nutrient is to be compared with a sports car with a full tank and three wheels. Wheels would be the limiting factor in this case. No matter how much fuel you add, the speed of the car is limited by the number of wheels. The same is true for plants. No matter how much nitrogen or phosphate you add to a plant, it will still not grow optimally if it’s lacking in potassium.

Nitrogen and phosphorus must be also be balanced. Plants are generally thought to become nitrogen-limited when the N:P ratio falls below 14, and phosphorus-limited when ratios are above 16.

A diverse, stable, and resilient roof

Plant diversity matters! Without plant diversity, the roof is unable to buffer for environmental changes: ruderal plants for fast colonialization and deep-rooted plants for higher drought resilience.

Also, diversity makes the roof more resilient to pests. If all plants are of the same type and an illness strikes, all of them might die. If there is a mixture of species, there will most likely be some survivors that can take the opportunity to spread and thrive.

But, how can we prevent pests from striking in the first place?

green roof plant diversity
Succession will cause different sedums to take over at different times. This is why diversity matters.

Pests and green roof plants

During which parts of your life did you get the most infections? If you’d allow me to make a qualified guess, I’d probably say that it has been during times of high stress, mentally and/or physically. If we sleep little, eat poorly, and stress a lot, we tend to get sick.

In a way, this is also the case for plants, through their stress looks different. Plant stress can be things like too little (or too much) water, high UV radiation, too much shade, too little (or too much) nutrients, or nutrients in the wrong ratios.

Usually, one moderate stressor won’t crash a system, but if several of them come together, well, then you end up with sick plants.

Often, we apply pesticides and hope for the best. However, if the stressors aren’t being dealt with, we are just postponing the situation.

As an example, we were once asked to remove grubs that were eating up the green roofs in Montgomery County MD. Instead of applying pesticides, we recommended that we needed to remove plant stress. You remove plant stress, and the plants themselves can deal with the grubs. Pesticides fix the system short-term but mess it up even more long-term.

Pesticides are not only expensive but end up in downstream water bodies, which may destroy local ecosystems and risk violating local environmental regulations.

Furthermore, the symbiotic relationships between the plants and microbes such as mycorrhizae might be killed by broad-spectrum pesticides. Destruction of this type of symbiosis means lower water and nutrient availability for the plants, which in turn means more stress.
It is like applying a band-aid on a gaping wound.

We are much better off taking a close look at what is stressing the plants in the first place: let’s focus on sustainability.

green roof plants mealybug
Mealybug infection of succulents

How can I de-stress my plants?

So, where do we start? How can you know if your plants are stressed?

Most of us already have enough other stress in our lives to worry about a roof full of “pet plants.” Continuously having to check your roof is simply not acceptable as it would transfer the stress from the plants to you. Stressing the building owners with high maintenance costs and time-consuming checks is not a good solution.

The solution is instead to create a roof that is resilient, stable, and less prone to “crash” or collapse. A sustainable roof takes care of itself and stays that way for decades with only minimal maintenance. The key is to have a healthy soil ecosystem that is matched with vegetation selected for your climatic region.

Let’s make nature work for us – for FREE!

sedum form green roofs on a field
A diverse and healthy sedum nursery

The making of a resilient roof

How do we make the plants happy? We can’t do anything about the rainfall, how the seasons come and go, or when the sun sets and rises, but we can adapt to the conditions. We can choose the right plants for the right region and, most importantly, we can improve the soil.

There is so much opportunity ahead! So much opportunity to make green roofs go from great to fantastic! Currently, German set standards (FLL) determine how one-size-fits-all green roof soils are being put together in climates ranging from the Prairie of the Chicago, to the welcome lake effect of Toronto, to the snow and cold of Montreal, to the humid subtropical climate of Virginia.

We can do better.

Improving soil health will increase stormwater retention and detention and ensure stable and resilient vegetation. It is a crucial, and often forgotten, factor in the green roof industry.

Luckily, this is an upcoming topic that we think will bring much good to the industry as a whole.

Reading tip: green roof plants

Oberndorfer, Erica, et al. "Green roofs as urban ecosystems: ecological structures, functions, and services."BioScience57.10 (2007): 823-833.

Green Roof Ecosystems (book)