GRP 004: Two Contrasting Approaches to Green Roof Design

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In this episode: We discuss the “landscape” versus “native plants” approach to green roof design.

Links mentioned:

GRP 004: Two Approaches to Green Roof Design (right click and select “save” to download this episode as an mp3 file)


Will: This is the Green Roof Podcast, Episode 4. Hi everyone, welcome to the Green Roof Podcast, the podcast dedicated to helping you getting you get your home or business on the road to a beautiful, energy-saving, living roof, even if you’re just starting today. My name is Will Ward.

Kevin: And this is Kevin Songer from sunny Florida.

Will: Today on the podcast we’re going to be talking about two approaches to green roof design, the pros and cons of each, and factors to think about when you’re trying to make a decision about how to approach designing your green roof from the very earliest stages. And if you stick around until the end of the podcast, on the quick tips segment we’re going to be giving you three very valuable websites that we’ve come across for helping you build the data profile of the climate and the setting that you’re going to be doing your project in, which will help you make some of the fundamental choices about how you approach your green room project. So please stick around for that. So Kevin, what’s going on?

Kevin: Well Will, it’s a beautiful day here in Florida. I just came back from a green roof project that we have in progress currently, and the plants, green roof plants, always look so perky and good in the morning. And it’s lovely, it’s going to have a food component, a native plants component to support biodiversity, good storm water plants and good landscape plants, and I think as we move through the next couple podcasts, we’ll see the advantages and disadvantages of choosing the right plants for the right place.

Will: Awesome, and things are pretty quiet up here in DC. We’re keeping our fingers crossed that we’ve finally broken over from that kind of stage where it’s spring for two days and then snaps back to a winter. So we’re hoping that we’ve broken through and it’s going to be spring for good now, but we’ll see how that happens. But I’m excited about our topic for today, which is going to be really getting at the first stages of planning a green roof and talking about two fundamental approaches to thinking about planning a green roof and some of the choices that go along with those two different approaches. And of course, we start with the usual caveat that these are not written in stone, there are a lot of different approaches, and there are a lot of different hybrids and combinations to different approaches. So, take these as generally guidelines and starting points for your own research.

Kevin: Absolutely, Will. Each roof is a living organism. I like to think of each green roof as an ecosystem, and I love Wikipedia’s definition of ecosystem. If I quote Wikipedia correctly, it goes something like this: an ecosystem is a web, a community, a network of individuals arranged in a self-sustaining and complex hierarchy of a pattern and process. And what that means for green roofs is that green roofs are very complex, and before we decide and choose what plants to put on the roof, be they food plants or storm water plants or native species, we need to understand some fundamental variables, characteristics of our rooftop ecosystem. So, understanding that roofs are complex ecosystems is the first step in successfully designing a green roof that’s going to last you for a long time.

Will: Yeah, and the way we approached this podcast, and this is based off of an article that Kevin published, and we’ll have a link to that in the show notes, about two approaches to thinking about the green roof ecosystem and the way that a green roof does or does not integrate with that ecosystem. And so the two approaches to green roof design that we’re going to be talking about today: approach number one we’re going to call the landscape approach, so how would you break that down Kevin?

Kevin: The landscape approach, Will, is typified by the use of very hardy, vigorous, drought-tolerant, pest-resistant landscape plants, those plants that have proven themselves year in and year out to survive. Not necessarily native plants, they can come from just about anywhere in the world.

Will: Okay. And approach number two we’re going to be calling the native plants approach, which is pretty self-explanatory, but what’s the general thesis or philosophy behind the native plants approach?

Kevin: Well, the big push to use native plants in landscapes and on green roofs is a fact that there is an abundant amount of scientific evidence that supports the theory that the native wildlife, the insects, the pollinators, the honeybees, etc, those wildlife species that are endemic to an area or are naturalized in the area can best utilize the chemical and phytochemical constituency of native plants. Now, there’s still a lot of data that has to be assimilated before we can say that with 100% certainty. However, there is some good showing, data-wise, that local wildlife actually benefits from the use of native plants in a landscape or green roof.

Will: Okay, and so going back to the landscape approach, what are the general benefits? So, you know that a certain species of plants generally does well on a roof and so that’s how, that’s what we’re going to go with.

Kevin: Right, it’s a plug-and-play approach Will, you’re absolutely approach. The landscape plants are very readily available at most big-box retailers, most local nurseries, because they’re grown and used on a regular by nurseries, plant nurseries, nation-wide and world-wide. So number one, landscape approach plants are very readily available, and they would include your species such as, here in the southeast Mexican petunia, some of the grasses, ornamental grasses, some of the lilies, African iris, etc. And these plants, you can pretty much drop them on the roof and walk away, and they’ll continue to spread and grow without any kind of care hardly. So they’re very durable, very reasonable from a cost perspective, because demand is out there and supply is there to fulfill the demand, so there’s a sense of comfort in knowing that if I buy these plants for my green roof, I can install them and walk away and not come back in a week to find dead plants. The landscape plants have that tried-and-true component and they’re very readily available. Whereas the native plants, on the other hand Will, it’s going to be hard sometimes to find native nurseries because the demand is not there. It’s growing across the nation, but we’re still seeing a lack of supply of native plants, and, so, you may have more difficulty in finding those native species that you want to put on your roof, and because the supply is limited they may be slightly more expensive.

Will: What about just gathering them from your local area, or getting them from the neighbor or that type of approach? Is that something that’s readily doable?

Kevin: That’s a very good question, Will. You always need to make sure that you’re abiding by local regulations with respect to collecting wild plants. The states and municipalities always typically have a regulation that prohibits the digging of threatened and endangered species. A lot of the more vibrant, colorful native species are on the threatened and endangered plant list because they’re so attractive and people want them for plants in their yard, so make sure you abide by all local regulations, but yes, if you have a big yard, you might want to dedicate a spot in the backyard to grow some native plants and then harvest those occasionally. Or if your neighbor has a weedy spot, go check out those weeds, they may be some great native plants suited for your green roof.

Will: I’m really curious about this movement to move toward native plants, and the sort of economics behind that. You said that more and more nurseries are beginning to offer native plants, and that’s springing up around that country. What is really behind that movement, and how has that been developing in the last couple of years?

Kevin: Will, there’s quite a few local, native plant society chapters around the United States and the world, and these chapters are typically advocacy groups to, with a mission to help preserve endemic plants species and encourage water conservation by use of drought-tolerant native species. And over the past five-ten years, I’ve seen these groups grow in size and number, as the interest has increased in preserving local habitats. And with the advent of the internet, the explosion of web capabilities, I’m really seeing a lot of educational information come out now on the benefits of using native species to support biodiversity pollinators, bees, etc. So there’s a growing , a definitely growing focus to use native plants on green roofs and in landscapes.

Will: Okay, so say we’ve decided to go one way or the other, either we’ve found a native plants supplier that’s appropriate for us, or decided to go with the landscape approach that uses more proven plants that you can find at a lot of different nurseries. What about the maintenance requirements and things like fertilization – how do the two schools break down on fertilization?

Kevin: Well, as you mentioned earlier, it does not have to be an either-or situation. You can have a green roof, and have a small patch of natives and a small patch of landscape plants, and see what does best in your situation, and then adjust your plantings in the future accordingly. Or, you can mix the two together. But, from a fertilization and maintenance standpoint, there’s some definite differences. Typically I see on landscape plants planted green roofs, the use of slow-release fertilizer, and the advantages and disadvantages of slow-release fertilizer include that slow release is great at keeping plants alive. However, it does contribute nitrogen and potentially phosphorus to storm water, which could cause algae problems. So there’s upsides and downsides to putting fertilizer on a roof. From a native perspective, native plant perspective, I find it very interesting that a lot of science I’ve seen incorporate what I call nitrogen-fixing plants into the native plant design, instead of the use of fertilizer. And the nitrogen-producing plants are like legumes. There are some nice native legumes, they’ll grab the nitrogen out of the air, and convert it into a form that can be used by plants for fertilization, and put it into the soil so you have fertilizer in a plant, which is very interesting. There’s some good information on the web about both, but definitely if you’re going to fertilize landscape plants use a slow release, and if you’re going to put natives always consider a nitrogen-fixing native like a legume. Also, with respect to maintenance, you have watering issues. Native plants can be very drought-tolerant, and so can some landscape plants.

Will: Okay, so we’ve heard a little about fertilizer requirements. What about the general theory around irrigating with these two different types, the landscape versus native plants approach?

Kevin: Well, I’m very pleased to see that both schools of thought are now trending towards micro or drip irrigation. And there are some wonderful systems for green roofs, irrigation systems that you can find on the web, that are micro-based and drip-based, that don’t spray and lose a lot of water to evaporation, but apply the water directly where it’s needed and ultimately conserve, Will. So, there’s some great systems out there, again you can find those on the web. There have been in the past, just standard, “when the plants look bad” theory to flood the planting area with water to hope we perk the plants up, but that approach is rapidly disappearing with these new irrigation technologies. Rainwater collection is very important for both landscape and for native plant planted green roof, and collecting water is simple. You have rain barrels or underground cisterns that store the storm water collected off the roof, and that water can be recycled at a later date to irrigate the plants when it’s dry. The one downside I would caution about a rainwater collection system is in certain areas of the world, you may have a whole lot of rain during the year, but it comes in the summer months, you may have some very dry winter months, so don’t think that you’re going to irrigate your green roof all year round with a rainwater cistern. You may be able to six or nine months out of the year, but there’ll be times of drought in certain areas of the US and the world where you’ve got to supplement that irrigation with well water or potable water. And I bring that point out to mention the importance of looking at your plants during design. You don’t want to put plants that can use rainwater irrigation nine months of the year, but then require potable irrigation water the other three months, because potable water we should conserve, we do have water crises in the US and around the world, and we should not be wasting water on a roof. So it’s important to understand – we’ll get into some of these factors later on, in understanding the climatic issues of your roof. But irrigation-wise, try to take advantage of all the nature-provided irrigation – dew, fog, rainwater precipitation, recycle your rainwater, use appropriate drip and micro-irrigation techniques, and, most important of all, design your green roof to use those plants that can survive without an excess of irrigation, Will.

Will: And one other point I thought we should hit with respect to water, is that we’re thinking of native plants as part of your local ecosystem, and of course the groundwater that’s already present in the soil is part of that for native plants, but up on the roof, that’s going to be a totally different game. How do you deal with some of those issues?

Kevin: Exactly. You need to understand your plants and that goes back to the saying right plant, right place. You would not want to put a native species that is happy in wet, soggy soils on a roof because, unless you’re pumping a lot of irrigation up there, it’s going to be dry and hot at times. So, understanding soil science, and understanding the soil that your native plants grow in, and your landscape plants, is important because you don’t have native hydrology on a green roof. You have a very alien, foreign type of soil hydrology on green roofs.

Will: Okay. Now, I’m really interested in this question – I think we’ve been talking from the point of view of an individual homeowner, someone who’s thinking about installing a green roof. But let’s zoom back out to the level of society, and we’re trying to promote green roofs, trying to increase the number of green roofs, it seems like the landscape approach where you have a certain number of plants, and you know they’re going to work very well across a large number of projects, might be a little more scalable, so it might be better suited to creating a large number of new green roof projects in a short amount of time. Whereas the native plants approach is going to take a little bit more time, because you need to be intimately familiar with the local setting, the plants that are going to survive there, you need to gather all this data about your setting. So how do you think about that trade-off between scalability and the native plants approach seems a little bit more, not one-size-fit- all, more project by project?

Kevin: Right, and you bring up a very valid point, Will. Because there’s so much knowledge about these landscape plants, because there’s so many of them, and because they’re used so often, the economics are very profit-friendly to use landscape plants. Again, they don’t cost much, are very readily available, and you can fill your truck-bed up with landscape plants and go to a green roof and bang! You’ve got a lot of green, a lot of texture, and a lot of color. So for most people, there’s an economic friendliness to using landscape plants. On the other hand, native plants can be thought of the some way, it’s just the supply is not as readily available. Once you understand native plants, sometimes they’re just as economically friendly, but they do require a little bit more thought because they haven’t been used as often. And we as a population and we as green roof designers, we don’t have that familiarity with native plants that we do with landscape plants. So, there’s a definite benefit, economic benefit for using landscape plants. You are right, they’re more scalable, they’re plug-and-play, very quick.

Will: Okay, well I think we’ve given our listeners plenty of food for thought for thinking about the two approaches and which might be right for them, but once you’ve made this decision, or maybe to make you before you or as you are making this decision, we recommend doing something that we call “building a site profile,” which, simply put, is just gathering as much data as you can about your local climate, your local conditions, what native plants might do well or might not do well on a green roof, and we just wanted to run quickly through these points and maybe point you to some resources or some tips to help you think through these issues. So, Kevin do you want me to just run through each one and we can discuss each one quickly?

Kevin: Absolutely, Will.

Will: Okay, so let’s start with wind speed and direction.

Kevin: Okay, well let me back up just a second. You know, in the morning when I get up, I always look at the weather because I want to know how to dress accoringly: is it going to rain, is there going to be sunshine, cold, hot, etc. And we should think about green roofs before we design and install in the same manner. What are we dealing with climatically? What kind of climate conditions are we dealing with? We need to know the fundamental basics of that ecosystem so we can chose the proper plants for that roof. There are so many different climate variables, and we are going to talk, Will and I, we are going to talk about those variables. But the two primary design variables that impact a green roof choice the most are (a) light and (b) wind. And so, those are the two primary design variables. Now we’ll also discuss secondary design variables, which will be temperature and many others.

To answer Will’s question, understanding wind exposure is critical to green roof design. Understanding light exposure is critical to green roof design. We’ll get into these more in detail in future podcasts, but in a nutshell, wind can kill a green roof. What happens is if you have a consistently strong desiccating wind, dry wind, you remove the water from the leaf, and without water your plant cannot conduct photosynthesis and it will die. A lot of approaches that I’ve seen in the past to alleviating wind issues is to pump more irrigation on the plants roots, but what happens is that you have if you have a strong, continual wind, Will, the water is going to be be desiccated so quickly from the leaf that the vascular system of the plant cannot keep up pumping enough water to the leaf to keep the plant alive. So, understanding how much wind you have on your roof, what kind of wind exposure, is crucial. We’ll talk about some of the ways to limit that wind exposure, like parapets or wind breaks, how to use different types of plants to serve as wind breaks. So wind is a very important factor in green roof design to understand, Will.

Will: Okay, so you’ve got data about your wind and you need to know not just at one given time, but you need a profile of how that changes through the year and the seasons, is that right?

Kevin: That’s important, absolutely, because you will have in some areas strong winds in the winter, strong winds in the summer, or vice versa, and you can design your plants to accommodate both.

Will: Okay, and so primary factor number one when you’re creating your site profile is wind. What about the second one you mentioned, which is light?

Kevin: All plants need light, Will, to live. So you need to know: do I have a southern exposure? Eastern, western, northern exposure? How many daylight hours do I have? What is the solar intensity? And light availability, like wind, without light, your plant’s not going to conduct photosynthesis, it will die. Light exposure and wind exposure are the two primary design variables in a green roof. We’ll talk more in detail as to why these are the most important. Some people would think that maybe temperature is more important than light or wind, and temperature is very important, but we have found that it’s not one of the two primary design variables such as wind and light. We’ll about talk briefly a few resources that are available, and Will’s going to point these links out that will allow someone to look at the historical data trends on available light and wind exposure, for any given point in the US and the world.

Will: Alright. So, we’ve got wind and we’ve got light, and now, what about some of the secondaries? What about temperature, rainfall, that sort of thing?

Kevin: Yeah, Will, those are important, and we’ll discuss those later. They help fine tune the plants for your system. The primary variables like wind really point us in the direction of the different families of plants to use on a green roof. The temperatures and precipitation, or available water vapor amounts, will allow us to fine tune and look into the genus and the species of which plants to use, and allow us to fine tune the colors and the textures that ultimately will decor our green roofs. So water vapor and temperature, adjacent plants that could potentially impact the growth habit of green roof plants, leaf litter, smog, there’s a variety, a number of secondary factors that we’ll explore.

Will: Okay, Kevin and I just took a little break, and we were just marveling at how much more there is here, and how much detail there is that we don’t have time for unfortunately in this episode. But I just had one more question about building a site profile that might help some new listeners. Say you were dropped in a different state: you’re in Florida, say you were dropped in California and you were walking around a neighborhood where you were going to install a new green roof there. What are some of the things you would look at when you’re looking at a site? What are the things you would look for, what would you write down in your notebook, what would you notice and not notice?

Kevin: First of all, I’d try to visit the site as often as possible, Will. It’s important to understand the site not only at one day out of 365, but understand the site in the spring, the summer, the fall and the winter. Of course that’s not always feasible, and that’s why the historical data on the web is important. But get out to the site as quick and as often as you can. Look for the sun’s paths, look for adjacent trees that could cast shade on your roof, look for anything that could ultimately impact your roof, and then, most importantly, what we’ve talked about before, look at plants that are growing in the gutters. They’ll give you a good indication of a lot of information valuable for green roof design.

Will: Alright. And now, we’re going to go directly to the quick tip today, which is a few resources that will help you gather some of this data, some of this longer term climate data about your site, which will help you when you’re making these choices, these fundamental choices about what type of approach you’re going to use when you start your green roof and select your plants. So the first one is What can we find there?

Kevin: It’s a wonderful portal for climate data and climate resources. If you’re wanting to know how much rain your green roof project in Portland, Oregon, will receive in any given month of the year, or in the Caribbean, or in Africa, or in Europe, then this is a wonderful site to just get good general information. Am I dealing with a site that’s going to be exposed to a lot of sun, or be very dry, or have a lot of water, and if so how much rainfall can I expect?

Will: Alright, that’s number one. Number two is

Kevin: This is some good technical data here, Will. Lots of charts, lots of maps, lots of graphs, lots of data, and also other links to technical resources. So if you are a designer and you’re wanting to collect some good data to support your ultimate green roof design, then this site can be very helpful.

Will: Alright. And the third one has a very long URL, so I’m going to try to read that out here, but I’ll have a link to that in the show notes. And it’s the US Drought Maps and Data.

Kevin: Sure, Will. We want to know, any time we design a green roof, if we’re going to be exposed to harsh conditions, and this site really focuses on the history of droughts in regional areas of the US. It’s a great site, some great graphics, and very good data.

Will: Alright! And that is going to wrap it up for today’s podcast, episode number four. We want to thank you all very much for listening. And one little breakthrough for us is this is our fourth episode and we’ve gotten our podcast approved in the iTunes store, so we’re both going to have links to that in our show notes, and I’ll give Kevin a link for his site, so we encourage you all to grab our podcasts in iTunes so that you can get each episode as it comes out. We’re going be aiming at doing one podcast a week roughly, and we look forward to hearing what you all have to say and your thoughts on the podcast. Until next time, have a great week!

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