Evaluating Zeolite in an Extensive Green Roof Substrate
Research in Colorado shows that the addition of zeolite to green roof substrates may produce mixed results, which can be improved by adjusting blend rates and watering schedules, and by selecting the appropriate plants for the site.
Green roofs provide the nursery industry with unique opportunities to propagate, produce and market unique plant taxa that have been proved to be suitable for growth in such challenging systems. Improving extensive substrate in ways that enhance green roof plant performance will make this type of system more attractive for use on buildings in urban communities. Increasing the desire for green roofs will in turn increase the demand for suitable plant taxa, thus expanding the market.
Extensive green roofs, which are characterized by shallow-depth substrate generally less than 6 inches deep, traditionally utilize a substrate composed predominantly of lightweight aggregate such as expanded clay, expanded shale, heat-expanded slate and pumice (volcanic rock). These materials allow for rapid drainage but have low nutrient holding capacity. While lightweight aggregates are beneficial for green roof substrate drainage and for satisfying building structural requirements, on their own these lightweight aggregates do not make ideal substrate for most plants. In a recent study, for example, various percentages of heat-expanded slate were evaluated as green roof substrate. As the percentage of heat-expanded slate in the substrate increased, performance of the Sedum species used, in general, decreased.
Organic matter is well known to be beneficial for root growth, although extensive green roof substrate with high organic matter content (more than 20 percent by volume) has resulted in shrinkage over time. This shrinkage is due to the gradual breakdown of the organic matter. Even coarse organic materials, such as coir and peat moss, will eventually break down over the life of the green roof. Thus, use of green roof substrate with high organic matter content would require replenishment of the organic matter component, which is not time- or cost-effective for most green roof systems. For that reason, most extensive green roof substrates are composed primarily of mineral-based materials.
One material that has been used in shallow, well-drained golf greens to improve nutrient- and water-holding capacities is an expanded potassium-calcium clinoptilolite product, commonly referred to as zeolite. ZeoPro™, a product of ZeoponiX Inc., Boulder, Colo., is a type of zeolite that is mined from volcanic deposits. The granules in ZeoPro™ have a diameter range of 0.02 to 0.09 inch, and a lattice structure suitable for plant-extractable nutrient and moisture retention.
Table 1. Chemical and physical characteristics of the four substrates.
Incorporating ZeoPro™ into a typical mineral-based green roof substrate may improve nutrient- and water-holding capacities. The objectives of our study, which was conducted during the 2008 and 2009 growing seasons, were to evaluate plant growth response to a series of extensive green roof substrate blends containing various percentages of ZeoPro™.
How it was done
Rooftop experiments were conducted on the roof of the eighth floor of the building that houses the U.S. Environmental Protection Agency Region 8 Headquarters in Denver. A 3.94-inch-deep extensive modular (tray) GreenGrid® system was installed in the fall of 2006. Research modules were placed among the existing modules in the spring of 2008.
Fig. 1. Image of one of the ten blocks (taken on July,1, 2008) showing response of the four Sedum taxa to the four substrates (moving counter-clockwise): a) 0% ZeoProT, (counter-clockwise) b) 33% ZeoProT, c) 66% ZeoProT and d) 100% ZeoProT.
The species used to evaluate the ZeoPro™ amendment were the Sedum taxa already in use on the green roof: Sedum acre (goldmoss stonecrop), S. album (white stonecrop), S. spurium (two-lined stonecrop) 'Dragon's Blood' and S. spurium 'John Creech'. The Sedum were planted as a mixed stand (one plant per taxa per module) in 24-by-24-by-3.9-inch modules on 12-inch centers from 128-cell plug trays. Modules were filled with one of four substrate blends:
- 3:0 GreenGrid® substrate to ZeoPro™ (0 percent ZeoPro™ H-Plus)
- 2:1 GreenGrid® to ZeoPro™ (33 percent ZeoPro™)
- 1:2 Green Grid® to ZeoPro™ (66 percent ZeoPro™)
- 0:3 Green Grid® to ZeoPro™ (100 percent ZeoPro™).
Modules for each of the four substrates were replicated 10 times. The GreenGrid® substrate is a proprietary blend that is lightweight, drains well and is designed for use in this modular system. It contains various percentages of expanded clay, peat, perlite and vermiculite. (Chemical and physical characteristics of the substrate blends can be found in Table 1.)
Substrate physical properties were analyzed at a professional soil-testing lab and reported on March 2, 2010. Analytical methods included organic matter, dry density, particle density, saturated hydraulic conductivity (permeability), total porosity, and air- and water-filled porosity at maximum water capacity and at pF 1.8 (pF 1.8 is equal to field capacity on the substrate moisture retention curve).
Table 2. Mean monthly weather data for the 2008 and 2009 growing seasons.
Planted modules were hand-watered every 48 hours to saturation and maintained at 75°F daytime and 65°F nighttime temperatures. Modules were moved from the greenhouse outdoors to acclimate on March 20, 2008, and fertilizer (Scott's Osmocote Pro 19-5-8) was applied at the rate of 1.46 ounces per tray. On March 26, 2008, the trays were installed on the green roof.
During the 2008 growing season, irrigation was supplied by 0.92 gallon-per-hour drip emitters spaced every 12 inches. At the beginning, irrigation was provided at 0.74 inch per week and then reduced to 0.31 inch per week on August 15. In order to provide more uniform coverage of water, the irrigation system was changed to an overhead MP rotator system during the 2009 growing season. With emitters spaced 8 to 12 feet apart, irrigation was provided at 0.25 inch per week starting July 9. Irrigation initiation in 2009 was delayed due to an unusually moist spring, with precipitation 81.3, 14.2 and 64.4 percent above normal for April, May and June, respectively. Weather for the 2008 and 2009 growing seasons is summarized in Table 2.
Fig. 2. Plant cover of four Sedum taxa in response to four substrates as determined by digital image analysis. (DIA) over eight dates during two growing seasons. ?Days 49, 91, 133 and 174 are in 2008 and days 413, 455,497 and 538 are in 2009.
What were the results?
All four Sedum taxa responded to the addition of ZeoPro™, however, not all in the same growing season or at the same percentage of ZeoPro™ (see Figure 2). For example, by the end of 2008, S. acre had the highest plant cover in the mixed blends (33 and 66 percent ZeoPro™) and the lowest in the uniform blends (0 and 100 percent ZeoPro™). S. album increased in plant cover with increasing ZeoPro™ content of the substrate.
However, both S. acre and S. album had low overwintering percentages, determined as presence or absence of individual plants, as ZeoPro™ content of the substrate increased (see Table 3). While winter survival as a percentage was higher in the treatment with no ZeoPro™ than the treatments with ZeoPro™, the S. acre and S. album plants that did survive had less plant cover over the 2009 growing season when compared to the 2008 growing season (see Figure 2). This is consistent with research that showed that some plants that were not fertilized during the previous growing season survived over the winter but were smaller in size compared to those that were fertilized. In the current study, all plants were fertilized at the beginning of the study; however, the ZeoPro™ treatments have higher nutrient levels, especially potassium, than the treatment with no ZeoPro™ (see Table 1).
Due to the fact that so few individual plants for either S. acre or S. album survived over the winter, and those that did survive were small, no significant differences in plant cover existed between treatments by the end of the 2009 growing season. Researchers in Michigan have noted good overwintering success for these two species of Sedum in the short term, even in some cases noting the dominance of these two species specifically. Over a long-term study, S. acre was relatively more dominant among a mixed stand of species in shade than sun. Due to the contrasting results, apparently there are enough climatic differences between regions to influence survivability of these species of sedums such as temperature fluctuations in fall, lack of snow cover during most of winter and early spring in Colorado compared to Michigan.
Table 3. Overwintering survival data for each Sedum taxa at each treatment; ten plants of each taxa were planted in 2008.Overwinter survival was calculated as the number of plants that exhibited regrowth determined on Day 413 (May 13, 2009) of the study.
In Canadian research, substrate depth greatly influenced overwintering success rates of S. × hybridum. Similarly, a Swedish study showed dominance of these two species, except that S. acre decreased in area covered coming out of the second winter of the study. This is similar to the results of the current study. The two S. spurium taxa ('Dragon's Blood' and 'John Creech') showed different results than the other two taxa (S. acre and S. album). At the end of the 2008 growing season, no significant differences in plant cover existed between treatments for either S. spurium cultivar.
The devil's in the details
Data collection. Plant area covered was determined by taking digital images, similar to a concurrent study evaluating plant species on the green roof. A FujiFilm FinePix S3000 (6x optical zoom 3.2 megapixel lens) camera was mounted to a 190xprob tripod with an extendable horizontal arm. Digital image analysis (DIA) was performed using SigmaScan Pro 5.0 image analysis software. The DIA data were analyzed to evaluate the progression of plant cover over time. DIA data were collected on eight dates over two years and were analyzed to determine plant cover. Four dates in 2008 at six-week intervals [May 14 (Day 49), June 25 (Day 91), August 6 (Day 133) and September 16 (Day 174)] and four dates at six-week intervals in 2009 [May 13 (Day 413), June 24 (Day 455), August 5 (Day 497) and September 15 (Day 538)] were evaluated. Winter survival was determined for each plant on May 13, 2009, and was determined via visual observation of the absence or presence of plant growth.
Additionally, volumetric moisture content (VMC) of the substrates was quantified using a ThetaProbe ML2x. The ThetaProbe was inserted into the substrate up to the depth of the probe (1.97 inches).
Three readings per module per date were recorded. For the VMC data, four dates in 2008 [May 14 (Day 49), June 25 (Day 91), August 6 (Day 133) and September 16 (Day 174)] and three dates in 2009 [May 27 (Day 426), August 19 (Day 510) and September 15 (Day 538)] were evaluated. Note: the 2009 dates for VMC data are different than the dates for DIA data due to technical difficulties with the ThetaProbe.
Experimental design and data analysis. The experiment was laid out as a randomized complete block design. There were 10 blocks with each of the four treatments per block (see Figure 1). All data sets were analyzed using a repeated measures analysis of variance procedure (GLIMMIX) in SAS® version 9.02. The GLIMMIX procedure was performed using t-tests for multiple comparisons of means to show differences in plant cover and VMC. The DIA data were transformed for analysis to the log scale to equalize and normalize the residuals; no transformation was performed on the VMC data. Since a few of the overwintering data were 0 and 100 percent, chi-square tests were used to make pairwise comparisons. All significant differences are at the p (less than or equal to) 0.05 level.
Although overwintering showed 100 percent survival across treatments for both S. spurium cultivars (see Table 3), plants in the 100 percent ZeoPro™ treatment were reduced in size at the beginning of the second season (note the decrease in plant cover on Day 413 in Figure 2), which is clearly an effect of overwintering survival. The two cultivars of S. spurium may have survived in greater numbers than S. acre and S. album because S. spurium is semi-evergreen, while the other two are evergreen.
The 2009 results for the S. spurium cultivars show significant differences by treatment, due in part to this overwintering phenomenon. For S. spurium 'Dragon's Blood', the 100 percent ZeoPro™ treatment had significantly lower plant cover from all other treatments through the 2009 growing season except on the last day (Day 538) compared to the 0 percent ZeoPro™ treatment (see Figure 2).
Sedum spurium 'John Creech' showed a similar pattern, but the 100 percent ZeoPro™ treatment recovered in plant cover more quickly than the 'Dragon's Blood' cultivar. Therefore, the 100 percent ZeoPro™ treatment was significantly lower in plant cover from the 33 and 66 percent ZeoPro™ treatments early in the season, on Days 413 and 455. The 0 percent ZeoPro™ treatment was only significantly different on Day 413 from the 100 percent ZeoPro™ treatment.
There are many possible factors that affected survivability of these green roof plants in these different substrate blends, especially during the winter season. Winter volumetric moisture content (VMC) and diurnal temperature fluctuation related to media color may influence plant survival. In a greenhouse study, the three species of sedums demonstrated variable rates of dry down with both S. acre and S. album drying down more rapidly than S. spurium 'John Creech'. The mean daily minimum temperature of the GreenGrid® substrate during the winter months (December 2008 through March 2009) was 27°F. However, minimum temperature alone may not be the only problem, as S. spectabile has been shown to not survive 27°F temperatures in September, but depending on the cultivar can survive conditions at less than - 4°F in January.
Table 4. Substrate volumetric moisture content (VMC) on seven dates over two growing seasons.
Additionally, the root hardiness of these species is unknown in this type of shallow, well-drained system. A longer trial period (greater than two years) may also ultimately affect results such as these. Finally, while it has not been formally documented, root size in relation to top growth for some of these species (such as S. acre and S. album) has been found to be noticeably less in luxury nutrient and moisture content situations compared to drier and lower fertility substrate.
Substrate VMC. Results of the VMC data indicate that moisture-holding capacities of treatments varied by their relative proportion of ZeoPro™ (see Table 4). During the first three evaluation dates of 2008, the trend is that the least amount of moisture was present in the 0 percent ZeoPro™ treatment, and the highest was in the 100 percent ZeoPro™ treatment, which is not consistent with the data provided in Table 1. The field results are consistent with research in turfgrass, which shows higher moisture contents in substrates that contain clinoptilolite than in sand alone. The difference between field and laboratory (Table 1 data) results is likely due to the higher ZeoPro™ substrate blends forming a thin crust at the substrate surface, therefore reducing the evaporative losses in the field compared to the 0 percent ZeoPro™ treatment.
Why green roofs?
Green roofs are an increasingly utilized device to help mitigate many environmental problems associated with urban communities. They have been effectively used worldwide as a mitigation tactic for urban stormwater management and urban heat island (UHI) effect, and for increasing the amount of green space available in urban communities. As a mitigation tactic for urban stormwater management, green roofs help to slow down the rate and reduce the total volume of water running off rooftops after precipitation events; due to the cooling effect of evapotranspiration, green roofs and the air above these vegetated surfaces are cooler when compared to nearby nonvegetated roofs. Green roofs also provide vegetated areas among expanses of asphalt, concrete, stone and glass; these vegetated "islands" add green spaces to urban communities and provide habitat for a variety of insect and animal species.
A qualitative comparison between irrigation application methods can be made as there were two different systems used in the two years of the study. As noted above, a drip irrigation system was used in 2008, and an overhead rotator system was used in 2009. This means that the overhead rotator system is equally, if not more appropriately, suited to this type of extensive green roof system because it effectively supplies parallel VMC for the plants while only using one-third as much water as the drip irrigation system (see Table 2). This observation is in agreement with similar irrigation observations discussed in other regions of North America.
Our research shows that the addition of ZeoPro™ to the substrate on an extensive green roof improved establishment year growth for S. acre and S. album, but higher concentrations of ZeoPro™ hindered overwintering success of these two species. Conversely, the two cultivars of S. spurium did not show benefit from the addition of ZeoPro™ in the first year, but did the second year. Therefore, the data indicate that addition of ZeoPro™ to extensive green roof substrate is beneficial for certain species of sedum. In general, VMC increased with increasing ZeoPro™ content of the substrate, but laboratory results showed decreasing water holding capacity as ZeoPro™ percentage increased.
Additionally, the overhead rotator irrigation system was apparently more efficient than the drip irrigation at supplying similar VMC to plants. Based on the finding of this study, we suggest that if ZeoPro™ is used to amend green roof substrates, it should consist of no more than 33 percent of the substrate blend.
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Jennifer M. Bousselot is a former graduate research assistant, and James E. Klett is a professor in the Department of Horticulture and Landscape Architecture at Colorado State University. Ronda D. Koski is a research associate in the Department of Bioagricultural Sciences and Pest Management at Colorado State University. They can be reached at email@example.com; firstname.lastname@example.org and email@example.com, respectively.
A summary of this research was reported in the Horticultural Research Institute's Journal of Environmental Horticulture in December 2012. Funding and support for this study was provided by the U.S. Environmental Protection Agency through a Cooperative Agreement (83350101-0).
Supplies were provided by:
- ZeoponiX Inc., Boulder, Colo. - ZeoPro™
- Weston Solutions Inc., West Chester, Penn. - GreenGrid® substrate and modules
- Hunter Industries, San Marcos, Calif. - MP Rotator® irrigation heads.