D. Bradley Rowe

Green roofs as a means of pollution abatement.

Green roofs involve growing vegetation on rooftops and are one tool that can help mitigate the negative effects of pollution. This review encompasses published research to date on how green roofs can help mitigate pollution, how green roof materials influence the magnitude of these benefits, and suggests future research directions. The discussion concentrates on how green roofs influence air pollution, carbon dioxide emissions, carbon sequestration, longevity of roofing membranes that result in fewer roofing materials in landfills, water quality of stormwater runoff, and noise pollution. Suggestions for future directions for research include plant selection, development of improved growing substrates, urban rooftop agriculture, water quality of runoff, supplemental irrigation, the use of grey water, air pollution, carbon sequestration, effects on human health, combining green roofs with complementary related technologies, and economics and policy issues.

The role of green roof technology in urban agriculture

Urban agriculture is a global and growing pursuit that can contribute to economic development, job creation, food security and community building. It can, however, be limited by competition for space with other forms of urban development, a lack of formalized land use rights and health hazards related to food contamination. The use of green roof technology in urban agriculture has the potential to alleviate some of these problems, without adversely affecting the benefits provided by urban agriculture. It would not only enable the use of land for development and agriculture, but may also facilitate the formation of formal space and water use agreements, and enable redistribution of ground-level resources among urban farmers. This could decrease the use of contaminated land and water at ground level and alleviate health concerns. Before green roof technology can be incorporated into urban agriculture on a larger scale, installation costs must be reduced, roof weight limitations should be assessed, and appropriate management practices should be developed which will ensure that the benefits of green roofs, such as energy savings and storm water management, are still provided to urban communities.

Evaluation of Vegetable Production on Extensive Green Roofs

Rooftop vegetable gardening is a production system in urban agriculture based on green roof technology. To broaden the scope of this practice, the use of relatively shallow substrate depths must be explored, since most existing flat roofs are unable to support much added weight. Three growing systems—a green roof, raised green roof platforms, and in-ground—were evaluated for vegetable and herb production over three growing seasons (2009–2011). Tomatoes (Solanum lycopersicum), green beans (Phaseolus vulgaris), cucumbers (Cucumis sativus), peppers (Capsicum annuum), basil (Ocimum basilicum), and chives (Allium schoenoprasum) were studied because of their common use in home gardens. All plants, except pepper, survived and produced biomass in all growing systems and yielded crops large enough for analysis in 2009 and 2010. Overall, yields and basil biomass were higher and of better quality in-ground during 2009, the only year irrigation was applied, and similar on the roof and platforms. Variability in success was partially due to annual weather variation with the greatest impact on cucumber. Yields of chive, a perennial crop, were not affected by growing system after the first year. Results suggest that, with proper management, vegetable and herb production in an extensive green roof system is possible and productive.

Nitrogen nutrition of hedged stock plants of Loblolly Pine. II. Influence of carbohydrate and nitrogen status on adventitious rooting of stem cuttings

Hedged stock plants of four full-sib families [27-2 × 27-5, 27-3× 27-1, 27-2 × 27-1, and 27-6 × 27-1 (designated B, G, R, andW)] of loblolly pine (Pinus taeda L.) were fertilizeddailywith a complete nutrient solution containing N at either 10, 25, 40, 55, or 70mg·L−1. May (spring softwood), July(summersoftwood), and January (winter hardwood) terminal stem cuttings were taken fortissue analysis and rooting studies. Spring cuttings rooted in the highestpercentages (59.5%), followed by winter (40.5%), and summer (34.7%). Maximumrooting for spring (70.0%), summer (48.6%), and winter (55.6%) occurred withcuttings taken from hedges that received N at 55mg·L−1. Genetic differences among familieswere evident as families G and W rooted in greater percentages at lower appliedN rates and family B was the best rooting family at the highest applied N rate.When internal N levels were considered families G and W also rooted best at lowinternal N levels, while family B was the best rooting family at high internalNlevels. Neither total nonstructural carbohydrates (TNC) nor any of severalspecific carbohydrates were correlated with rooting and an optimal TNC : Nratiofor rooting was not found. Root counts generally increased with increases inapplied N. Root number was weakly correlated with TNC (R = 0.29, P ≤0.01) and several specific sugars, but was not correlated with internal N.Totalroot dry weight, total root length, and total root area responded similarly.

Comparison of stormwater runoff from sedum, native prairie, and vegetable producing green roofs

Stormwater retention is one of the well-studied benefits of green roofs. A roof’s ability to retain stormwater depends on factors such as the intensity and duration of the rain event as well as substrate depth, substrate moisture content at the start of the rain event, and vegetation type, health, density and water use efficiency. Extensive green roofs used for crop production differ from traditional Sedum and prairie-covered extensive green roofs in plant density and water use efficiency, but their impact on stormwater retention has not been well studied. Three vegetation types (unfertilized Sedum and native prairie species mixes, and a fertilized vegetable and herb species mix) were compared for stormwater runoff quantity over three growing seasons and stormwater runoff quality during one growing season. The prairie covered green roofs had the lowest increase in runoff as precipitation increased, almost half that of Sedum or vegetable producing green roof treatments. Vegetation type had no effect on runoff nitrate-nitrogen (NO3−) concentrations, but NO3− concentrations decreased over the course of the growing season. Runoff phosphorus (P) concentrations also decreased over time in the Sedum and prairie treatments, which were lower than P concentrations from the vegetable green roof throughout the growing season. This is likely a result of the difference between amounts of NO3− and P applied to the vegetable green roof and the needs of the crop plants in that treatment. The similarities in water retention and water quality between vegetable producing extensive green roofs and Sedum green roofs suggest that vegetable production with careful nutrient management will not have a negative impact on stormwater retention or runoff water quality.

Nitrogen nutrition of hedged stock plants of Loblolly Pine. I. Tissue nitrogen concentrations and carbohydrate status

Hedged stock plants of four full-sib families [27-2 × 27-5, 27-3× 27-1, 27-2 × 27-1, and 27-6 × 27-1 (designated B, G, R, andW)] of loblolly pine (Pinus taeda) were fertilized dailywith a complete nutrient solution containing N supplied fromNH4NO3 at either 10, 25, 40, 55, or 70mg·L−1. In May (spring softwood), July(summer softwood), and January (winter hardwood) terminal stem cuttings weretaken for tissue analysis and rooting studies. Averaged over families, meantissue concentrations of N were higher in spring (1.8%) and summer (1.7%) thanin winter (1.3%). Concentrations of N increased linearly with increasing levelsof applied N. Concentrations of total nonstructural carbohydrates (TNC) inwinter (32.8%) were twice those in spring (17.1%) or summer (16.3%), but wererelatively unaffected by N application. In contrast, starch concentrations weresignificantly higher during spring and summer than winter. The greatest numbersof orthotropic shoots ≥ 9 cm in length were produced for thespring, followed by summer, and then winter hedgings. Number of shoots producedper hedge increased with increasing applied N rates, and family B producedsignificantly more shoots than the other three families at all applied N levels.Genetic differences among families were evident as several interactions with Nrates were observed.

Evaluation of nutrient management and mulching strategies for vegetable production on an extensive green roof

ABSTRACT Substrate nutrient and moisture management are two major concerns in green roof agriculture, especially when using extensive systems, but there are currently no recommendations or best management practices. The purpose of this study was to explore three mulching strategies (pine bark, living sedum, and no mulch) and three fertilization regimens (25, 50, and 100 g∙m−2 of 14-14-14 N-P-K slow release fertilizer applied twice each growing season) over two growing seasons to determine their benefits to rooftop agriculture. Tomatoes (Solanum lycopersicum), beans (Phaseolus vulgaris), cucumbers (Cucumis sativus), peppers (Capsicum annuum), basil (Ocimum basilicum), and chives (Allium schoenoprasum) were grown. The 2010 crops outperformed those grown in 2011, likely because of the more extreme temperature and precipitation variations during 2011. When there were differences among mulch treatments, pine bark usually resulted in higher productivity than live sedum mulch. The effects of live sedum mulch on crop production were mixed, consistent with previous literature on the use of live mulches in agriculture. With the exception of whole plot, tomato, and cucumber grades, there was a positive dose response to fertilizer. Further research into more types of mulch, their effects on the green roof microclimate, and fertilizer composition and release rates is required.

Salt tolerance of common green roof and green wall plants

Detrimental effects of road deicing salt on vegetation are well known and have been well studied, with the exception of typical green roof plants, which could experience damage on green roofs with public access and green walls near roadways in cold climates. Two studies were conducted comparing salt tolerance of five Sedum species, two Allium species and a mixture of turf grasses when exposed to six levels of salinity applied either as foliar spray or as liquid applications to the soil. A third study compared salt tolerance when plants were placed at three distances from a major highway. Response variables measured included survival, a health score from 0 to 5, and a growth index. Allium cernuum, A. senscens and S. ellecombianum were relatively tolerant of both saline spray and soil inundation at high saline concentrations in terms of survival, mean health scores, percentage of healthy plants and growth index. Sedum reflexum was much less tolerant of saline spray at higher salinity concentrations and soil inundation regardless of salinity levels. Distance from the road had no effect on plant survival rates but plants farthest from the road had higher mean health scores and a greater percentage of healthy plants than plants closer to the highway.

Does Compost Selection Impact Green Roof Substrate Performance? Measuring Physical Properties, Plant Development, and Runoff Water Quality

ABSTRACT Six green roof substrate blends were created by using composts sourced from local suppliers and the Michigan State University Student Organic Farm. Bulk density, field capacity, total porosity, and saturated hydraulic conductivity were determined for each substrate and compared to an un-amended expanded shale aggregate. Significant differences were detected in all measured physical properties. A plant growth study was conducted in a greenhouse. Ocimum basilicum (basil), Sedum floriforum (sedum), and Carex eburnea (bristleleaf sedge) were grown in a depth of 10 cm of all six substrates for 6 months. The greatest dry shoot masses in bristleleaf sedge and sedum were twice those of the smallest masses. The largest wet harvest of basil was four times greater than the smallest harvest. Runoff water was collected after simulated precipitation events on regular intervals during the plant growth study and analyzed for nitrate and phosphate concentrations. Ion concentrations were greatest on the first measurement date and decreased rapidly with time. Compost selection had a strong impact on initial nitrate and phosphate concentrations, but the influence of compost on concentrations diminished with time. Overall, compost selection was found to have measureable and meaningful impacts on green roof substrate performance.