Success of extensive green roof vegetation depends primarily on associated plant species' ability to survive the low moisture content of the substrate. As a result of the well-drained nature of the substrate, plants adaptable to dry, porous soils are primarily used in extensive green roof applications. Although Sedum species have dominated the plant palette for extensive green roofs, there is growing interest in expanding the plant list for extensive green roof systems. To effectively select suitable plants, species need to be evaluated in terms of their response to gradual and prolonged dry down of the substrate. A study to determine the relative rates of dry down for 15 species was conducted in greenhouse trials. During dry downs that extended over 5 months, the substrate of succulent and herbaceous species dried down at different rates. The change in moisture content of the substrate was not consistent among succulent and herbaceous plant species during the initial 18 d of dry down. Despite differences in rate of dry down, the succulent species, as a group, maintained viable foliage for over five times longer than the herbaceous species. The revival rates of the succulent species were nearly double those of the herbaceous species. Therefore, not only are succulent species more likely to survive during periods of drought, but these species are more likely to resume growth soon after water is again made available.

This research examined plant area covered (plant cover) for six plant species on an existing modular extensive green roof in a semiarid climate. Species evaluated were Antennaria parvifolia Nutt., Bouteloua gracilis (Kunth) Lag., Delosperma cooperi (Hook. f.) L. Bol., Eriogonum umbellatum Torr. aureum ‘Psdowns’, Opuntia fragilis Nutt., and Sedum lanceolatum Torr. Many methods for measuring plant cover are subjective and based on semiquantitative measurements. This study compared digital image analysis data (DIA) with manually collected converted two-dimensional data (C2D) for plants grown on an extensive green roof. For each plant in the study, digital images and manual two-dimensional measurements were taken on four dates (at 6-week intervals) in 2008 and on four dates (at 6-week intervals) in 2009. Additionally, comparisons between DIA data and final biomass, and C2D and final biomass, were performed. Plant cover increased for all six species during the 2008 growing season. However, E. umbellatum aureum ‘Psdowns’ had a low overwintering rate (12.5%) and was removed from analysis in 2009. In the spring of 2009, four of the five remaining species exhibited decreased plant cover as a result of winter dieback; the one exception was O. fragilis. In terms of plant cover, both quantification methods (C2D and DIA) revealed that B. gracilis and D. cooperi had more plant cover than A. parvifolia, O. fragilis, and S. lanceolatum by the end of the study. Thus, five of the six species evaluated in this study are appropriate for use in extensive green roof applications in semiarid regions. High levels of correlation were found between the DIA and C2D data sets (r = 0.77) averaged over the five species on all eight data collection dates. The groundcover species (A. parvifolia, D. cooperi, and S. lanceolatum) had a higher correlation on average (r = 0.83) than the upright (B. gracilis, r = 0.70) and decumbent (O. fragilis, r = 0.65) species. Additionally, DIA and final biomass correlations showed parallel trends with groundcovers averaging r = 0.83, upright r = 0.64, and decumbent r = 0.41. Therefore, using DIA to evaluate plant cover and biomass accumulation is especially appropriate for groundcover species.

A multi-faceted research project was conducted on a modular green roof in semi-arid, high elevation Denver, Colorado U.S.A. A photovoltaic (PV) array ran along the southeastern edge of the research area and visibly influenced the plant growth, cover and biomass. Plants grown near the PV prospered compared to plants in the exposed area. Average summer temperatures in the modules under the PV array were cooler with less temperature variation compared to the modules located in the exposed areas of the green roof. Shading structures integrated on green roofs may produce effects that resemble natural ecotones tending towards greater plant coverage and biomass, and therefore greater green roof resilience.

Five preemergence herbicides were applied to seven herbaceous perennials to evaluate weed control efficacy and phytotoxicity. Different species were used each year. The species used during 1992 were coneflower (Rudbeckia fulgida Ait. `Goldstrum'), common foxglove (Digitalis purpurea L. `Excelsior'), Shasta daisy (Leucanthemum ×superbum Bergmans `Alaska'), Stokes's aster (Stokesia laevis Greene `Blue Danube'), and avens (Geum Quellyon Sweet `Mrs. Bradshaw'). The species used in 1993 were woolly yarrow (Achillea tomentosa L.) and woolly thyme (Thymus pseudolanuginosus Ronn.). The herbicides and rates were napropamide (Devrinol 10G) at 4 and 8 lb a.i./acre; metolachlor (Pennant 5G) at 4 and 8 lb a.i./acre; oxyfluorfen+oryzalin (Rout 3G) at 3 and 12 lb a.i./acre; trifluralin (Treflan 5G) at 4 and 8 lb a.i./acre; and oxadiazon (Ronstar 2G) at 4 and 8 lb a.i./acre. Plants were grown in no. 1 containers and weed seeds were sown onto the substrate surface. Two control treatments, no herbicides but with weeds (weedy control), and no weeds or herbicides (weed-free control) also were evaluated. Weed control was effective and similar for all herbicides tested. Napropamide at 8 lb a.i./acre caused stunting in foxglove (20% to 45% less growth compared to weed-free control). Oxyfluorfen + oryzlain at 12 lb a.i./acre caused severe phytotoxicity (≈80% to 95% of plant injured) and stunted the growth (70% to 80% less growth, sometimes plant death) of woolly yarrow. Woolly thyme was stunted by all herbicides when applied at the recommended rates (42% to 97% less growth compared to control) except for oxadiazon and oxyfluorfen + oryzlain. Woolly thyme appeared to be more susceptible to phytotoxicity due to its less-vigorous growth habit and shallow, adventitious roots that were in contact with the herbicide.

Six different preemergence herbicides were used with eight different plant species to evaluate weed control efficacy and possible phytotoxicity. The species used were Stachys byzantina C. Koch (lamb's ears), Campanula persicifolia L. (peachleaf bellflower), Achillea millefolium L. ‘Summer Pastel’ (common yarrow), Coreopsis lanceolata L. (Lance coreopsis), Gypsophila pacifica Kom. (baby's breath), Wisteria sinensis Sims (Chinese wisteria), Syringa vulgaris L. (common lilac), Phlox paniculata L. (perennial phlox), Dahlia Cav. x hybrida (garden dahlia). Herbicides were applied to the soil surface at rates of lx and 2x as recommended by the label. The herbicides and rates were as follows: Pennant (Dual) 7.8 Liquid, 4.6, 9.1 kg ai/ha (4, 8 lb ai/A); Gallery 75DF, 1.1, 2.3 kg ai/ha (1, 2 lb ai/A); Ronstar 2G, 4.5, 9.0 kg ai/ha (4, 8 lb ai/A); Rout 3G, 3.4, 6.8, 13.6 kg ai/ha (3, 6, 12 lb ai/A); Surflan AS, 2.3, 4.6 kg ai/ha (2, 4 lb ai/A); and Treflan 5G, 4.5, 9.0 kg ai/ha (4, 8 lbs ai/A). Weed seeds of Setaria glauca (L.) Beauv. (yellow foxtail), Echinochloa crus-galli (L.) Beauv. (barnyardgrass), Poa annua L. (annual bluegrass), Capsella bursa-pastoris (L.) Medik. (shepherdspurse), Senecio vulgaris L. (common groundsel), and Amaranthus retroflexus L. (redroot pigweed) were sown on the soil surface. Two control treatments (no herbicide and no weed seeds applied, or no herbicide but with weed seeds) also were evaluated. Plants were grown in #1 black plastic containers in a medium of clay loam soil, plaster sand and sphagnum peat (1:1:2 by vol). Weed counts at the end of the season indicated that weed control was variable according to the herbicide used. Rout and Ronstar at both the lx and 2x rates controlled over 99% of the weeds (compared to the control treatment with weeds). Weed control for the other herbicides were as follows: Surflan lx = 92%, Surflan 2x = 95%, Pennant lx = 93%, Pennant 2x = 98%, Gallery lx = 35%, Gallery 2x = 43%, Treflan 1x = 88%, and Treflan 2x = 96%. Evaluations also indicated that herbicides utilizing oryzalin resulted in phytoxicity or stunting to Phlox (appearance declined 88% at the 1x rate and 93% at the 2x rate compared to the control treatment), Gypsophila (dry weights for Surflan 1x was 27% less and Surflan 2x was 39% less compared to the control treatment), and Stachys (appearance declined 55% for 1x rate and 60% at the 2x rate). Gallery (isoxaben) resulted in stunting in Stachys (dry weights for Gallery 1x were 75% less compared to control plants and Gallery 2x was 80% less).

In June 1991, a two year field study was initiated to examine if three non-turf groundcovers with reputations for using low amounts of water actually use less water than Kentucky bluegrass (KBG). Irrigation treatments were based on percentages of ET (100%, 75%, 50%, 25%, 0%) and calculated by the modified Penman equation. Results from the 1991 season indicate that at the 100% and 75% treatments Potentilla tabernaemontani and Cerastium tomentosum were significantly better than the other species in terms of establishment and vigor but quality declined significantly at rates below 75%. At the 50% rate both KBG and Sedum acre maintained good quality although growth was slow. At the 25% rate, quality of KBG significantly declined while Sedum acre maintained good quality. Quality of Sedum acre declined only slightly at the 0% treatment and would be a good alternative to KBG if water conservation was a high priority in the landscape.

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Recycled wastewater (RWW) has become a common water source for irrigating golf courses and urban landscapes. Ponderosa pine (Pinus ponderosa) is commonly used in urban landscape settings in the Rocky Mountain West. To evaluate the effects of RWW irrigation on quality and needle ion accumulations of ponderosa pine, eight landscape facilities near metropolitan Denver, CO, were selected for the experiment. Among these sites, four had been irrigated exclusively with domestic RWW [electrical conductivity (EC) = 0.84 dS/m] for 5, 6, 15, and 20 years, respectively. The other four with similar turf species, age ranges, and soil textures had been irrigated with surface water (EC = 0.23 dS/m). Ponderosa pines grown on sites irrigated with RWW exhibited 10 times higher needle burn symptoms than those grown on sites irrigated with surface water (33% vs. 3%). Tissue analysis indicated that ponderosa pine needles collected from sites receiving RWW exhibited 11 times greater Na+ concentration, 2 times greater Cl−, and 50% greater B concentrations than samples collected from the control sites. Stepwise regression analysis revealed that the level of needle burn was largely influenced by leaf tissue Na+ concentration. Tissue Ca level and K/Na ratio were negatively associated with needle burn symptoms, suggesting that calcium amendment and K addition may help mitigate the needle burn syndrome in ponderosa pine caused by high Na+ in the tissue.