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Moisture Requirements, Drought Tolerance, and Irrigation

Greenhouse studies on drought tolerance

The first greenhouse study consisted of seven species combinations subjected to five different watering regimes. Species included individual trays of a native perennial (Coreopsis lanceolata); a native grass (Schizachyrium scoparium); Sedum album; Sedum reflexum; Sedum kamtschaticum; a mixture of seven sedum species; and a control with no plants. Sedum album, S. reflexum, and S. kamtschaticum were chosen because they possess small, medium, and large sized leaves, respectively, traits that may influence evapotranspiration. Irrigation regimes consisted of watering every other day, once a week, once every two weeks, once every four weeks, and never. Each treatment was replicated eight times for a total of 280 trays arranged in a completely random design.

The second greenhouse study consisted of two species combinations (a mixture of seven sedum species and a control as described above), three substrate depths, and five watering regimes (same as above). The three substrate depths included 2.5 cm of substrate with 1 cm of water retention fabric, 2.5 cm of substrate with 2 cm of fabric, and 5 cm of substrate with 1 cm of fabric. Each treatment was replicated eight times for a total of 240 trays arranged in a completely random design. All trays in both studies included the XeroFlor green roof drainage system and substrate carrier (Behrens Systementwicklung, Groß Ippener, Germany). The growing substrate was composed of expanded slate, USGA specification sand; peat, and aged compost.

During both greenhouse studies, evapotranspiration was measured by obtaining tray weights. Initially tray weights were obtained daily and thereafter on a periodic basis. Substrate moisture content was recorded with a Theta Probe Soil Moisture Sensor ML2X (Delta-T Devices, Ltd., Cambridge, U.K.). In addition, plant stress was monitored with chlorophyll fluorescence (Fv/Fm) measurements taken on randomly selected leaves with a Plant Efficiency Analyzer (PEA) fluorometer (Hansatech Instruments Ltd., Norfolk, England, UK). Since chlorophyll fluorescence levels are tied to the maximum dark-adapted photochemical efficiency of photosystem II (PSII), they can serve as a general measurement of plant photosynthetic potential. At the conclusion of each experiment, dry mass accumulation was calculated from shoot and root dry weights.

Results of the first study indicate that even after the four-month period, Sedum spp. survived and maintained active photosynthetic metabolism, relative to Schizachyrium and Coreopsis.  Furthermore, when Sedum was watered after 28 days of drought, chlorophyll fluorescence (Fv/Fm) values recovered to values characteristic of the 2 days between watering (DBW) treatment.   In contrast, the non-CAM plants required watering frequency every other day in order to survive and maintain active growth and development.  Regardless of species, the greatest increase in total biomass accumulation and fastest growth occurred under the 2 DBW regimes. 

In the second study, substrate volumetric moisture content could be reduced to 0 m3 • m-3 within one day after watering depending on substrate depth and composition.  Deeper substrates provided additional growth with sufficient water, but also required additional irrigation because of the higher evapotranspiration rates resulting from the greater biomass.  Over the 88 day study, water was required at least once every 14 days to support growth in green roof substrates with a 2 cm substrate depth.  However, substrates with a 6 cm substrate depth could do so with a watering only once every 28 days.  Although vegetation was still viable after 88 days of drought, water should be applied at least once every 28 days for typical green roof substrates and more frequently for shallower substrates to sustain growth. The ability of Sedum to withstand extended drought conditions makes it ideal for shallow green roof systems.

Complete results are published in:

Durhman, A.K., D.B. Rowe, and C.L. Rugh.  2006.  Effect of watering regimen on chlorophyll fluorescence and growth of selected green roof plant taxa. HortScience 41(7):1623-1628.

VanWoert, N.D, D.B. Rowe, J.A. Andresen, C.L. Rugh, and L. Xiao. 2005. Watering regime and green roof substrate design affect Sedum plant growth.  HortScience 40(3):659-664.

Many succulents growing on tables inside the MSU greenhouses
Greenhouse evaluation of succulents

Five Sedum Kamtschaticum samples in various stages of drought
Sedum kamtshaticum can live for at least 88 days without water

Comparison of Water Use Efficiency of Overhead, Drip, and Sub-irrigation

Healthy, actively growing plants are necessary to achieve optimal stormwater retention, energy conservation, aesthetics, and other benefits that green roofs can provide.  Depending on the plant species, supplemental irrigation is sometimes necessary. Because substrate depths are often shallow, there must be adequate pore space to allow for drainage which translates to less water holding capacity and little if any capillary movement of water.  For this reason, there are challenges to utilizing drip and sub-irrigation despite the increasing trend to specify these for green roofs. Increasing substrate depth can alleviate some of these problems, but shallow depths are often desirable because buildings must be structurally strong enough to support the added weight of the green roof.

Our overall objective was to determine water use efficiency of overhead, drip, and sub-irrigation methods on coarse aggregate substrates utilized on green roofs and to measure plant growth and health for various substrate types subjected to these irrigation methods.  The study was conducted in the MSU Plant Science Greenhouses in LiveRoof green roof modules and consisted of two phases: (1) determining the physical properties of various substrates and systems and (2) quantifying plant health when subjected to different irrigation methods. 

In Phase I, five commercial substrate types or systems were subjected to three irrigation methods. Each treatment was replicated eight times. All substrates were analyzed to determine bulk density, capillary pore space, non-capillary pore space , infiltration rate, water holding capacity, pH, conductivity (EC), and organic matter by LOI (loss on ignition) at 360 °C.  Measurements included volume of water applied (30 minutes), water retention at container capacity, runoff (wasted water), volumetric moisture content measured before and just after field capacity was observed, and water dispersal (distance water front moved horizontally from emitter for drip and sub-irrigation).

Substrates subjected to overhead irrigation or those with a moisture retention fabric retained the greatest amount of water.  Sub-irrigation resulted in the least amount of water retention and the most wastewater, except when a water retention fabric was present.  Substrate volumetric moisture content followed the above readings.  The moisture retention fabric was effective in retaining water, but for sub-irrigation a visible water front was not visible as water did not reach the surface via capillary action (Figure 1). Differences can be attributed to the fact that overhead irrigation distributed water over 100% of the area, whereas in many cases the water front radiating from the drip or sub emitters never merged leaving dry areas in between emitters.  Adding the moisture retention fabric improved water retention while greatly reducing the percentage of wasted water for the drip (83% down to 67%) and sub-irrigation treatments (92% down to 78%). For overhead irrigation, the reduction was not statistically significant, but waste water runoff was still reduced from 22% to 19%.  The presence of vegetation also reduced the percentage of waste water runoff as the sedum mix reduced waste from 83% to 77% when irrigated by drip lines.

Drip irrigation test trays after 5 minutes Drip irrigation test trays after 15 minutes Drip irrigation test trays after 25 minutes

Representative horizontal movement of wetting front for Live Roof substrate subjected to drip irrigation after 5, 15, and 25 minutes, respectively.

sub irrigation test trays after 10 minutes sub irrigation test trays after 20 minutes sub irrigation test trays after 30 minutes

Representative horizontal movement of wetting front for Live Roof substrate subjected to sub-irrigation after 10, 20, and 30 minutes, respectively.  Orange flags show location of sub-surface emitters

In Phase II, we compared how plant health was influenced by irrigation method. Sedum album and Sedum floriferum grown in Renewed Earth substrate with and without a moisture retention fabric were subjected to overhead, drip, sub, and no irrigation treatments.  Every three weeks, repeated measurements were recorded for plant survival, growth index [(L x W x W)/3], chlorophyll fluorescence to quantify plant stress, and substrate volumetric moisture content.  After 12 weeks plants were harvested, separated into roots and shoots, dried, and weighed to determine biomass, root:shoot ratios, and carbon sequestered.  

Results show that overhead was the most favorable for plant growth and health. Total plant dry weights averaged 0.99 g, 0.77 g, 0.40 g, and 0.09 g for Sedum album subjected to overhead, drip, sub and no irrigation, respectively, when no water retention fabric was used.  Likewise, Sedum floriferum had dry weights of 1.64 g, 0.83 g, 0.66 g, and 0 g for the same treatments. The inclusion of water retention fabric generally improved results for drip and sub-irrigated plants. It is interesting to note, that for S. floriferum, the root:shoot ratio was highest for the sub-irrigation treatment with no MRF.  With limited water availability, this species partitioned greater growth to the root system relative to shoots.  Chlorophyll fluorescence values were also the lowest for plants subjected to sub- or no irrigation.

a person's hands are pictured using a theta probe a person's hands are pictured measuring chloro-flourescense

Measuring volumetric moisture content

Measuring chlorophyll fluorescence

In conclusion, for the green roof substrates tested, those subjected to overhead irrigation maintained the highest volumetric moisture content and had the least amount of runoff or wasted water compared to drip and sub-irrigation. Overhead applications also generally resulted in greater plant growth and plant health as measured by chlorophyll fluorescence.  The fact that there is little capillary movement of water from drip and sub-irrigation emitters raises the question as to the efficiency of these methods for establishing and growing plants.  Overhead, drip, and sub-irrigation all have their place, however, if irrigation is to be used then one must choose the most cost effective and environmentally friendly method of irrigation depending on a combination of climate, design intent, plant selection, and substrate depth and composition.

Complete results are published in:

Rowe, D.B., M.R. Kolp, S.E. Greer, and K.L. Getter. 2014. Comparison of irrigation efficiency and plant health of overhead, drip, and sub-irrigation for extensive green roofs.  Ecological Engineering 64:306-313. (http://dx.doi.org/10.1016/j.ecoleng.2013.12.052)

Banner caption: Sedum kamtshaticum can live for at least 88 days without water