Growing media

Three different substrates were used in the experiments (see Appendix for properties of all substrates). To replicate ‘real-life’ conditions of a green roof, an industrial green roof substrate, Meadow Roof Medium (hereafter referred to as MRM; Vital Earth GB Ltd., Ashbourne, Derbyshire, UK) was used in Experiments 1, 2, 4 and 5. Due to changes in the manufacturing processes, it was not possible to obtain MRM in 2015, and a ‘model’ situation, using peat-based compost (Vitax Ltd., Leicestershire, UK), was therefore set up in Experiment 3.

MRM was chosen as a test substrate as it is a commercial green roof substrate manufactured and used in the UK, composed predominantly of crushed brick with added organic matter. This therefore represents ‘real-life’ green roof conditions and the water-retention characteristics and the behaviour of plants growing in this substrate are of practical interest. However, nutrient concentrations in the MRM are inherently high, resulting in high levels of phosphorus leaching from the substrate during the greywater (GW) experiment (Experiment 5). Additionally, the nutrient content added through GW application was small compared to the inherent substrate nutrient content. This made it difficult to analyse the uptake and storage of tracer nutrients in the substrate and plants, and to assess differences in runoff quality between treatments.

It was therefore decided to also test the impact of GW under ‘model’ conditions using a nutrient- poor substrate, so that the nutrient content added through GW application was an appreciable proportion of the total nutrients in the system. Thus, several substrate mixtures with varying nutrient concentrations were tested in a pilot study (data not shown) to formulate a substrate that was low but not deficient in nutrients, and would not be too free-draining. A 50:50 mix by volume of vermiculite and peat-based compost (hereafter referred to as VC mix) was identified as the most suitable substrate, and was used in Experiment 6. The VC mix was prepared at the time of planting by manually combining equal volumes of vermiculite (William Sinclair Horticulture Ltd., Lincoln, UK) and peat-based compost (Vitax Ltd., Leicestershire, UK) in a bucket and mixing thoroughly before filling containers. As nutrient concentrations in the VC mix were low, uptake and accumulation of the tracer nutrients in the substrate, plant tissues and runoff was easier to identify, thus allowing better assessment of differences between species and irrigation treatments. To enable quantification of all nutrient inputs to the plant/substrate during Experiment 6, no fertilisers were added to the VC mix either during planting or throughout the

41 experiment. Irrigation with GW was the only additional nutrient input and therefore may have acted as a fertiliser; plants irrigated with tap water (TW) received no additional nutrients during the experiment.

2.4.1 Preliminary trials to characterise substrate moisture properties

Preliminary trials were carried out with each substrate at the start of the experimental seasons (March 2014 with MRM; March 2015 with peat-based compost; and May 2015 with VC mix), in order to determine the moisture characteristics of each substrate and determine daily plant water use. This information was then used to identify typical substrate moisture content (SMC) values under well-watered and dry conditions for each substrate, and to inform the irrigation volumes and scheduling in Experiments 5 and 6.

In the MRM and VC mix tests, three replicates of each species in 2 L containers, plus three control containers with bare, unvegetated substrate, were saturated to container capacity by submerging in water for 15 minutes and then draining for 30 minutes. Each container was then weighed and SMC was measured as described in Section 2.5.1. Thereafter, containers were weighed every 24 hours for 4 – 5 days to determine daily water loss from each container, and SMC was also measured daily. Containers received no further irrigation during this period. The substrate moisture characteristics of the peat-based compost used in Experiment 3 were tested in the same way, using 2 L control containers with bare, unvegetated substrate only. The mean daily water loss per container in each of the preliminary trials is presented in Table 2.2. It is worth noting that plants in the VC mix test were generally larger than those used in the MRM test, particularly

Sedum, and tests were carried out at different times of the year (March for MRM and May for VC

mix), which may partly account for the greater water uptake from the VC mix (in terms of absolute volumes), especially as water loss from bare, unvegetated substrate was similar.

SMC of the MRM tended to range between 0.300 and 0.400 m3 _m-3 _{when the substrate was}

saturated and decreased to as low as 0.100 m3 _m-3 _{after 4 or 5 days of drying, depending on}

species (data not shown). With the VC mix, SMC was around 0.550 – 0.600 m3 _m-3 _{when the}

substrate was saturated and typically above 0.120 m3 _m-3 _{after 5 days of drying, depending on}

species (data not shown). The SMC of unvegetated peat-based compost varied from 0.600 m3 _m-3

when saturated to 0.500 m3 _m-3 _{after 5 days of drying (data not shown). Based on the results of}

the preliminary tests, plants in all three substrates were considered ‘well-watered’ during Experiments 1 – 6 when SMC was > 0.250 m3 _m-3_{, whilst SMC < 0.150 m}3 _m-3 _{was considered low.}

42

Table 2.2: Mean daily water loss per 2 L container for each treatment in the three substrates. Data are the mean daily water loss from 3 replicate containers per species/control measured over 4 or 5 days following saturation, with associated standard error of the means (SEM).

Treatment

Mean (± SEM) daily water loss per 2 L container (mL)

Meadow Roof Medium VC mix Peat-based compost

Heuchera 56 (± 4.4) 67 (± 6.4) -

Salvia 72 (± 2.6) 114 (± 11.5) -

Stachys 68 (± 3.8) 117 (± 12.5) -

Sedum 39 (± 1.7) 164 (± 17.9) -

Control 52 (± 4.3) 53 (± 7.9) 43 (± 4.0)