Introduction In greenhouse production of potted plants, the percentage of evaporation on the total evapotranspiration varies widely, depending on development stage, growing media used and plant species. Considering the entire production period in pot roses, the proportions of water discharged from the pots due to evaporation varied between 42 and 46% from a total of 1.45-1.57 L per plant (Budke, 2013) . Thus, evaporated water significantly contributes to increasing the humidity in greenhouses.

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... xperiments with insulation glasses in greenhouses showed a high efficiency to save heat energy (Bettin et al., 2012) . However, due to the effect of decreased condensation on the cold panes, the humidity in these greenhouses was generally higher. The higher humidity increases the risk of calcium deficiency due to reduced transpiration (Bradfield and Guttridge, 1979) and the risk of infection by Botrytis (Volpin and Elad, 1991) . Also, the plant material is often softer and more pliable which is usually not desired in ornamental plants (Mortensen, 1986) . Even though experiments in highly insulated greenhouses showed a higher humidity but lower evapotranspiration, still about 90 L m -2 water were needed for evapotranspiration of bedding plants from mid-February to end of May. With an enthalpy of 2,444 kJ L -1 water for evaporation at 25°C (Haynes et al., 2014) and an average evaporation of 45%, these water losses also imply an additional energy demand of 99 MJ m -2 in the mentioned period. Covering containers with different materials is often done in nursery plant production. However, the reason is mainly weed control and not a reduction in evaporation. In ornamental crop production under protected conditions container cover layers or a stack of substrates with different physical properties to reduce evaporation are not common in practice. Under most conditions, water flow to the surface of a growing media followed by evaporation will take place as capillary flow of liquid water. This assumption is justified for the majority of applications in soils, except if the soil is relatively dry (Scanlon et al., 2003) . A pine bark mulch cover layer on peat substrate may reduce the flow of water in the liquid phase considerably if getting dry, but vapor flow might considerably contribute to water flow in the rather coarse material. Therefore, vapor flow may be an important process which might have to be considered when modeling evaporation in containers covered with coarse materials. Basis for any evaluation of the efficiency of cover layers with respect to evaporation are the physical properties of the materials used. The physical properties of growing media describing the quality of the substrate for different horticul- German Society for Horticultural Science Summary Evaporation from growing media significantly contributes to increasing the humidity in greenhouses. The effects of a pine bark mulch cover on substrate evaporation was evaluated with different pot experiments. The obtained data have been tested within the water balance model HYDRUS-1D, which was originally developed for mineral soils. Objective of this study was to test the performance of HYDRUS-1D to describe evaporation in plant containers and to evaluate the effect of pine bark as cover layer or layers within growing media. Application of pine bark in combination with peat substrate reduced evaporation up to 50% depending on position, thickness of mulch layer and water content of the substrate. The highest reduction in evaporation was measured in a dry substrate which is covered with 4 cm pine bark. The HYDRUS-1D model describes evaporation from growing media in combination with layers of pine bark correctly as long as hysteresis of the water retention curve and vapor flow is considered in the model. Significance of this study What is already known on this subject? • Evaporation can be reduced by applying coarse materials on top of and between fine materials by interrupting capillary continuity. Simulation models are valuable tools to describe and optimize physical processes in soils and growing media. What are the new findings? • The optimum thickness of cover layers of pine bark to significantly reduce evaporation from growing media is 2 to 4 cm. The physical processes are sufficiently well described with the HYDRUS-1D model (R² between 0.36 and 0.90). What is the expected impact on horticulture? • The results of the investigation show that pine bark cover layers may help to decrease the negative effect of high evaporation in greenhouses (between 37 and 54% in our investigation corresponding to a reduced energy demand of approximately 1.5 MJ m -2 day -1 ). The physical system consisting of irrigation technology, growing media and coarse materials can be optimized with the help of simulation models. 50 E u r o p e a n J o u r n a l o f H o r t i c u l t u r a l S c i e n c e Anlauf et al. | Reduction of evaporation from plant containers with cover layers of pine bark mulch tural uses are usually measured at standard conditions, such as the container capacity and the air capacity, both measured at a matric potential of -10 hPa. These methods are standardized (DIN EN 13041, 2010) and are widely accepted. The standard parameters describe static conditions whereas in horticultural practice dynamic systems are present where the actual water and air content fluctuate considerably with time mainly due to irrigation practices. Therefore, the use of methods to describe the dynamic behavior of water and air in peat substrate, such as simulation models, seem to be a promising tool to describe the actual situation in a container more realistically (Fonteno, 1993; Heinen and De Willigen, 1995; Palla et al., 2008; Anlauf et al., 2012; Anlauf and Rehrmann, 2013) . The use of simulation models to calculate water uptake, evaporation and redistribution in plant containers depending on irrigation strategy and type of material is a cost effective method. Computer models are frequently and effectively used to simulate water movement in mineral soils. Model applications for growing media are much less frequent and the quality of the simulation is usually worse than for mineral soils. Possible reasons are shrinking and swelling processes and the hysteretic behavior of the hydraulic functions. Both processes are of much higher significance in growing media compared to mineral soils (Raviv and Lieth, 2008) . The objectives of this study were (i) to measure the effect of pine bark mulch layers on top and inside plant containers with growing media on evaporation in greenhouses; (ii) to test the application of the HYDRUS-1D simulation model (Simunek et al., 2008) to describe water content and evaporation under different conditions in plant containers; (iii) to investigate if the physical processes of capillary water transport (in the growing media) and vapor flow (at least partly important in the pine bark cover layer) are described correctly by the simulation model; and (iv) to develop strategies for the use of layering techniques to reduce evaporation. Because only evaporation was considered there were no plants in the containers in all experiments. Materials and methods Experimental setup The experimental setup was divided into two parts: in a first step the evaporation of growing media in plant containers was measured depending on the initial water content of the peat substrate and two different heights of pine bark mulch as cover layer. Main objective of this part was the evaluation of HYDRUS-1D to describe evaporation (including vapor flow) correctly. The growing media was filled into containers (10 cm diameter, 10 cm height) with a specific bulk density and three different initial water contents (wet, average 0.71 cm 3 cm -3 , medium, average 0.52 cm 3 cm -3 , and dry, average 0.39 cm 3 cm -3 ), each in three replicates. In addition, the "wet" and "dry" versions were covered with a 4 cm thick layer of pine bark mulch, whereas the "medium" version was covered with 2 cm pine bark mulch, each in three replicates. The containers were randomly positioned on electronic scales in a climate constant laboratory (20°C, ca. 50% relative humidity) and the weight was recorded continuously by data logger for a period of 5 days (dry versions) and 12 days (medium and wet versions). The actual evaporation was calculated from the daily weight losses. In a second experiment evaporation was measured in plant containers (10 cm diameter, 10 cm height) filled with the peat substrate (7.5 cm total height) plus pine bark mulch (2.5 cm total height) installed at different depths in the container positioned randomly on ebb-and-flow tables. Main objectives of this experiment were to measure the effect of the position of the pine bark mulch on the evaporation under conditions close to horticultural practice (ebb-and-flow irrigation on flooded tables), and to describe this effect with HYDRUS-1D.