Water resources can be used more efficiently by including sustainable substrate components like coir that increase water-holding capacity. The first objective of this study was to evaluate the impact of coir amendment rate on plant available water and plant gas exchange, with the goal of optimizing substrate available water and determining the optimum coir amendment rate in a greenhouse environment. The second objective was to establish the optimum method of determining plant available water using either plant gas exchange parameters or substrate physical properties. Greenhouse experiments were conducted with Hydrangea paniculata ‘Jane’ (Little Lime^® hardy hydrangea) potted with one of five different coir rates (0%, 10%, 25%, 40% and 65%) mixed with pine bark on a volume basis. Plant gas exchange parameters and substrate water content were measured daily over a range of increasingly drier substrate moisture contents. Actual photosynthetic rates increased with increasing coir amendment rate and were highest with 65% coir amendment. Amending pine bark with coir increased the water storage capacity, plant available water, and plant gas exchange parameters. Results suggest that 65% coir amendment rate was the optimum amendment rate among those tested in a greenhouse environment and plant photosynthetic rate was the better method of determining plant available water.