Increasing amounts of coir dust in substrates do not improve physical properties or growth of tree seedlings in a novel air-pruning propagation tray1

Abstract Air-pruning can improve tree seedling root quality in propagation by subjecting root tips to desiccation, thereby avoiding deflections, but also increases substrate dry-out rates. Several studies have indicated that coconut (Cocos nucifera L.) coir dust can enhance water holding properties, possibly benefiting trees grown in air-pruning trays. However, water availability characteristics are influenced by particle size. In this experiment, coir dust was added into a sphagnum peat-perlite substrate mix at rates of 10, 15 and 20%. An industry standard peat-perlite mix was tested as a fourth substrate type. Red oak (Quercus rubra L.), red maple (Acer rubrum L.), quaking aspen (Populus tremuloides Michx.) and eastern white cedar (Thuja occidentalis L.) were grown from seed in these four substrate types. Physical and chemical properties of all substrate types were analyzed pre-experiment. The particle size distribution was finer and more even in the peat-perlite mix compared to the three coir mixes. The higher proportion of coarse particles in the 20% coir mix may have reduced water availability. Seedlings grown in the 15 and 20% coir mixes had lower above and below-ground growth compared to the 10% coir and peat-perlite mixes in all species except red oak. Index words: soilless media, water holding capacity, air space, particle size distribution, chlorophyll content, tree growth. Species used in the study: red oak (Quercus rubra L.); red maple (Acer rubrum L.); quaking aspen (Populus tremuloides Michx.); eastern white cedar (Thuja occidentalis L.).