Evaluating alternative growing media to peat for ornamental horticulture

While peat has traditionally been used as an ideal growing medium for horticultural plant production, draining and destruction of peat bogs for use in growing media depletes a huge carbon sink. The horticultural industry urgently needs alternatives to peat based growing media, not only to exercise corporate social responsibility but increasingly to adhere to government guidance and/or legislation. While many horticultural industries have opted to use coir, this practice is questionable in terms of carbon balance since it must be transported from tropical regions to the UK and there are also concerns over the ethics of harvesting such products and periodic supply chain issues. In the UK, wood-based growing media has potential to be an effective replacement for peat, but whether its use can result in similar ornamental plant quality under typical nursery watering and fertilisation practices is uncertain. The porosity of these media, while ideal for drainage, may limit root-substrate contact necessary for plant water and nutrient uptake. Adding surfactants to these growing media may improve the distribution of soil moisture within them, but their effects on plant physiological responses to water deficit are relatively unknown. To evaluate the viability of alternative growing media, factorial experiments were set up with an ornamental species (Dianthus plumaris) and media with different peat contents (with and without surfactants). Above-ground physiological measurements (leaf area, biomass and plant water relations) and below-ground measurements (root architecture and rhizosheath development=medium adhering to the root system) were made throughout the growing cycle and at harvest respectively. The addition of surfactants increased above ground dry biomass in the substrates. It was determined through measurements of leaf water potential and biomass that the high peat content substrate performed better than the peat reduced substrate.