Abstract: A system controlling plant growth conditions in hydroponic growing systems, the system for controlling plant growth conditions comprising: at least one detector for measuring at least one property of a plant growth substrate or environmental condition; first and second data processing means; data storage means and the or each detector being arranged to measure a property or properties of a plant growth substrate or environmental condition and to transmit a detector identifier and the measured property or properties over a communications link to the first data processing means; the first data processing means being arranged to: hold in a memory predefined growth data including irrigation data, environmental data, plant data, harvest data, crop data and/or climate data defining a relationship between: plural values for one or more of temperature, pH level, water content, nutrient content, oxygen content of the substrate, air temperature, humidity and light level, climate conditions, plant parameters, harvest an