Although ozone is toxic to plants, it is less toxic when dissolved in water. Aqueous ozone is already used to control microbial contamination in many situations and has the advantage that it leaves no chemical residues.
In the experiments described here, five species of woody nursery stock (including Hydrangea, Spiraea and Weigela) were brought into a glasshouse for treatment. Aqueous ozone was prepared by generating the gas from oxygen and dissolving it in water. The concentrations tested ranged from 10.4 to 125 microlitres of ozone per litre and were applied for seven and a half minutes each day through overhead spray lines.
The solutions were prepared when required as aqueous ozone is short-lived. Any gaseous ozone that was released was mopped up by carbon filters in the glasshouse.
Plant damage was observed at the highest ozone concentration but not with a concentration of 31 microlitres of ozone per litre. This concentration should also control the build-up of algae and various other microbes in recycled water and destroy many plant pathogens.
Phytotoxicity of Aqueous Ozone on Five Container-grown Nursery Species by Graham, Zhang, Zheng and Dixon (2009). HortScience 44 (3): 774-780. Members of ISHS can view HortScience from the website www.ishs.org.