As the cost of tap water rises and its use for horticultural operations is increasingly questioned, the use of recycled water disinfected with chlorine is gaining attention.
Chlorine is already widely used to disinfect the public water supply. In the experiments described here, five container-grown nursery stock species were irrigated daily with water containing various concentrations of free chlorine. The plants used were Spiraea japonica 'Gold Mound', Hydrangea paniculata 'Grandiflora' Award of Garden Merit, Weigela florida 'Alexandra', Physocarpus opulifolius 'Summer Wine' and Salix integra 'Hakuro-nishiki'.
The foliage of every plant was wetted daily by irrigating it from overhead. The control treatment used well water, containing 0.04mg of free chlorine per litre.
After a month of treatment, all plants irrigated with water containing 20mg of free chlorine per litre showed some leaf damage. This included mottling and chlorosis, as well as premature leaf fall and growth inhibition. None of these symptoms occurred if the water contained less than 2.5mg of free chlorine per litre. Tap water should contain less than this.
The results suggest that disinfected, recycled water can also be used to irrigate hardy nursery stock, provided that the water contains no more than 2.5mg of free chlorine per litre.
Sensitivity of Five Container-grown Nursery Species to Chlorine in Overhead Irrigation Water by Cayanan, Zheng, Zhang, Graham, Dixon, Chong, and Llewellyn (2008). HortScience 43 (6): 1882-1887. Members of ISHS can view HortScience from the website www.ishs.org.