May 2011. A question was asked whether the application of pesticides to a crop reduces water loss – in irrigated cropping systems, presumably.

One member thought that a very high-volume spraly of, say, 1000 litres per hectare would amouint to 0.1 mm rain and could be thought of as foliar irrigation. Needless to say at this rate the role of pesticides in reducing water use is limited.

On the other hand, herbicides which work by increasing metabolic rates in weeds may, theorectically, increase water uptake in the first instance.

There were some other thoughts:

• Some nurseries apply plant growth regulators (PGRs) to make bedding plants – poinsettias, potted mums, etc. – shorter and bushier. This may reduce their relative water loss from evapotranspiration. [PGRs are considered pesticides and registered as such by U.S. Environmental Protection Agency.]
• Horticultural oil sprayed on plants to control scale insects, or a petroleum product used as a diluent in a pesticide mix, could inhibit water loss from stomata, or plant parts not covered by a thick cuticle.
• if leaves of plants are wet, either through rain or dew, there is more run-off than from dry leaves, and chemical is washed to the soil.