Aerial spraying of phreatophytes with antitranspirant

An antitranspirant (AT) can retard excessive groundwater consumption by phreatophytes without eradicating them. To determine whether aerial spraying (the only realistic application method for most sites) of a 6% (vol/vol) wax‐based AT emulsion would provide adequate spray coverage and reduce transpiration, multiple passes were made with (1) a fixed wing plane on salt cedar, cottonwood, and willow and (2) a helicopter on salt cedar. Spray coverage on the ground was 30–100% in the open (depending on the number of passes and wind drift) and 10–90% under the canopies (depending on vegetation density). Average coverage on tags ranged from 13% in the lower canopy to 75% in the upper (depending on species and application rate) after spraying by the fixed wing plane and 47 to 98% after spraying by the helicopter. Coverage in the center of dense bushes was 0–20%, but transpiration is only minimal there. Scanning electron microscope photomicrographs showed considerable AT on foliage in the upper canopy and lesser amounts in the lower; the film was detected even 24 days after spraying. Aerially applied AT increased resistance to leaf water vapor diffusion by 150% during the first few days and by 80% thereafter. Transpiration of outer foliage of salt cedar was reduced 50% initially and 20% after two weeks without phytotoxicity.