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Sensor-guided application technologies are needed to achieve constant spray deposition for the rapid growth of nursery liner trees during a growing season. An experimental real-time variable-rate sprayer was developed to adjust spray outputs automatically based on the liner canopy size. The developed sprayer consisted of two vertical booms, an ultrasonic sensor detecting system coupled with a spray flow rate controlled unit, a microcontroller, and a spray delivery system. Two booms were integrated with five opposing pairs of equally spaced spray nozzles. The sensors were mounted 0.35 m ahead of the spray nozzles to ensure sufficient time for signal processing. The accuracy of the sprayer in triggering spray against detected targets moving at 3.2 km h-1 was evaluated by use of a high-speed camera. An outdoor laboratory plot consisting of six different-sized tree species was used to test the sprayer performance consistency. Test results revealed that the spray nozzles were triggered from 4.5 to 12.5 cm ahead of detected targets. Seventy-five percent of test runs for detecting canopy volume of the six tree species produced significant (p < 0.05) Pearson correlation coefficients from 0.43 to 0.88 when the sprayer travel speed was from 3.2 to 6.4 km h-1. While the sprayer applied variable output rates from 0 to 20.4 L min-1 at travel speeds from 3.2 to 8.0 km h-1, the mean spray coverage inside canopies of six tree species was 12.0% to 14.7% and the mean spray deposit was 0.72 to 0.90 L cm-2. Effects of travel speed on both mean spray deposit and coverage were insignificant. Therefore, the newly developed sprayer offered a possibility to achieve uniform spray deposition and coverage for nursery liner applications despite variations in liner canopy size and sprayer travel speed.