Abstract: A vehicle-mounted machine for preventing frost damage to growing crops comprises an upright shell that defines a plenum chamber, having an upwardly opening air inlet in its top and having plural rearwardly extending tubular heating chambers opening from its bottom portion. A fan coaxially mounted in the front of each heating chamber draws air from the plenum chamber and propels it across a propane burner that is coaxially mounted in the heating chamber behind the fan. Discharged air, mixed with combustion gases from the burner, has a temperature of about 45.degree. F. and is emitted rearwardly downwardly and obliquely to both sides of the machine to mix with cold surface air. A field or grove protected by the machine is divided into a grid of equal-area zones, each having a high pole atop which there is a light that is turned on when a sensor near ground level detects a temperature above but near freezing. The machine moves to where it is needed, as indicated by lighting of the lights.