A method includes measuring an occurrence frequency of elastic waves generated by cavitations in vascular tissues in vascular plant, before and after a change in water stress to the vascular plant, respectively by an elastic wave reception sensor fixed to an axis of the vascular plant, calculating a change rate of the occurrence frequency, from the occurrence frequency of the elastic wave measured before and after the change, respectively, and determining whether or not an embolism in the vascular tissue arrives at an unrecoverable level of the embolism, from the calculated change rate. And then, one can determine irrigation timing and quantity to the vascular plant, using an index based upon the above-determined result.
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