A device and method for automated, mechanical harvesting of plants having differing heights and stem lengths, wherein a stem portion of the plant must be severed at a uniform, predetermined length from the top of the plant. The inventive method comprises the steps of (i) guiding the stems of the plants into a first severing blade (ii) severing a lower portion of the stems at an excess length to the free the plant from its root and form a precut stem length greater than the predetermined length required for the harvested plant, (iii) guiding the precut stem along a guide track toward a second severing blade which is positioned at a separation distance below the guide track which will cause severance of the stem at the predetermined length, (iv) vertically positioning the precut stem within the guide track such that the top of the plant is at a predetermined elevation with respect to the second severing blade, and (v) severing an additional portion of the stem from the plant by advancing the vertically positioned precut stem to the second severing blade to yield a harvested plant having a uniform, predetermined stem length. Structural components for implementing these steps are disclosed as well as the features of a conveyor belt useful for controlling stem movement, orientation and position displacement through the process.