PURPOSE: To automatically detect the growth of a tree and display the pruning time of the tree without damaging the beautiful appearance of the tree, by disposing a detector having a net shape, etc., at a position near to the tree, connecting a switch member to the detector and operating a display member with the switch member.

CONSTITUTION: A detector 2 comprising a net and a supporting, wire both being prepared of e.g. transparent nylon filament is disposed at a place near to a tree 1, and one end of the supporting wire 4 is fixed to the wall of a building and the other is connected to a press plate 11 and to a spring 12. A limit switch 7 is composed of the press plate 11, the spring 12, a switch element 10 and a switch cover 9, and the signal of the switch is transmitted to a display member 14 through a signal-transmitting wire 13 to display the information of the growth of the tree on the display.