What we need is a device that illuminates a plant or tree to the
degree in which it receives an amount of water. For example, as
the plant or tree is being watered, the outside surface of the tree
will 'glow' with a purple phosphorus as the water level within the
plant's roots rises. When sufficient water has been given to the
plant, the entire plant itself will then glow with an amber phosphorus
-- indicating enough water has been given.

How to make the plant or tree glow, however, is open to debate.

Paul