The following diagram shows an overview of the solution components that will be developed to solve John's problem: 

A connected device will be attached to John's plant, and on a scheduled basis will retrieve measurements from the plant and environment (soil moisture and temperature) and publish the event to the IBM Watson IoT Platform. The device will also subscribe to trigger commands (actions).

An application deployed to the IBM Cloud (Bluemix) will subscribe to John's events, and whenever a configured minimum level of soil moisture or a high temperature threshold is reached, the application will send an action command to water the plant for a certain duration.