How does the user know how many plants/sensors there are?I would set up a webpage which all the plants/sensors listed. Next to each would be an empty box to enter an email address and a box to enter a telephone number. Perhaps a drop down box to determine frequency of notification.Then click submit.The Arduino would take those values and use them.If you want to send SMS you will need a GSM shield. To send emails, you would need an Ethernet shield.Just some initial thoughts.
Were you planning on writing the Android app?You can communicate Android with Arduino via wifi or Bluetooth.
Can the GUI be programmed in a way that it is only required for first time use and then the arduino remembers the data it passed to it?