Plant Tracker is a new app for your mobile phone (Android and iOS available) which allows you to rapidly report the presence of invasive alien species (including rhododendron) on your walks out and about. You can upload a photo if you aren't sure of the ID, which helps confirm the ID and ensure records are accurate. Joint initiative between University of Bristol and Environment Agency. You can also report online via a PC if you prefer.
http://planttracker.naturelocator.org/