Which IUCN list should I choose?
The IUCN Red List of Threatened Species is an online global resource providing current conservation assessments developed using rigorous categories and criteria.
Content for plants is growing for the current IUCN Red List, but there is a need to incorporate more data on the status of plants around the world. In the meantime, the 1997 IUCN Red List remains the most comprehensive listing of the global threat status of plants but is based on 1997 data.
IUCN's Red List FAQ
There are many national and regional red list assessments available which provide additional information on threatened plants.