Email me if you would like an Access Database.
My Approach consisted of the following Steps :-
1) Google for 'Botanic Databases' and 'Botanic Data Models'
2) Find specific Data Models covering some aspect of Botanics or Botanical Gardens.
3) Create an ERD by taking the best common things from the available Data Models.
4) Publish the results for review and discussion by other people.
I found a wide range of references when I Googled 'Botanic Data Models'.
Many references turned out to be disappointing - either simply lengthy discussions without any Data Models, or addressing problems of Taxonomy.
Here is a small sample that are worth looking at, based on the time I spent researching the problem :-
Harvard University - an excellent list of Botanical Databases.
Plant Collection Data Model - recommended.
US Geological Survey Best Practices Data Model (PDF) - worth a look.
Worldwide Botanical Knowledge Base - Abstract Data Model for Taxonomy (UML Class Diagrams).