Someone I follow on YouTube recently linked a PDF on Archive.org for correspondences of many floral plants, a tradition dating back hundreds of years. The book itself is not directly magic-related, but it does contain meanings people used to associate with these flowers, to convey messages. Many of these meanings are used in literature, but also in spells. This book, I think, might be a good help to decode spells others have written, as well as to craft one's own around available items.
The book is in the public domain, so no worries about copyright infringement.
For the main page with different file format options: