Decide what plant you will be writing dialogue for, and assess its personality. When Robert Burns wrote "my Luve's like a red, red rose," he was envisioning a woman very much like the "nun demure, of lowly port" Wordsworth describes in "To the Daisy." If a rose if talking, it will sound cantankerous and regal, a daisy pleasant and familiar.
Research the anatomy of the plant whose dialogue you will be writing. If you are composing a speech for a hyacinth, you will want to make sure that your other characters are close enough to the ground to hear it talking, while if your plant is a talking jasmine, you will need to set the scene somewhat higher up.
Consult famous literary examples of plant personification. In Chapter 2 of Lewis Carroll's "Alice Through the Looking-Glass, and What She Found There," Alice holds a conversation with a tiger-lily, a rose, a violet, a larkspur and several daisies. J.R.R. Tolkien's The Lord of the Rings" features a race of ancient tree-shaped giants called "Ents," and C.S. Lewis featured talking trees in many of his Narnia novels.
Decide to whom the plant will be talking. If the plant will be speaking with another plant, you will need to repeat the above steps in order to personify it. If instead you are writing a conversation between a plant and a human, you will need to determine how this conversation is taking place. Your story could take place in a fantastic setting, for instance, or your dialogue could simply be part of a dream sequence.