Various folk cultures and traditions assign symbolic meanings to plants. Although these are no longer commonly understood by populations that are increasingly divorced from their old rural traditions, some survive. In addition, these meanings are alluded to in older pictures, songs and writings. New symbols have also arisen: one of the most known in the United Kingdom is the red poppy as a symbol of remembrance of the fallen in war and some display white poppies, also for remembrance and to show opposition to warfare.
More information on the Wikipedia page , which has a list of plants.
It is not obvious whether there is any symbolism to the plants in the Voynich Manuscript
See also Language of flowers