Q: Explain the symbolism of certain 'Marian flowers'.
A: Let me begin with pointing to a very informative and beautiful book which contains many of the answers you are looking for: Vincenzina Krymow, Mary's Flowers; Gardens, Legends and Meditations, St. Anthony Messenger Press, Cincinnati, 2002. A book worth purchasing!
As for the flower symbols related to Mary's virtues, evangelical virtues as you call them, there is the white flowered gourd which covers them all. It is called "Mary's Virtues." There are many flowers to symbolize Mary's purity: White Rose, Edelweiss, the Virgin or Madonna Lily, St. Mary's or Christmas Holly. I am not aware of a flower specifically highlighting Mary's prudence. Her humility is seen in the Sweet Violet or Our Lady's Modesty; also the Lily of the Valley, and Wild Thyme. Mary's Faith is assimilated with Crimson Clover and Veronica. Our Lady's Praises or Blue and White Petunia could be used for devotion. There are several possibilities for Mary's obedience: one signifying spiritual openness would be the Tulip, the other praising her submission to law (purification) the Boxwood. Poverty comes true in Our Lady's Bedstraw which she had to use to prepare the bed of the child; we find allusions to patience in the Patient Lucy or Dear Mother's Love, as well as in the Yellow Clover or Lady's Hope. Mercy suggests various meanings: there is Our Lady's Keys or Cowslip which signals her role as mediatrix; the White Purple Orchid is called Mary's Hand of Pity. Again, many flowers reflect Mary's sorrow, among them Mary's Heart or Bleeding-Heart; also the Common Dandelion or Bitter Sorrow. Enjoy Our Lady's flowers!