"It is a long time since my last visit. ...I must say, your agapanthus are flourishing."
-- Albus Dumbledore to Vernon Dursley (HBP3)
Mundane plant in the lily family used for decorative plantings.
- Dumbledore mentioned that the agapanthus planted near the Dursley's front door was thriving (HBP3)
Ancient Greek agápē, “love, affection” + ánthos, “flower”
Dumbledore's chit-chat remark about a non-magical flower on Privet Drive actually may have a much deeper symbolic meaning. He may have been drawing attention to the fact that Petunia planted a lily in her garden, close to her door where she would see it every day. Was it there to honor her sister, Lily, who died trying to protect Harry? Was he once again reminding Petunia of her promise to care for her sister's child?
The agapanthus is an African member of the Lily family with blue flowers, sometimes called "Lily-of-the-Nile." Like baby Moses from the Old Testament who was left in the Nile River for Pharoah's daughter to find, Harry was left on the doorstep for Petunia to find with the milk bottles (PS1). When she agreed to Dumbledore's request that she take him into her home, Harry's protection was sealed from the bond of blood between them (Op37).
The name "agapanthus" literally means "Love Flower" (or you could say "Love Lily") in ancient Greek. The first part of the word comes from the ancient Greek word "Agape," meaning unconditional or selfless love; those values formed the teachings of Jesus in the early Christian Church. Lily Potter personified that type of love - the willingness to sacrifice herself for her child -- just as Harry later had to sacrifice himself to save his friends (PS17, DH34).
Did Petunia know anything about the symbolism of the agapanthus, or was it just another suburban flower to help the Dursleys win a a gardening award? Perhaps, but it is interesting that the two people who grew up with Lily and knew her best are associated in some way with lilies. Petunia has the agapanthus, while Severus Snape mentioned the asphodel, another Greek-named lily used in the Draught of Living Death - during Harry's first Potions class (PS8). And to connect Snape back to the agapanthus, when he saw a letter in Lily's handwriting at Grimmauld Place, he wept at first then tore off the part that said "Love, Lily," literally the meaning of the agapanthus lily in Petunia's front garden (DH33).