there is a similar question to this on wiki answers. The question asked was:

What flowers start with the letter p?

The answer was: Pansy, petunia, phlox, polyanthus, Pasque-flower, peony, periwinkle, primrose. pelargonium, penstemon, plumeria, pimelia, poppy.

Other than that you can refer to this website: http://www.romantic-ideas-for-life.com/flower-names.html
Just keep scrolling down until you get to the letter p. If you can't find it, here is the list:

Flower names with "P":

• Paeonia: Bashful

• Palm leaves: Victory and Success

• Pansies: Recipient is being thought about. The flower's name comes from the French word pensée which means thought or remembrance. Associated with the first wedding anniversary.

• Paper White: Aphrodisiac due to its intoxicating scent. It is also the national flower of Wales because it often blooms on March 1st which is St. David's feast day, Wales' patron saint.

• Peony: Bashfulness; Shame; Happy life; Happy marriage; Good health; Prosperity. Associated with the twelfth wedding anniversary.

• Peppermint: Cordiality.

• Periwinkle: Friendship.

• Petunia: Resentment; Anger; Your Presence Sooths Me

• Phlox: A good partnership; Harmony; Unanimity

• Pine: Hope; Pity

• Poppy, General: Eternal Sleep; Oblivion; Imagination; Associated with ninth wedding anniversary

• Poppy, Oriental: Silence is golden

• Poppy, Red: Consolation; Pleasure

• Poppy, Scarlet: Extravagance

• Poppy, White: Tranquility; Consolation; Sleep; Rest

• Poppy, Yellow: Wealth; Success

• Primrose: I Can't Live Without You