The White Poinsettias are a crop just like the Black Roses from Halloween, that must be collected in seed packets before they can be planted. You can click on the "Get" button next to the White Poinsettia crop in the store in order to "Ask for more" seed packets, with this allowing you to post a general request to your wall for your friends to see. Since this might be looked over by many players, you might try asking some of your closest neighbors personally to send you some (they are available to send in packets of 20 from the game's free gifts page).
Once you have packets of seeds, you'll be able to plant them and harvest them 16 hours later. You'll get 200 coins for each harvest, and you'll earn 2 experience points for each seed that you plant. As with other crops, the White Poinsettia can be mastered, with the first star of mastery coming at 250 harvests.
The White Poinsettias will only be available to grow for the next 22 days, so what are you waiting for? Start asking your friends for seed packets soon so you have a chance at mastering this limited edition crop!