This one is a biggie.
A lot of the conventionally grown flowers sold in the U.S. are grown in greenhouses in countries where environmental regulations are more lax. Workers there often have little protection from pesticides, many which are banned in the U.S. After the flowers are cut they are packed up and shipped thousands of miles to your local flowershop, where they sit — still soaked in pesticides — waiting for you on the day before Valentine's Day.
Don't be a part of the conventional cog; this year, buy organic. Organic flowers help support organic farmers and come with the added benefit of not increasing your cancer risk.