The expression Say it with flowers is so overused that we forget it is actually an advertising slogan and not an English language idiom.
Actually, the slogan was created a very long time ago, in 1917, by the Florists’ Telegraph Delivery group (FTD). Almost a century later, the company is still using it successfully, which shows that it’s not a good idea to change a message when it is a good message!
According to the US Library of Congress blog, the tradition of sending flowers actually started back in the early 1900’s, even before Mother’s Day was proclaimed.
So, whether 100 years ago or 100 years from now, if you don’t know how to say it…