Flower Power was a hippie concept in the mid 60s that made it’s way into mainstream American culture. The phrase is credited to poet Allen Ginsburg to express opposition to the Vietnam war, and later as a symbol of the non-violence ideology, but it was probably rooted in a 1961 Pete Seeger folk song:

Where have all the flowers gone?
Long time passing
Where have all the flowers gone?
Long time ago
Where have all the flowers gone?
Girls have picked them every one
When will they ever learn?
When will they ever learn?

Flower Power and it’s companion, Make Love, Not War, became the antiwar slogans of the times, and were given a boost by a famous photo from the 60s shows a teenager placing flowers in the barrels of National Guardsmen rifles.  Flowers were the symbol for Passive Resistance and non violence.

Ginsburg coined the phrase in a 1965 article “How to Make a March Spectacle”.  The concept was that protestors, rather than appearing threatening, should hand out “masses of flowers” to the police, authorities, and spectators.  The protests would then seem more like street theater and become more appealing to the mainstream.

Abbie Hoffman added to Flower Power during his 1967 Workshop in Nonviolence:”The cry of ‘Flower Power’ echoes through the land. We shall not wilt. Let a thousand flowers bloom.”

And of course, John Phillips of The Mamas & The Papas wrote San Francisco (Be Sure to Wear Flowers in your Hair) which is known as the unofficial anthem of the counterculture movement”.

At first it was just hippies and hippie wanna-bees that wore flowers but it rapidly spread mainstream.  The late 60s saw flowers from Flower Power merged into pschedelia and flowers began showing up everywhere. Pop artist Peter Max added day glow colors to a stylized design and flowers went mainstream.