"Roses are red" can refer to a specific poem, or a class of doggerel

Doggerel

Doggerel is a derogatory term for verse considered of little literary value. The word probably derived from dog, suggesting either ugliness, puppyish clumsiness, or unpalatability in the 1630s.-Variants:...

 poems inspired by that poem. It has a Roud Folk Song Index

Roud Folk Song Index

The Roud Folk Song Index is a database of 300,000 references to over 21,600 songs that have been collected from oral tradition in the English language from all over the world...

 number of 19798. It is most commonly used as a love poem.

The most common modern form of the poem is:

Roses are red,

Violets are blue,

Sugar is sweet,

And so are you.

Origins

The origins of the poem may be traced at least as far back as to the following lines written in 1590 by Sir Edmund Spenser

Edmund Spenser

Edmund Spenser was an English poet best known for The Faerie Queene, an epic poem and fantastical allegory celebrating the Tudor dynasty and Elizabeth I. He is recognised as one of the premier craftsmen of Modern English verse in its infancy, and one of the greatest poets in the English...

 from his epic The Faerie Queene

The Faerie Queene

The Faerie Queene is an incomplete English epic poem by Edmund Spenser. The first half was published in 1590, and a second installment was published in 1596. The Faerie Queene is notable for its form: it was the first work written in Spenserian stanza and is one of the longest poems in the English...

(Book Three, Canto 6, Stanza 6):

It was upon a Sommers shynie day,

When Titan faire his beames did display,

In a fresh fountaine, farre from all mens vew,

She bath'd her brest, the boyling heat t'allay;

She bath'd with roses red, and violets blew,

And all the sweetest flowres, that in the forrest grew.

A nursery rhyme significantly closer to the modern cliché Valentine's Day

Valentine's Day

Saint Valentine's Day, commonly shortened to Valentine's Day, is an annual commemoration held on February 14 celebrating love and affection between intimate companions. The day is named after one or more early Christian martyrs named Saint Valentine, and was established by Pope Gelasius I in 496...

 poem can be found in Gammer Gurton's Garland, a 1784 collection of English nursery rhymes:

The rose is red, the violet's blue,

The honey's sweet, and so are you.

Thou are my love and I am thine;

I drew thee to my Valentine:

The lot was cast and then I drew,

And Fortune said it shou'd be you.

Victor Hugo

Victor Hugo

Victor-Marie Hugo was a Frenchpoet, playwright, novelist, essayist, visual artist, statesman, human rights activist and exponent of the Romantic movement in France....

 was likely familiar with Spenser, but may not have known the English nursery rhyme when, in 1862, he published the novel, Les Misérables

Les Misérables

Les Misérables , translated variously from the French as The Miserable Ones, The Wretched, The Poor Ones, The Wretched Poor, or The Victims), is an 1862 French novel by author Victor Hugo and is widely considered one of the greatest novels of the nineteenth century...

. Hugo was a poet as well as a novelist, and within the text of the novel are many songs. One sung by the character, Fantine

Fantine

Fantine is a character in Victor Hugo's 1862 novel Les Misérables.- Backstory :"Fantine was one of those beings which are brought forth from the heart of the people... She was called Fantine because she had never been known by any other name...""All four were ravishingly beautiful. As to Fantine,...

contains this refrain, in the 1862 English translation:

We will buy very pretty things

A-walking through the faubourgs.

Violets are blue, roses are red,

Violets are blue, I love my loves.

The last two lines in the original French are:

Les bleuets sont bleus, les roses sont roses,

Les bleuets sont bleus, j'aime mes amours.

(Les Misérables, Fantine, Book Seven, Chapter Six)

Folklore

Numerous satirical and transgressive versions have long circulated in children's lore among them:

Roses are red.

Violets are blue.

Onions stink.

And so do you.

Roses are red.

Violets are blue.

Cashews are nuts.

And so are you.