Let's begin with two poetic devices that can only be appreciated in the poem's original Spanish.
1. RHYME SCHEME:
The poem's rhyme scheme is ABBA, CDDC.
For example, Line 1 ends with "blanca," which rhymes with the end of Line 4, which ends in "franca." Lines 2-3 end with the rhyming words "enero" and "sincero." Lines 4-8 have a similar pattern.
2. ALLITERATION: the repetition of a consonant sound at the beginning of words, such as "Peter Piper picked a peck of pickled peppers."
In Lines 4-8, there is considerable repetition of the hard "c" sound, which is also represented in Spanish with the letters "qu." The following words begin with this sound: cruel, corazon, con, que, cardo, cultivo (2x).
Another device that is used is anaphora, or repetition of a phrase. The most obvious repeated phrase occurs at the beginning and at the end of the poem: "I have a white rose" ("Cultivo...rosa blanca"). Another repeated phrase is "I give." Both these phrases are used in reference to the "true friend," and to "the cruel one." By using the same phrases to describe his actions toward both his friend and his enemy, the poet emphasizes that he treats both with the same love and respect.