"Farmers can grow dates and even bananas" is a mnemonic device used for teaching the sequence of the appearance of sharp notes in a musical key signature for a diatonic scale. Each word represents the next sharp note in the sequence: if a diatonic scale has exactly one sharp, that sharp will be F (as is the case for G Major (modern ionian mode), E natural minor (modern aeolian mode), or D mixolydian mode. If there are exactly two sharps, they will be F and C. If there are five sharps, they will be F, C, G, D, A.
The sequence of flat notes is derived by reversing this list: the first flat to appear in a key signature is B, then E, then A, D, G, C, F.
Non-diatonic scales such as the harmonic minor and flamenco (major phrygian) mode do not have mnemonic devices of this nature, because the appearance of sharps and flats in these modes is non-sequential. For example, A harmonic minor features a G sharp and no other sharp or flat keys, but D harmonic minor features C sharp, B flat, and does not have G sharp present in the sequence. Non-diatonic scales are therefore derived from diatonic scales through a series of pitch modifications to specific scale degrees (e.g. going from natural minor to harmonic minor requires raising the seventh scale degree by a semitone, and going from natural minor to flamenco mode requires both raising the seventh by a semitone and lowering the second scale degree by a semitone. Consequently, non-diatonic scales are less often memorised in terms of key signature, and more often they are memorised as the paradigm of which scale degrees to convert, or else a musician will simply memorise the exact notes of scales they personally use the most often, and rely on ear or sheet music to account for gaps in their knowledge.
Iron Noder 2021, 24/30