I have been unable to track down this poem so far, so it seems probable that the author composed it specifically for the novel. However The Mocking Fairy, a poem by Walter de la Mare (1873 - 1956), contains the following line: "Quoth the Fairy, nidding, nodding in the garden;". The line " And horsemen, clad in scarlet bright, appeared" is found in The Maid of Orleans, Canto II (1730), an unfinished poem by Voltaire (1694 - 1778). It is possible that these lines had some influence.
The Song of the Flowers
We are little rose-buds gay,
Nidding, nodding through the day
Some are pink, and some are white,
And some are clad in scarlet bright
See us scatter petals sweet
Like confetti at your feet