Milton, Paradise Lost, VIII.504.

The first three lines of Wordsworth’s “Address to Kilchurn Castle, upon Loch Awe.”
Wordsworth began this poem during the tour of Scotland he took with his sister, Dorothy,
in 1803, but it wasn’t published until the 1827 version of his Poetical Works.

A renowned portrait painter and the first president of the Royal Academy, Reynolds
(1723-1792) was one of late-eighteenth-century Britain’s most influential aesthetic
theorists. Reynolds’s observation on the color of houses does not appear in any of
his published works, so it seems likely Wordsworth heard it anecdotally.

In Observations on the River Wye, and Several Parts of South Wales (1782), the famed landscape theorist William Gilpin offers a lengthy denunciation
of the use of white in landscapes. In the midst of this peroration, Gilpin includes
a long quotation on the subject from William Lock (or Locke), a renowned eighteenth-century
patron of the arts.

The species Wordsworth has in mind is prunus padus, most widely known today as the hackberry tree.