Mulberries grow by the vineyard where Ascanio comes to live, as pointed out to the unnamed first-person narrator by the old man.
“Still it could be seen that the poor lady pined for company, and her waiting women, who loved her, were glad when the Cavaliere Ascanio, the Duke’s cousin, came to live at the vineyard across the valley—you see the pinkish house over there in the mulberries, with a red roof and a pigeon-cote?”[1]
[1] Wharton 1901, p. 8