The Magazine of Horticulture, Botany, and All Useful Discoveries, 10(6): 227 (June 1844)

Charcoal.—Last year I potted two standard hydrangeas in 16-sized pots, (7 inches broad,) with two thirds turfy loam and one third sifted charcoal; the drainage, which was three inches deep, I formed of coarse pieces of charcoal. When the plants bloomed, I found the color of the flowers to be a beautiful bright blue, and so they continued to flower the whole of the season. Every plant seems to delight in charcoal; since I have laid charcoal merely on the surface of the mould, round the stems of many of my large orange and lemon trees, the alteration has been very striking; the foliage assuming a dark rich hue, and the plants being altogether most luxuriant. It is my opinion that large beds of blue hydrangeas might easily be obtained by the above treatment.—(Gard. Chron. 1844, p. 69.)