I think most useful section in Elsie Davenport's Your Yarn Dyeing is the set of directions on when to harvest dye plants: "Leaves in early summer; flowers just coming to full bloom, fruits when just ripe, roots after plant has died down, whole plant when flowers are in bloom, bark in spring." (Tarzana, CA: Select Books, 1953, p. 113)
I also want to remember that she says you can get grey and black from alder (alnus glutinosa) with iron mordant–alder is considered a waste tree on Vancouver Island–dark grey from fresh young blackberry shoots mordanted with iron, blue grey from ripe blackberries with alum mordant, black from acorns (she notes this is hearsay), and crimson from St. John's wort buds with vinegar (again, second-hand knowledge for Davenport).
Davenport states that you use hard water to dye with madder, a confirmation of what I came across and posted yesterday: "Dyeing processes demand a plentiful supply of good water, either rain or soft water. The one exception to this is the dyeing of tomato red with madder." (p. 48)
There's an astringent tone to this book, unlike the other two of hers that I've read on handspinning and weaving. Davenport throws out zingers and uncompromising advice in the same breath: "Good dyeings, in addition to being fast, should be level, and the idea that uneven dyeing is necessarily desirable, and indicative of the work of a handcraftsman, merits little respect." (p. 58) It's funny to come across this perspective in a time when braids of fibre in mottled colours are among the most desirable products for handspinners to buy.