The part decides the method
Leaves and flowers hold their character in surface oil glands and thin tissue, so hot water reaches it in minutes. Roots, barks and hard seeds hold theirs inside dense woody structure that a three-minute steep barely touches. Everything else in this guide follows from that one difference: steep the first, simmer the second.
Leaves
The largest category and the most varied. Some are farmed to be drunk — rooibos, yerba mate, sideritis. Some are kitchen aromatics with a second life — sage, bay, curry leaf. Some come off trees planted for fruit: olive leaf from pruning, mulberry from silk plantations, persimmon and loquat from orchards. Which kind you have tells you how consistent it will be, because a by-product harvest is graded by whoever happened to collect it.
Flowers
The category where appearance carries the most information and the least flavour. Whole intact heads that open in water are a visible quality signal; dust is the same plant milled to nothing. But several flowers in commercial blends — calendula, cornflower — contribute colour and essentially no taste, and recognising them is most of what reading an ingredient list involves.
Roots and rhizomes
Dense, slow to give anything up, and almost always sold cut or sliced because whole roots are impractical. Twenty minutes at a simmer is a normal instruction here and three minutes in a cup is not. Roots also keep far better than leaves: a two-year-old packet of dried leaf is usually inert, while sliced root is often still fine.
Bark
The smallest category and the one with the clearest authenticity problem. Cassia and Ceylon cinnamon are different species with visibly different quills, and the word cinnamon covers both almost everywhere. Bark needs simmering for the same reason roots do.
Seeds and spices
The category where whole beats ground by the widest margin. Nearly every seed here loses most of its aroma within weeks of grinding and nearly every one is sold ground. Several also want toasting before use, and one — fenugreek — goes from nutty to acrid in about ten seconds of over-toasting.
Fruit, peel and berries
Drying takes the aroma and leaves the sugar, the acid and the tannin, which is why so many fruit infusions are flavoured as well as fruited. Peel is a separate case again: the aroma is in the coloured outer layer and the white pith beneath it carries only bitterness.