Where boiling is right
Roots, barks, hard seeds and roasted grains. These are dense or already cooked, there is little volatile aroma to drive off, and full heat is what gets anything out of them. For most of the catalogue’s roots the water should still be moving.
Where boiling costs you
Aromatic flowers and delicate leaves. Chamomile, chrysanthemum, honeysuckle, jasmine, plum blossom and the scenting flowers all lose aroma and gain bitterness at a full boil. Water off the boil — roughly 85 to 90°C — keeps more of what you bought.
The cold-water cases
Three, and each has a reason. Marshmallow root gives up far more of its mucilage cold than hot. Schisandra brewed hot turns harshly bitter, so omija-cha is steeped twelve hours cold. Sumac is steeped cold into a sour drink for the same reason. In all three the tradition specifies cold and the chemistry explains why.
Cold brewing as a choice rather than a rule
Almost anything in this catalogue can be cold-brewed and most things improve slightly: less bitterness, less astringency, more aroma retained. It takes hours rather than minutes, which is the trade. Honeysuckle is the clearest beneficiary — most of what puts people off it disappears.