Whenever the herbal material is hard and woody, making a decoction increases the likelihood that the soluble contents of the herb will actually be extracted into the water. Roots, rhizomes, wood, bark, nuts, and some seeds are hard and have strong cell walls, so to ensure an effective transfer of active constituents to the water, more heat is needed than for infusions.
Infusions are appropriate for softer plant tissue such as leaves, flowers, or green stems, where the desired substances are easily accessible. If an infusion is to be made of bark, roots, seeds, or resin, it is best to powder them first to break down some of the cell walls, which will facilitate extraction. Seeds-for instance, fennel or anise seed should be slightly bruised to release the volatile oils from the cells. All aromatic herbs should be infused in a pot with a tight-fitting lid to minimize loss of volatile oils.