Extracts can be made from both freshly harvested or dry plant material. On drying a herb, only the volatile substances are removed, the dissolved and suspended substances being deposited within the now dry and dead cell wall. In this state the herb may be stored until required for extraction. In most cases the aqueous extract is the same whether derived from the dried or freshly harvested herb since deposited water-soluble substances undergo little change in the dry state.

An alcoholic extract may, however, differ considerably with the age and storage conditions of the plant material since the deposited oils, (which form an important part of an alcoholic extract), may change in character during lengthy storage, leading to a change in odour of the extract. Similarly, prolonged storage often allows evaporation of the essential oils, and the alcoholic extract from such exhausted material could, at worst, be odourless.