This section of the book is from "The Complete Herbalist" by Dr. O. Phelps Brown. Also available from Amazon: The Complete Herbalist: The People Their Own Physicians By The Use Of Nature's Remedies.
A Physician who would cure diseases, or seek to assist Nature to throw off all morbid accumulations from the body, should have a single eye to the perfection, purity, or quality of the remedial agents he may feel called upon to employ. Plants should be gathered at a proper period, and under correct climatic influences, and always chosen from those in a wild or uncultivated state.
The roots of an annual plant will yield their most active medical properties just before the flowering season, whereas this class of roots are erroneously gathered after the flowering season; in consequence, they are less active, and do not retain their qualities for any reliable time. The roots of the biennial plants are most energetic if gathered when the leaves have fallen from the plant, in the autumn of the first year; while the roots of perennial plants are most active when gathered between the decay of the flowers and leaves and the renewal of verdure of the following Spring. Bulbs are to be collected as soon as matured, or soon after the loss of the foliage, in order to secure their most active principles
Herbaceous stems should be collected after the foliage, but before the blossoms have developed themselves, while ligneous or woody stems should be collected after the decay of the leaves and previous to the vegetation of the succeeding Spring.
Barks are to be gathered in the Spring previous to flowering, or in Autumn after the foliage has disappeared. Spring is the best time to gather resinous barks, and Autumn for the others.
Leaves are best when gathered between the period of flowering and maturation of the fruit or seeds. Biennial plants, however, do not perfect themselves the first year, consequently their leaves should be gathered only during the second year of the growth of the plant.
Flowers are to be collected when about to open, or immediately after they have expanded, although I prefer the buds. Flowers, buds, and leaves, are to be gathered in dry weather, after the dew is off from them, or in the evening before it falls, and freed from all impurities. Aromatics should be collected after the flower-buds are formed, while stalks and twigs are best if gathered soon after the decay of the flowers. Berries, succulent fruits, and seeds are to be collected only when ripe, except in some few cases where the medicinal virtue is contained in the unripe article. Roots are to be well washed, rejecting all worm-eaten or decayed portions. Bulbs are cleaned and dried as roots. Barks, stems, twigs, and woods are best dried in a moderate sun-heat, and should be taken every night into a well-ventilated room, where the dew or rain will not touch them, and laid upon sticks, slats, or boards which are some few inches apart, so that the air may be well circulated through. The best method of drying leaves is to strip them from the stem, lay them loosely upon a flooring where the sun shines moderately and the air circulates sufficiently to avoid mould -- keep them well stirred. The custom of steaming or moistening leaves in order to pack them more solidly after having been dried, is exceedingly improper, as the articles become thereby much deteriorated in quality and soon get musty.
Seeds are dried in the same manner as stems and leaves. Aromatic herbs and annual plants are dried as advised for leaves similarly prepared.