(1) Gather only common plants, and remember that the medicinal or nutrient content of a plant will vary from year to year and from plant to plant, even between those growing side by side.
(2) Never take more than 5% of the population – that is, one plant in 20 or one berry in 20. Even then, such gathering can deplete a resource if too many people gather in one area. Plants growing in harsh, mountain climates, for example, will not have enough energy to produce flowers or fruit each year. Not only that, but the wildlife also depend on this same source for vital food. Survival of many animals depends on access to the roots, shoots, and fruits you are harvesting.
(3) Never gather plants from protected or heavily used areas like parks or nature preserves. Doing so is not only illegal, but morally wrong, causing permanent destruction to ecosystems.
(4) Know which part of the plant you need. Do not kill the entire plant if you only need a few leaves or flowers, for example. Take only what you need and damage the plant as little as possible. If you are gathering rootstocks or bulbs, leave smaller pieces buried in the ground for them to reproduce. If you want to grow a plant in your garden, gather the seeds for propogation rather than transplant them from the wild.
(5) In this day and age when plants are becoming increasingly rare, it is best not to gather wild plants for food and rarely for medicine. It is best to grow your own for this purpose.
(6) Gather plants only if you are certain of their identity. Many irritating or poisonous plants are look-a-likes for edible or medicinal ones. If you are not certain, do not gather.
(7) Gather plants carefully. Bag and label immediately as accidents have occurred by mixing leaves or rootstocks or by gathering from nearby plants which could be poisonous.
(8) Dry herbs quickly and away from bright sunlight in order to preserve the aromatic ingredients and to prevent oxidation of other chemicals. Good air circulation is also required. Avoid drying them in a garage as they can become contaminated with gasoline fumes.
Aerial parts are best collected in the midst of flowering, giving a mixture of leaves, stems, flowers, and seed heads. Skullcap, for example, can be collected when about half the flowers have formed the characteristic cap-shaped pod. Large leaves, like burdock, can be harvested and dried individually. Smaller leaves, like lemon balm, are best left on the stem. Gather deciduous leaves just before flowering. Evergreen herbs, like rosemary, can be harvested throughout the year. If using all aerial parts, harvest in mid-flowering, giving a mixture of all the parts. Remove obvious dirt, grit, and insects!
Bark is generally best collected in the fall when the sap is falling in order to minimize damage to the plant. Never remove all the bark, or even a band of bark completely around a tree. This will kill the whole tree. Remove thin, vertical strips of no more than a few inches. Dust or wipe the bark to remove moss or insects, and avoid oversoaking in water. Break into manageable pieces, spread on trays, and let dry.
Bulbs are harvested after the aerial parts have wilted. Collect garlic bulbs quickly as they tend to sink downward after the leaves have wilted, and are then difficult to find.
Flowers should be collected when fully open and handled carefully as they are easily damaged. Harvest after the morning dew has evaporated and when fully open. Cut the flower heads from the stems and dry whole on trays. Small flowers, like lavender, can be dried on their stems; but, if the stem is fleshy, like mullein, the flowers must be removed and dried individually.
Fruit berries and other fruits should be gathered when just ripe, but before the fruit becomes too soft or pulpy to dry effectively. Spread on trays to dry turning fleshy fruit frequently to ensure even drying. Discard any with signs of mold.
Leaves, if large like burdock, can be gathered individually; but, if smaller,like lemon balm, are best collected on the stem. Leaves of deciduous herbs should be gathered just before flowering. Evergreen leaves, like rosemary, can be gathered throughout the year. Sometimes young leaves are collected separately for cooking, soups, salads, and spring tonics, as in the case of nettles or dandelions. A second crop of herbs can then be taken closer to flowering for the more mature leaves.
Roots are generally gathered in the autumn when the aerial parts of the plant have died down and before the ground becomes too hard, making digging difficult. An exception is dandelion whose roots should be gathered in the spring. Some roots reabsorb moisture from the air and must be discarded if they become soft. Large roots should be chopped into smaller pieces when fresh since they can be difficult to cut when dry.
Sap and resin are collected from the tree in autumn by making a deep incision in the bark or drilling a hole and collecting the sap in a cup tied to a tree. Sometimes a sizeable bucket is needed. A large amount of birch sap, for example, can be collected overnight at certain times of the year. Squeeze sap from such latex plants as wild lettuce over a bowl. Many saps can be corrosive, and wearing rubber gloves is necessary when handling them. For collecting aloe gel, simply cut along the center of the leaf and scrape out the gel.
Seeds should be collected when ripe or, with larger seed heads like fennel, when about two-thirds of the seeds on a particular head are ripe and before too many have been dispersed by birds or the wind. Hang the entire stalk of seed heads upside down over a paper-lined tray or in a paper bag away from direct sunlight. Seeds will fall off when ripe.