Plant Harvest: Time Is Of The Essenc
by Dean Gray

The term development refers to a step or stage in growth and maturation. All life forms go through general stages of development, and for plants in particular, their medicinal chemicals can be more or less abundant depending on the developmental stage.

Through a temperate growing season, a perennial plant goes through vegetative and repro-ductive stages of development. During the vegetative stage, the plant moves stored resources from the roots to new stem and leaf growth. The new leaves quickly become the photo-synthetic factories transforming solar energy into carbohydrates. During the reproductive stage, the leaves are the primary sources of energy for the juvenile flowers, in addition to root uptake of nutrients and water.

Following successful pollination, the leaves continue to trans-locate sugars to the developing seeds. Following this, the leaves send excess essential elements and starch back to the roots for storage over the winter, in order to start the process again in the spring. In a medicinal plant, chemical concentrations vary considerably from tissue to tissue during these various developmental stages. Younger leaves and flowers generally contain greater levels of phenolic acids and terpenoids than more mature leaves;1,2 while seeds tend to accumulate fatty acid derivatives through maturation. Not only do the concentrations of plant chemicals fluctuate through the season, but they can also be short-lived and experience rapid turnover. Therefore, in terms of the chemical quality of a plant extract, the developmental stage of the plant is crucial in helping to determine the optimum time of harvest.
It is widely believed that root crops such as ginseng (Panax quinquefolius L.) should be harvested in the fall after aerial growth, photosynthesis, and transport of nutrients from the fading leaves to the roots has finished. At this time, root weight and the concentrations of active constituents have been reported to be maximized.3 Species such as St. John's Wort (Hypericum perforatum L.), where the youngest leaves and flowers are harvested, are generally collected at the very beginning of the flowering period prior to fertilization.4

While these are suitable protocols to follow, it is not a given that every species follows these rules. Changes in chemical concentrations through the growing season have not been fully investigated for many of the plant species that are valued for their use as medicines. In addition, other growing parameters that directly affect developmental stage, such as temperature, water, light, nutrition, and plant age are all critical variables to consider.

Every plant reaches it's peak potency at a particular developmental stage and time of the season. Much of our research is focused on determining when that peak occurs. It is this kind of research that allows us to improve the quality and potency of herbal extracts.

References Cited
1. Mato M., and N. Ishikura (1993). Flavonol changes in seedlings of Vigna mungo during growth. Journal of Plant Physiology, 142:647-650.
2. Gershenzon J. (1994). Metabolic costs of terpenoid accumulation in higher plants. Journal of Chemical Ecology, 20:1281-1328.
3. Smith R., D. Caswell, A. Carriere, and B. Zielke (1996). Variation in the ginsenoside content of American ginseng, Panax quinquefolius L., roots. Canadian Journal of Botany, 74:1616-1620.
4. American Herbal Pharmacopoeia and Therapeutic Compendium (1997). St. John's wort: Hypericum perforatum. American Herbal Pharmacopoeia, 32 pgs.