Agroforestry systems have the advantage over standard agricultural systems in that there are a wide range of microclimates ranging from near full sun (in the center of wide alleys in alley cropping systems) to partial sun (the edges of alleys, between trees in orchards or forest gardens, on the northern side of clearings in forest gardens forest farms) to full shade (in the understory of forest gardens and farms, or on the north side of a plantation or E-W alley tree line). A specific crop should thus be able to be located in near ideal conditions as close to its natural habitat as possible.
On a field scale, planting may present some problems. Sowing of seed is possible for some species (using precision drills used in vegetable production), but leads to almost certain use of undesirable herbicides. For plants propagated by stolons (root cuttings eg. mints), planting machines must be adapted. The simplest method is to machine plant young plants grown in blocks.
Good weed control for low-growing species is essential to ensure that the harvested product is pure and not contaminated with weeds. Most growers of medicinal plants do not follow organic principles and apply numerous doses of herbicides for weed control. Instead of this we advocate the use of mulches, which is more labor intensive and more expensive, but leads to uncontaminated plants and soils. There is an increasing market for organically-grown herbs with higher returns to offset increased costs.
For trees and shrubs, permanent mulches of leaves, bark or other organic matter are easily used and will maintain optimal soil conditions. For herbaceous perennials, if they are to be cut high enough to be quite clear of any mulch, permanent mulches should also be used. However, if they are notably susceptible to slug and snail damage, then seasonal mulches from late spring to autumn are a better idea.
Low growing perennials are difficult to mulch without the mulch itself contaminating the aerial parts of the plant. It is best not to use high-nitrogen materials for mulching. Other methods of weed control used by organic growers include the use of brush hoes, growing plants through porous polythene mulches, and using ridges as for potato growing.
Use of varieties
Several of the more common perennial medicinal plants (eg. mints) have had varieties selected for higher amounts of medicinal compounds or purer ratios of essential oils. Most of the selection has been made by medicinal plant growers themselves and few of them are easily available on the market, but they are worth considering if a source can be found.
Ecological factors affecting yields
With many agricultural crops, increasing yields through biomass production is the main aim, but this principle does not translate directly to secondary plant products (i.e., medicinal compounds in plants). Increasing the plant biomass per unit area does not necessarily increase the amount of these compounds per unit area. The following factors should be taken into account to understand the ecological influences on medicinal plants:
- The generation of active substances is basically connected to the metabolism of the plant,
- Differences of ecological requirements occur between species and often within species, leading to differences in the active compounds.
- The influence of ecological factors on the generation of active substances is complicated by the fact that the amounts and ratios of active substances change over time during the growing year and the life of the plant.
The intensity and duration of light available has a pronounced effect on the production of active substances.
In general, for species which are naturally sundemanding, reduced light intensity (e.g., by being part shaded) or duration (e.g., by cultivating further North than naturally found) reduces yields of active substances and can also alter their relative amounts in the plant. For example, in peppermint, yields of essential oils fall in proportion to the light intensity. Cultivating it in 50% shade conditions will still yield significant quantities of oils but not enough for economic viability. This has important consequences for agroforestry systems in which medicinal plants are being grown: sun-demanding medicinal plants are best situated either in the upper canopy layer i.e. tree crops or should be grown in the sunniest sites, in the center of alleys between trees or in sunny clearings.
For shade-tolerant medicinal plants the situation is quite different. In these, the active substances are produced in conditions of low light intensity and the duration of light is less important. Increasing the light available to these plants may increase the proportion of active substance, but it will also stress the plants and may well reduce biomass production. If so, overall active substance production may not increase and the plant may also be more prone to pests and diseases. It obviously makes sense to site shade-tolerant plants in situations to which they are adapted. These plants, then, can be used more widely in agroforestry systems in the shrub and perennial layers and in shadier sites such as the edge of alleys and the northern edges of clearings.
The influence of temperature is relative, depending on the optimum temperature of a given species. Above or below that temperature, amounts of active substances are likely to fall. The assumption that herbs of Mediterranean origin will only produce high quality essential oils under stressed conditions (i.e., high temperatures, drought, low fertility) is unfounded.
The connection between active substances and the water supply depends strongly on the species. Deep rooted species likes trees and comfrey, and plants of dryland origin like many Mediterranean herbs, do not respond greatly to irrigation. A good site for dryland plants may be a sunny southern edge where tree roots make it a dry location. Other plants, especially those which need a moist site, may require irrigation depending on the climate and season. In continental Europe, most commercial medicinal plant cultivation uses irrigation but in British or other temperate regions it will not be so necessary. Shade-tolerant plants grown in agroforestry systems are unlikely to require irrigation.
The soil has a complex influence on plants through its physical, chemical and biological properties. A healthy soil usually leads to healthy plants and this adage can be applied to medicinal plants too. Soils should be tended to raise and maintain organic matter levels and to be able to supply sufficient nutrients for the plants. Nitrogen applications increase biomass yields, but effects on the essential oil quality and content are much smaller or negligible; the therapeutic strength of many medicinal plants is reduced by feeding.
Medicinal plants are harvested for use fresh, for drying or for the extraction market mainly the latter two. Commercial growers need to correctly time harvest to get both abundant and quality crop yields. Different species have specific stages of growth at which they should be harvested, and the content and composition of essential oils and other active ingredients varies during plant development. Oil content is usually highest in the morning, depending on the light and temperature conditions. For most species, peak oil content coincides with flowering. Plant materials for drying should be harvested on dry warm days after any dew has evaporated. Avoid crushing harvested materials.
Foliage is harvested by hand with sickles/scythes or in larger fields with mechanical cutters (e.g., sickle bar mower or disc mower) then raked up. For most species, leaves are harvested before the flowers are fully open; they should be clean and free of pests. When harvesting perennials or small shrubs, it is best to leave some basal leaves to help them recover and enable a further crop later in the season.
Flowers are usually picked by hand or sometimes with a simple hand harvester of the type used for picking berries. They are best harvested when first open fully and placed loosely in an open basket.
Fruits or berries require harvesting when fully ripe but not over-ripe; they should be dry and free from bits of bark or leaf. Seed crops are usually harvested with a combine when ripe. Some species, with seed pods that shatter, must be harvested slightly underripe. Roots can harvested with a hand fork or by using a mechanical digger such as a potato digger. They are usually harvested in the autumn, when the top of the plant is dying down.
If collecting medicinal plants from the wild make sure that you have the landowner's permission, that you have correctly identified the species, and never gather all the specimens of the species in a given locality leave enough to ensure its continued survival in the area.
Reprinted by permission from Agroforestry News, Volume 7, No 3, April 1999.