here's another li'l tidbit of interest from DJ..
Even with a known high-THC clone, THC level and cannabinoid ratios may change depending on environmental conditions.
What defines drug strain cannabis is the plant's ability to convert cannabidiol (CBD) or possibly cannabichromene (CBC) into THC.1 If we as growers do not provide the plant with reason to make this conversion it likely will devote its energy elsewhere, to aid in its survival.
Environmental Influence
It takes high quality genetics to produce high quality marijuana, but genetics is only half of the equation. The genetic structure (genotype) only plays 50% of the role in determining the appearance and quality (phenotype) of a given plant. The other half is determined by environmental conditions such as light, temperature, humidity and soil nutrition. All these factors play a role in both the physical and chemical nature of marijuana's trichomes.
The best way to take a look at how environment affects THC production is to look where on the planet cannabis has naturally adopted a high THC profile. As cannabis has spread around the world it has taken on many different traits to help in its adaptation to varied areas. The best drug varieties have always been found at equatorial or high altitude locations. The one thing which both of these variables have in common is high light intensity and a large amount of ultraviolet (UV) light in the spectrum.
Recent Swiss trials in outdoor plots of clones grown at different altitudes have shown that there is correlation between higher altitude and increased potency (although there seems to be a trade off in yield). This likely means that THC-rich resins act to protect the plant and its seed from both higher light intensities and ultraviolet presence. It's no surprise that cannabis has developed a chemical to protect itself against the Sun's damaging UV rays, as they can be injurious to all forms of life.
In a plant's search for survival, energy put towards unneeded processes is wasted energy. Therefore a high-THC plant grown in a low THC environment will likely produce a medium THC result.
Humidity also plays a role in plant resin production. Although some potent equatorial strains do seem to occur in high humidity areas, most high-test land races have evolved in drier areas, like Afghanistan. The aridity of the areas of Afghanistan where Indica strains have evolved is quite apparent by the trait of large dense flower clusters. This would only be an advantage in an area of low humidity, as flowers will mold in anything more.2
There are many examples of non-cannabis plants producing resins in order to protect themselves from drying out. The waxy coating on cacti and other succulent plants is a prime example.3 Marijuana flowered in humid conditions will often have a longer stalk on the glandular trichome than the same strain grown in drier conditions. While this may give the appearance of being very crystallized, it will likely contain less THC than the same plant grown in a drier environment. Another problem with longer trichome stalks is that the gland heads are more likely to break off during handling.
Flushing: pros and cons
Much time and thought has been put into the feeding needs of each part of marijuana's life cycle, yet for some reason the final stages of resin development always seem to be ignored. But the vegetative period of plant growth is only setting the platform for us to produce the trichomes that we are after.
Flushing in particular seems to be something that is over-emphasized by many of today's growers. Many growers "flush" their plants with straight water or clearing agents during the final weeks before harvest in an effort to improve taste and smokeability. The theory is that this forces the plant to use up stored nutrients that may affect these qualities. Although this is certainly true to some extent, what many are forgetting is that not all nutrients can be moved within the plant.
Nitrogen, which is the main factor in poor-tasting bud, can be moved within the plant. If not present in the root zone a plant will take it from the older leaves to support newer growth. Calcium, however, is a nutrient that cannot be moved within the plant, if it is not present in the root zone it is not available for growth. Little research has been done on nutritional requirements of cannabis during the final stages of flowering, but it seems likely that calcium is vital as it is crucial in cell division. A calcium deficiency at later stages could therefore adversely affect trichome production.
This is not as serious of a concern for soil-based growers, as lime or other calcium sources which are mixed into the soil likely will provide sufficient nutrition even while flushing with pure water. But hydroponic growers using very pure water sources with little naturally occurring calcium could have problems. Flushing is certainly a valid technique, but is easily overdone and is not a quick fix for overfeeding earlier in the flower stage.
Some studies have shown that high potassium levels have a negative influence on THC production,4 which would correlate to the general belief that while hemp crops uptake more potassium than phosphorous, the reverse seems to be true for drug and seed cannabis crops.2 A study on how to minimize THC levels in hemp crops showed that THC levels in newer leaf growth decreased as nitrogen levels were increased.5 As no THC measurement was taken from floral clusters we can only speculate that the same would likely hold true in buds. This would also explain the good results that most growers have flushing their plants, as nitrogen is the nutrient most easily flushed from the soil.
Companion planting
Much research is still needed on the interrelationships of plants in the garden. Little is known about common vegetable garden plants effect on each other, let alone how they may react with cannabis.
Growing certain plants in proximity to each other has been documented to cause noticeable effects on growth, both positive and negative. The main companion plant that has attracted interest with underground marijuana researchers is stinging nettle (Urtica dioica) which has been said to increase essential oils in many plants.6
