Effects of the Environment on Potency

We have emphasised the heredity is the most important factor that determines potency. Potent marijuana grows from seeds of potent marijuana. A healthy, mature plant bears an abundance of flowers, guaranteeing you a potent harvest. Some researchers have investigated the impact of the environment on relative potency, since this question is of interest to officials concerned with marijuana control as well as to marijuana growers. Their primary goal has been to discover the gross effects of different environments rather than to single out the effects of any particular factor. A consensus is that the impact of environment on potency is small relative to that of the plant's heredity. Nevertheless, where scientists have commented on this question, the common denominator for higher potency has been stress.


Stress in anything that detracts from the plant's health or vigour. Environmental factors such as competition from other plants, low water availability, and poor soil conditions are examples of stress factors.

In many marijuana-growing cultures, farmers have practices that are stress-related; splitting the base, severe pruning, bending or contorting the stem, and water deprivation are common examples. Of course, the fact that marijuana-growing cultures have such practices does not mean that these practices actually increase potency, or that this is, or ever was, their intent. This original meaning may well have been forgotten centuries ago. For instance, cultivation of sinsemilla has been practiced for centuries, not for potency, but because the seedless product is easier to process or smoke.

There does seem to be some underlying relationship between stress and higher potency. Stress factors may slow growth in general, but at the same time, may not slow the synthesis of cannabinoids. Potency may be affected in much the same way by growth factors that are not considered stressful. As described previously, marijuana plants grow more compactly and have smaller leaves under conditions of relatively warm temperatures, or strong sunlight, or a dry atmosphere; they grow taller and have larger leaves when grown under cool temperatures, moderate light, or a humid atmosphere. Higher relative potency seems to correlate with conditions which favour compact development of the plant and its parts.

The rate of cannabinoids synthesis relative to photosynthesis may be affected in ways not apparent. Sunlight, for instance, is a growth factor. In almost all cases, the more sunlight the plants receive, the faster and larger they will grow. Yet plants grown with intense sunlight seem to maximise potency. Intense sunlight can raise plants' internal temperatures to levels that interfere with the photosynthesis cycle. Absorption of light energy and conversion to biochemical energy continues unimpeded, but the synthesis of sugars is impeded. (Under a midday sun, this phenomenon has been observed in other field crops. (206)) In marijuana, cannabinoids synthesis may continue unaffected at these higher temperatures. This might account, in part, for the slightly higher potency of plants grown in tropical zones.

The subject of potency is mired in confusion and mystery, largely because of fertiliser and soil ads, marijuana-growing books, and individuals who promise ways of increasing potency or growing super grass. There are no magic formulas or secrets to divulge that will make or break the potency of your crop. We have tried to play down this type of thinking throughout this book. Choice of seed, and a harvest of well-developed buds, far outweigh any other factors in determining potency.

We know of no one who has demonstrated that manipulation of any particular environmental factor leads to higher potency. This lack of demonstration probably exists for two reasons: (1) environmental effects on potency are relatively small compared to the effects of inheritable traits, and hence are not easy to discern; and (2) "increased potency" is difficult to prove ((Most scientific experiments are observations, and test effects or compare results rather than try to prove something.)) The variations in potency within any variety, and within each plant, require stringent methods of sampling for comparative tests. And since potency also changes with time, meaningful comparisons can be difficult to make. Scientific papers reflect this difficulty.

An experiment on potency must account for inherent variations in potency before environmental effects can be analysed. Samples would need to be equivalent in terms of variety, growth stage and development, sex, plant part, and the position of the part on the plant. A simple way to do this would be to harvest females when each reached full bloom and then compare the uppermost buds from each plant.

Most of the research on potency done to date either has not reported sampling techniques or did not account for certain inherent variations. In the extreme case, all vegetative matter from one plant was mixed together, and the THC concentration in a sample of this matter was compared with that in a similar sample of mixed marijuana from another plant. Such practices can give misleading results. Consider the fact that an unhealthy plant will have dropped many of its lower, less-potent leaves. A healthy plant has more leaf overall and retains more of its lower leaves. There is a good chance that the unhealthy plant will test higher in average THC content, because proportionately shoots, which are relatively more potent. Such a sampling error could create the impression that stress is positively correlated with potency.

Marijuana scientists have recognised the need for testing equivalent samples and for setting standards for testing. In 1974, the English scientist John Fairburn (68) published a number of well-controlled experiments concerning potency and light. This is a hopeful sign that more meaningful experiments will be forthcoming.

We have said that the common denominator for increasing potency is stress. Let's put that in perspective.

You cannot go wrong if you grow the largest and healthiest plants possible. Our experience has been that the most potent plants are more often the healthiest and most vigorous in the garden.

Factors that limit growth rate are probably related to potency, and if growth rate is relatively slower, cannabinoids concentrations may be higher. Plants whose average yield is six ounces may be slightly more potent than plants whose average yield is eight ounces. Factors related to potency affect growth rate rather than ruin the plant's health. When a plant is so traumatised that it is barely surviving, potency as well as growth rate declines.

Obviously, if the growth rate is slower, the harvest will be smaller. Any difference in potency due to stress is quite small, but the difference in yield can make the difference between harvesting an ounce and harvesting several pounds (ie., don't get carried away with the practice of stressing the plants). If you wish to stress the plant, wait until it is firmly established and growing well.

Outdoors, don't stress the plant until at least the middle of its life. You want the plant to be large enough to bear a good harvest of buds. Water deprivation is a good method of limiting growth outdoors. However, wilted plants must be watered, or they will die.

Competition from other weeds has been correlated with higher potency in two recent studies (71,74). You might prefer not to weed your patch after the seedling stage. (Initially, weeding is necessary because indigenous weeds generally outgrow marijuana seedlings.)

Another safe way of applying stress is to remove all large leaves from the plant once it has begun to flower.

Indoors, the plants are already in a delicate state. We advise indoor growers to grow the largest and healthiest plants possible for best results.


Most growers show a keen interest in fertilising, since it is one factor over which they have some control. Most growers also feel that nutrients, which play such an important part in plant growth, probably have a relationship to the potency, and this is a reasonable assumption. In marijuana lore, potency is sometimes attributed to particular soil types (for instance, red dirt, which is iron-rich) or to presence or lack or certain nutrients (for examples, nitrogen or potassium deficiency).

The relationship of potency to soil conditions, in particular the nutrient content, has been looked at recently by several research groups. In two such studies, (71,74) the cannabinoid content of naturalised weedy hemp stands in the Midwest was examined. Variations in potency were then correlated to soil properties, such as N, P, and K content. The two papers came to similar conclusions. First, stands growing in areas where they were under stress tended to produce less biomass (yield) but were more potent overall. Second, when nutrients or other growth factors, such as height and weight of plants or root size, were correlated with potency, potency was almost always correlated positively with positive growth factors. That is, higher potency occurred when the plants were growing with adequate or high amounts of nutrients present, not when nutrients were inadequate.

What appears to be a contradiction (stress leads to higher potency, or good growing condition lead to higher potency) may be explainable in terms of what these experiments actually measured.

A basic assumption in these studies was that all the plants were relatively homogeneous genetically, since they may have originated from a single stock of hemp grown during World War II. Assuming this is true, then variations between stands would be due to differences in local environmental factors. However, since environmental condition differed locally for separate stands, one cannot tell whether variations in potency between stands are due to present environmental factors (phenotypic responses) or reflect thirty years of adaption by each stand to its local environment (genotypic shifts).

It may be that positive growth factors are associated with higher potency in phenotypes (plants now growing), whereas stress leads to higher potency in succeeding generations, because of selective pressure. It is interesting that both papers reported strong positive correlations between higher potency and competition from other weeds, since competition between plants does exert strong selective pressures.

The following list of possible effects of nutrients on potency has been adapted from these four studies. (63,71,74,231)


Nitrogen was positively correlated with higher potency. Once controversy in marijuana lore is whether a nitrogen deficiency during flowering increases potency. We have grown plants with N deficiencies, and they seemed no more potent than those grown with high amounts of nitrogen available. However, the N-deficient plants did produce a much smaller harvest.


P has been correlated positively with higher potency in all studies that have examined this factor. Phosphorus is necessary for good flower development and seed production. Give the plants a steady supply of phosphorus throughout growth and in particular during flowering.


K has been correlated both positively and negatively with potency. More often, it has been found to be negatively correlated. As discussed previously, plants that show some potassium deficiencies may grow well; so you may choose not to treat minor symptoms of K deficiency often grows vigorously with little harm other than the spotting and the loss of some lower leaves. It should not be necessary to fertilise with potassium during flowering unless deficiency symptoms are severe and the plant has ceased growing.


Abundant Ca levels have been consistently correlated with higher potency.


Mg has been negatively correlated with potency. However, this may have been dur to the interaction of Ca and Mg, and may reflect Ca's strong positive correlation to potency, rather than the negative effects of Mg per se. Plants that show Mg deficiencies must be fertilised, or they will quickly lose most of their leaves and barely remain alive.

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