New trends in illicit cannabis cultivation in the United Kingdom of Great Britain and Northern Ireland


Illicit products on the market in the United Kingdom prior to the 1990s
Limitations of clandestine outdoor cultivation of cannabis in the United Kingdom
The beginnings and origins of new cultivation practices
Hydroponic versus outdoor cultivation
The present: a high-tech industry
Hydroponic cannabis "factories"
Analytical findings from plants seized from hydroponic factories
Legal and forensic implications
Evidence in court
Potential impact on cannabis abuse
Future prospects


Author: C. BONE, S. J. WALDRON
Creation Date: 1999/12/01

New trends in illicit cannabis cultivation in the United Kingdom of Great Britain and Northern Ireland

The Forensic Science Service, Drugs Team, Birmingham Laboratory, United Kingdom of Great Britain and Northern Ireland

The Forensic Science Service, Drugs Team, Chepstow Laboratory, United Kingdom of Great Britain and Northern Ireland


Since 1993, indoor cultivation of cannabis has become increasingly common in the United Kingdom of Great Britain and Northern Ireland as a source of highly potent female flowering head material. The tetrahydrocannabinol (THC) content of samples of female flowering head material from indoor cultivation analysed by the Forensic Science Service has averaged about 8 per cent, with the highest THC levels measuring about 23 per cent. The potency is likely to depend on the variety of cannabis grown and the degree of sophistication of the cultivation techniques employed.

These THC levels are significantly higher than the THC levels attained from outdoor grown plants analysed in the United Kingdom: about 1 per cent from cannabis leaf; about 3 per cent from female flowers; and 3-5 per cent from imported cannabis.

The indoor cultivation technique generally employs some method of hydroponics, either active or passive, which allows a controlled supply of nutrients to the plants, together with use of powerful lights to allow optimum photosynthesis for growth or flowering purposes.

The taking of cuttings from female plants is generally the preferred method for producing the next generation of plants, as the plants will be female and will contain a THC level similar to that of the previous generation. The use of cuttings obviously allows the potential for a continual supply or several harvests of cannabis per year.

Illicit products on the market in the United Kingdom prior to the 1990s

Prior to the 1990s, the cannabis available for consumption in the United Kingdom could be divided into two types [1-8]:

( a) Homegrown: predominantly leaf material containing low levels of tetrahydrocannabinol (THC), the component mainly responsible for the characteristic effects of cannabis intoxication (approximately 1 per cent);

( b) Imported: usually compressed blocks of mature female tops, including seed and stalk, containing 3-5 per cent or more THC. It was generally imported from Africa and the West Indies. High-quality seedless cannabis from South-East Asia having mean THC levels of over 9 per cent was available in a form known as "Thai sticks".

Another major cannabis product frequently encountered prior to 1993, imported cannabis resin, had a THC content ranging from 1 to 10 per cent. Such resin was believed to originate from the Asian subcontinent and North Africa. Cannabis oil, which generally had high THC levels (15-50 per cent), was relatively rare in the United Kingdom.

Limitations of clandestine outdoor cultivation of cannabis in the United Kingdom

Cannabis grows readily in the United Kingdom, and the plants cultivated illicitly prior to the 1990s were usually grown from seeds that had been collected from imported material. It has been shown that the material produced by such plants contains THC levels comparable with those of imported cannabis, i.e. an average of 3 per cent in mature female tops. Such levels can be expected, as it has been shown that THC levels in plants grown from imported seed are comparable to the levels in the original plants [5, 6, 7, 9].

THC values are dependent upon the following: the genetic characteristics, maturity, sex and the particular part of the plant; the environment in which the plant is cultivated; the amount of time between harvests; and the storage and preparation of the final product. Varieties of cannabis grown outdoors in the 1970s were found to have THC levels as follows [9]:

  Average THC level
Range of THC levels
Leaf 1.0 0.01-4.5
Vegetative tops 2.7 0.04-6.8
Female flowers 3.0 0.07-7.0

Outdoor cultivation was limited not only because of the inherent danger of discovery but also because the climate in the United Kingdom reduced the length of the flowering period so that the extent of flowering was often reduced. The long hours of sunlight required to produce optimum growth were readily available during the summer months. Plant growth was thus encouraged, resulting in tall, bushy plants with high yields of leaf. However, days that were shorter in length were required for the plants to begin producing the more potent female flowering tops. As the shorter days did not come until late in the growing season, the flowering period was often curtailed and the extent of the flowering was limited. Although any flowers produced may have been comparable in potency to the imported material, only relatively small quantities were produced. Such plants, although quite large and bushy, typically 1-2 metres in height and producing between 50 and 300 grams of foliage per plant, [8] would often produce relatively few mature flowering tops, if any, before night frosts killed the plants.

Simple, indoor cultivation on a small scale, consisting of potted plants on window ledges or in greenhouses, was also carried out. Such plants, however, generally produced little vegetation compared to plants cultivated outdoors, and even smaller yields of mature flowering tops were produced. Homegrown cannabis was usually easily recognizable as such and generally considered to be of inferior quality and virtually unusable. Consequently, illicit cannabis production in the United Kingdom was not deemed to be viable on a commercial scale.

The beginnings and origins of new cultivation practices

Towards the end of the 1980s, a change in cannabis production in the United Kingdom could be observed that appeared to be a direct consequence of developments in the Netherlands. One was the development of strains of cannabis that were bred to produce high levels of THC and higher proportions of mature female tops. Such strains were specifically designed to be suited to either indoor or outdoor cultivation. One of the first such strains of cannabis was called "skunk", owing to the odour it produced. The term "skunk" is now used in the United Kingdom, however, as a general term for good-quality homegrown cannabis, which is usually sinsemilla (seedless female flowering tops).

The potency of cannabis is attributed to its major psychoactive ingredient, THC. As the concentration of THC is highest in the flowering or fruiting tops of mature female plants, it has become common practice to ensure that only female plants are cultivated. Male plants are less desirable not only because the material produced by them is generally less potent but also because, if present, they may pollinate female plants and thus prevent them from attaining the maximum level of THC. Thus, the level of THC is influenced by the genetic characteristics of the plant as well as the growing conditions.

The cultivation of cannabis "naturally", i.e. from normal seed, is unsuitable for large-scale production because of the lack of uniformity of quality and because a certain proportion would naturally be male. It has become common for growers to cultivate plants using cuttings from selected female plants taken from specially selected mother plants. The use of cuttings, or clones, results in the production of daughter plants that exhibit all the desirable qualities of the mother plant and ensures that no male plants are produced.

It was not merely the new strains of cannabis available, however, that were causing the change in cultivation practices, but rather changes in the whole approach to indoor cultivation. Using sophisticated horticultural techniques, growers were able to cultivate continuously, producing several crops throughout the year. Such indoor cultivation increasingly involved varieties of cannabis, that were specially selected for their increased potency and were grown using hydroponic growing techniques.

Hydroponics is a system of plant cultivation that does not require soil as a growth medium. Plants and cuttings grow in an absorbent synthetic medium that is readily supplied with water and nutrients artificially, e.g. by means of an electric pumping system. The system can be operated indoors with powerful electric lights that provide consistent lighting conditions and substitute for natural sunlight. The hours of light and liquid and nutrients supplied can be controlled artificially by using an electric timer in order to provide optimum growth conditions. Carbon dioxide can be introduced into the system artificially to enrich the ambient level and help produce the optimum conditions for plant growth and flowering.

Initially, information on and equipment for indoor growing techniques were available primarily from sources in the Netherlands. It was not long, however, before outlets opened in the United Kingdom. At the outset, such enterprises were low-key and cautious, but growers have since become more open, having realized that it is possible to stay within the law by producing detailed information for "interest only".

The increase in the production and use of homegrown high-quality sinsemilla appears to have coincided with a shortage of cannabis resin on the illicit market in the United Kingdom. Currently, therefore, much of the cannabis abused in the United Kingdom is homegrown material, whereas previously imported cannabis was preferred.

Hydroponic versus outdoor cultivation

The move from illicit cultivation outdoors to hydroponic cultivation can be attributed to the following factors:

( a) Faster growth. The ability of producers to maintain a constant supply of nutrients of known composition and to control the number of hours of light from powerful electric lights allows them to accelerate the rate of vegetative growth and to induce flowering at will. Faster growth means earlier yields and more harvests a year;

( b) Higher yields. As less competition for nutrients and water results in less extensive root growth, more plants can be grown per unit area. Often, from three to four harvests are achieved annually;

( c) Cleaner and more sterile conditions. The use of an inert, sterile growing medium such as "rockwool" or expanded clay pebbles in a controlled environment makes it easier to maintain a clean, infection-free environment, thereby reducing the potential for pest diseases;

( d) Lower risk of over- or underwatering. The risk is automatically diminished in hydroponics because the root region is well aerated;

( e) Prolific flowering. The cannabis plant is able to complete its growth cycle and can thus produce more female flowers for harvesting;

( f) Uniform quality. In a process known as "cloning", plants can be grown from cuttings taken from specially selected mother plants that are known to be of a variety high in THC. The result is similar female plants with a high THC content. Cloning is well suited for hydroponics and makes it possible to have a continual supply of cannabis growing from one year to the next;

( g) Lower risk. The crop is out of immediate view, making it less likely that the clandestine cultivation of cannabis will be detected.

The present: a high-tech industry

By the early 1990s indoor hydroponic cultivations were well established, with the result that illicit cannabis cultivation may currently be the largest cottage industry in the United Kingdom. Attics, bedrooms, cellars, store cupboards and outbuildings have been found that had been converted to produce cannabis commercially, often on a considerable scale.

Hydroponic cannabis "factories"

The illicit cannabis "factories" encountered within the United Kingdom range in capacity from small-scale production of a few plants to large-scale commercial production of hundreds of plants in several rooms, disused barns or warehouses.

Typically the premises must be secure, must be supplied with a constant supply of electricity and water and must have an area for harvesting and drying the mature cannabis leaf and flowering head. Young cuttings are placed to root in a synthetic medium in a separate area that is often illuminated by fluorescent lights, since they are not too bright or hot and encourage root growth.

As the cuttings establish themselves, they are transferred to an area illuminated by powerful high-wattage lamps (metal halide), which encourage vegetative growth. The lights are often controlled by means of electric timers to ensure that the plants obtain sufficient hours of light per day to achieve fast growth. The plants will typically grow to a height of about a metre before they are transferred to another area to flower. There, other high-wattage lamps (high-pressure sodium lamps) are used, and the hours of light per day are reduced by means of electronic timers. This induces the plants to flower, and if they are not pollinated, they will produce prolific female flowers high in THC that are then ready to harvest. The growth areas are usually surrounded by a reflective material to maximize the availability of light and heat to the plant.

Nutrients are supplied to the growing medium in liquid form, either actively by a pumping system or passively by means of a wick-type system. If the process is performed correctly, three to four harvests a year are easily possible.

A well-equipped laboratory will also have the following pieces of equipment:

( a) Carbon dioxide cylinders to enrich the atmosphere with carbon dioxide, an essential ingredient for photosynthesis and plant growth;

( b) Equipment for testing pH and conductivity to ensure that the nutrients are of the correct pH and strength;

( c) Heaters, thermometers and hygrometers to ensure that correct temperature and humidity are maintained;

( d) Electric fans to circulate the air;

( e) Chemicals, such as plant hormones and pest control compounds, and predatory biological mites;

( f) Relevant books, documents and calendars.

See figures I-III.

Figure I.  Hydroponic cannabis laboratory with carbon dioxide supply and high-wattage lamps to encourage vegetative growth

Source:South-West Regional Crime Squad.

Analytical findings from plants seized from hydroponic factories

The amount of usable cannabis recovered per plant varies enormously, depending on the bushiness of the plant. Weights recovered have been found to range from 1 to 355 grams per plant. In the more mature plants, flowering head material may be 50-70 per cent of this weight.

Growers endeavour to maximize the production of the female tops and often discard the leaves, considering them to be of relatively poor quality. At maturity, plants usually consist of a very high proportion of large, densely packed female flowering tops. The product usually consists of light green, carefully dried, "manicured" individual tops in which many brown stigmas are visible. Glands are also clearly visible to the naked eye, even on the smaller, surrounding leaves. Many of the more potent strains of cannabis produce relatively few leaves, and some growers simply discard them as being worthless. Such leaves may, however, contain significant levels of THC comparable to those in imported material or in some sinsemilla. Therefore, some growers, not wishing to waste any of their precious harvest, save those leaves for use and sale, although it may be difficult to sell such material because its appearance makes it less desirable.

Alternatively, leaves may be used in cooking and beverages or to produce hash oil, although the latter is not common in the United Kingdom.

Examples of THC measurements of cannabis analysed by the Forensic Science Service Laboratories are provided in the table below [8].

Analysis of cannabis

Total THC Content (%)

Year Sample No of samples Range Mean
1995 leaf












1996 leaf












1997 leaf













Note: Each sample was carefully dried and stems were removed. Two ground samples of 250-500 milligrams were extracted into methanol containing an internal standard, by ultrasonic vibration. The extracts were then analysed by gas chromatography for their total THC content. (Method based on that described by Fairbairn and Liebmann [15].)

In addition, the harvested material recovered from two factories in 1997 were found to contain 22 and 23 per cent THC.

Legal and forensic implications

In the United Kingdom, cannabis is controlled by the Misuse of Drugs Act 1971. For the purposes of the Act, cannabis is defined as any plant of the genus cannabis or any part of a plant of the genus cannabis with the exception of the mature stalk or fibre produced from it, and seed, when separated from the plant [10].

The possession, supply and production of cannabis (unless under licence for fibre production) are all prohibited. Legally, therefore, all forms are referred to simply as cannabis and are controlled.

The supply and possession of cannabis seeds are not controlled in the United Kingdom, although legislation to that end is being considered for the future. The cultivation of cannabis, however, unless under licence for fibre hemp production, is controlled. A maximum of 0.3 per cent THC is allowed for fibre-producing plants in line with recommendations of the European Community [11, 12].

As long as there has been no direct incitement to produce, i.e. to cultivate, a controlled drug, no offence has been committed. While books, magazines and pamphlets containing detailed growing information referring openly and directly to cannabis cultivation are freely available, there has been only one successful prosecution in the United Kingdom for the writing of such a book.

Similarly, unless it can be shown that equipment has been provided with the knowledge that it was intended for use in the cultivation of cannabis, there has been no offence. Much of that equipment is used legitimately in horticulture to produce crops other than cannabis.

It derives from the above definition that legally all forms of cannabis are referred to simply as cannabis and are controlled. Simply using the term cannabis in a legal context could be misleading, however, as it may in fact refer to any of the following: poor quality leaf; leaf from a more potent strain of cannabis; compressed, seeded imported material; good quality mature flowering tops; or superior quality flowering tops.

It is necessary for the court to have an indication of the quality of cannabis so that its value and abuse potential can be considered at sentencing or when determining fines.

Typical THC values encountered, as described in the text, are as follows:

Material THC content
Leaf 1-5
Imported cannabis 3-5
Sinsemilla (outdoor, indoor) 3-23
Cannabis resin 1-10

Although it may be possible to visually assess the likely quality of confiscated cannabis, the only way to accurately determine the quality is by laboratory analysis of its THC level.

Growers, when caught, will often claim that cultivation is for their own use or so-called "social supply" as opposed to being a commercial enterprise. Medicinal use is also used increasingly as an explanation for cannabis abuse. Such instances are generally on a scale of around 100 or more plants. Proof of the intent to supply cannabis to others in such cases depends on the number of plants, quality of cannabis produced and the sophistication of cultivation. In such instances, investigating officers may require a scientist to visit the site to assess the degree and sophistication of the cultivation and select plants and other items for examination and analysis. It may also be necessary to consider any cannabis abuse patterns of the grower. Large-scale production may involve warehouses or farm buildings with thousands of plants. Relying heavily on automation, such production requires a significant financial investment. Although it is often all too apparent that a production site is commercial, it may be difficult to show an individual's involvement.

Evidence in court

Statements of evidence as well as description of the cannabis factory and relevant findings usually give some opinion of the potential yield of useable cannabis from the plants growing there and of their quality or THC content. Questions posed in court often relate to the estimated yield, e.g. whether it is a true estimate, whether it is an amount acceptable for personal use, taking into account the habit of the grower or whether such a yield is more likely to be supplied for commercial gain.

Questions regarding the quality and value of the cannabis are also important. The cost of the equipment used and the degree of sophistication of the laboratory is significant, as the grower is often unemployed. Similarly, technical questions such as whether the plants have been grown from cuttings or seeds may also be important, especially if the plants have not yet flowered, since 50 per cent of plants grown from seed may be male.

Potential impact on cannabis abuse

According to a survey published in 1981, the content of cannabis cigarettes varies widely, depending on the quality of the material [13]. On average, however, a cigarette contains approximately 0.2 grams of good quality cannabis. More recent analysis of cigarettes shows that even though the potency of the majority of cannabis available has increased significantly, the amount used in individual cigarettes does not appear to be significantly different. On the contrary, it appears that the plentiful supply of high quality material for many growers leads to increased frequency of abuse. Many abusers claim to take cannabis on a daily basis, and it is believed that regular heavy users might smoke five cigarettes per day [14]. It has also been reported that some regular smokers of cannabis might smoke between 2 and 8 grams a day [16].

Future prospects

Dronabinol, a synthetic form of THC that is currently exempt from certain controls in the United Kingdom, is prescribed to treat certain medical conditions. Many people, having become aware of such uses, claim to use large quantities of cannabis for medicinal purposes. While that argument may be used as mitigation, it is not a legitimate reason for abuse or cultivation.

It has been suggested that the control of cannabis should be extended to the separated seed under legislation in the United Kingdom. This may not have a significant effect on cultivation, however, as the established method of propagation currently uses cuttings rather than seed, and many growers have become self-sufficient in production. It is also believed that some cultivators specialize in supplying cuttings. It appears that indoor cannabis cultivation in the United Kingdom is now well established and the high level of interest on the subject is reflected by the vast range of printed literature available as well as information accessible on the Internet. The wealth of information attests to an interest in any horticultural developments that can be applied to cannabis cultivation, and the large investments in such ventures are an indication of the potential profits.


 1. E. Pitts, P. J. O'Neil and K. F. Loggo, "Variation in the THC content of illicitly imported Cannabis products 1984-1989", Journal of Pharmacy and Pharmacology, vol. 42 (1990), pp. 817-870.

 2. P. B. Baker, T. A. Gough and B. J. Taylor, "Illicitly imported Cannabis products: some physical and chemical features indicative of their origin", Bulletin on Narcotics, vol. 32, No. 2 (1980), pp. 31-40.

 3. P. B. Baker, K. R. Bagon and T. A. Gough, "Variation in the THC content of illicitly imported Cannabis products", Bulletin on Narcotics, vol. 32, No. 4 (1980), pp. 47-54.

 4. P. B. Baker and others, "Variation in the THC content in illicitly imported Cannabis products. part II", Bulletin on Narcotics, vol. 34, Nos. 3 and 4 (1982), pp. 101-108.

 5. P. B. Baker, T. A. Gough and B. J. Taylor, "The physical and chemical features of Cannabis plants grown in the United Kingdom of Great Britain and Northern Ireland from seeds of known origin", Bulletin on Narcotics, vol. 34, No. 1 (1982), pp. 27-36.

 6. Ibid., "The physical and chemical features of Cannabis plants grown in the United Kingdom of Great Britain and Northern Ireland from seeds of known origin. part II", Bulletin on Narcotics, vol. 35, No. 1 (1983), pp. 51-62.

 7. B. J. Taylor, J. D. Neal and T. A. Gough, "The physical and chemical features of Cannabis plants grown in the United Kingdom of Great Britain and Northern Ireland from seeds of known origin. part III: third and fourth generation studies", Bulletin on Narcotics, vol. 37, No. 4 (1985), pp. 75-81.

 8. Data collected by the Forensic Science Service, London (unpublished).

 9. J. W. Fairbairn and J. A. Liebmann, "The cannabinoid content of Cannabis sativa L. grown in England", Journal of Pharmacy and Pharmacology, vol. 26 (1974), pp. 413-415.

10. United Kingdom of Great Britain and Northern Ireland, Criminal Law Act 1977, sect. 52.

11. U. Avico, R. Pacifici and P. Zuccaro, "Variations of tetrahydrocannabinol content in cannabis plants to distinguish the fibre-type from drug-type plants", Bulletin on Narcotics, vol. 37, No. 4 (1985), pp. 61-66.

12. Council Regulation (EEC), No. 2059/84, 16 July 1984.

13. I. J. Humphreys and J. R. Joyce, "A survey of the cannabis content of unsmoked reefer cigarettes", Journal of the Forensic Science Society, vol. 22 (1982), pp. 291-292.

14. Institute for the Study of Drug Dependence, Drug Misuse in Britain (1996).

15. J. W. Fairbairn and J. A. Liebmann, "The extraction and estimation of the cannabinoids in Cannabis sativa L. and its products", Journal of Pharmacy and Pharmacology, vol. 25 (1973, p.150.

16. J. W. Fairbairn and other, "Cannabinoid content of some English reefers", Nature, vol. 249 (1974), pp. 276-278.

* The authors gratefully acknowledge the Strategic and Specialist Intelligence Branch of the National Criminal Intelligence Service, Spring Gardens, 2 Citadel Place, London, for supplying data on hydroponic cannabis laboratories for the preparation of the present paper.

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