Cannabis (botany)

Cannabis is an annual, dioecious, flowering herb. The leaves are palmately compound or digitate, with serrate leaflets.^[1] The first pair of leaves usually have a single leaflet, the number gradually increasing up to a maximum of about thirteen leaflets per leaf (usually seven or nine), depending on variety and growing conditions. At the top of a flowering plant, this number again diminishes to a single leaflet per leaf. The lower leaf pairs usually occur in an opposite leaf arrangement and the upper leaf pairs in an alternate arrangement on the main stem of a mature plant.

Leaves

The leaves have a peculiar and diagnostic venation pattern that enables persons poorly familiar with the plant to distinguish a cannabis leaf from unrelated species that have confusingly similar leaves (see illustration). As is common in serrated leaves, each serration has a central vein extending to its tip. However, the serration vein originates from lower down the central vein of the leaflet, typically opposite to the position of, not the first notch down, but the next notch. This means that on its way from the midrib of the leaflet to the point of the serration, the vein serving the tip of the serration passes close by the intervening notch. Sometimes the vein will actually pass tangent to the notch, but often it will pass by at a small distance, and when that happens a spur vein (occasionally a pair of such spur veins) branches off and joins the leaf margin at the deepest point of the notch. This venation pattern varies slightly among varieties, but in general it enables one to tell cannabis leaves from superficially similar leaves without difficulty and without special equipment. Tiny samples of cannabis plants also can be identified with precision by microscopic examination of leaf cells and similar features, but that requires special expertise and equipment.^[2]

Flowers

Cannabis normally has imperfect flowers, with staminate "male" and pistillate "female" flowers occurring on separate plants. It is not^[3], however, for individual plants to bear both male and female flowers.^[4] Although monoecious plants are often referred to as "hermaphrodites", true hermaphrodites (which are less common) bear staminate and pistillate structures on individual flowers, whereas monoecious plants bear male and female flowers at different locations on the same plant. Male flowers are normally borne on loose panicles, and female flowers are borne on racemes.^[5] 

The germination of cannabis seeds is triggered by darkness, moisture, and warmth. Thus in absence of the necessary conditions they may be stored for years and possibly decades, though this will decrease the germination rate. Germination can take from 12 hours to a week to occur, often slower the longer the seeds have remained dormant. When these conditions are met the seed shell is broken and a living plant emerges, as simply two round leaves and a single root tendril.

Growth medium

Soil is required, except for cannabis grown with hydroponics or aeroponics

• Sufficient nutrients—commercial potting soils usually indicate this as "N-P-K = x%-y%-z%" the percentages of the fundamental nutritional elements, i.e., nitrogen, phosphorus and potassium. Nutrients are often provided to the soil via fertilizers but such practice requires caution.
• pH between 5.9 and 6.5. This value can be adjusted – see soil pH. Commercial fertilizers (even organic) tend to make the soil more acidic (decrease its pH).

Warmth

The optimal day temperature range for cannabis is 24 to 30 °C (75 to 86 °F). Temperatures above 31 °C (88F) and below 15.5 °C (60F) seem to decrease THC potency and slow growth. At 13 °C the plant undergoes a mild shock, though some strains withstand frost temporarily.^[6][7][8]

Light

Light can be natural (outdoor growing) or artificial (indoor growing).

Under artificial light, the plant typically remains under a regime of 16–20 hours of light and 4–8 hours of darkness from the germination until flowering, although the plant can use a full 24 hours of light without harm. There is an ongoing debate over the importance of the "dark period". It has been suggested that, when subjected to a regimen of constant light without a dark period, cannabis begins to show signs of decreased photosynthetic response, lack of vigor, and an overall decrease in vascular development. Typically, flowering is induced by providing at least 12 hours per day of complete darkness. Flowering in cannabis is triggered by a hormonal reaction within the plant that is initiated by an increase in length of its dark cycle, i.e. the plant needs sufficient prolonged darkness for bract/bracteole ^[9] (flowering) to begin. Some Indica varieties require as little as 8 hours of dark to begin flowering, whereas some Sativa varieties require up to 13 hours.

Water

Watering frequency and amount is determined by many factors, including temperature and light, the age, size and stage of growth of the plant and the medium's ability to retain water. A conspicuous sign of water problems is the wilting of leaves.^[6] Too much water can kill young cannabis plants.

The additives in tap water can reduce plant growth and can be removed with reverse osmosis filtration but is expensive and produces poorer results than well or spring water.

Nutrients

Nutrients are taken up from the soil by roots. Nutrient soil amendments (fertilizers) are added when the soil nutrients are depleted. Fertilizers can be chemical or organic, liquid or powder, and usually contain a mixture of ingredients. Commercial fertilizers indicate the levels of NPK (nitrogen, phosphorus, and potassium). During the vegetative stage, cannabis needs more N than P and K, while during the flowering stage, P is more essential than N and K. The presence of secondary nutrients (calcium, magnesium, sulfur) is recommended. Micro nutrients (e.g. iron, boron, chlorine, manganese, copper, zinc, molybdenum) rarely manifest as deficiencies.

Because cannabis' nutrient needs vary widely depending on the variety, they are usually determined by trial and error and fertilizers are applied sparingly to avoid burning the plant.^[6]

Germination

Germination is the process in which the seeds sprout and the root emerges. In cannabis it takes from 12 hours to 8 days. Warmth, darkness and moisture initiate metabolic processes such as the activation of hormones that trigger the expansion of the embryo within the seed. Then the seed coat cracks open and a small embryonic root emerges and begins growing downward (because of gravitropism), if placed in a proper growing medium. Soon (after 2–4 days) the root is anchored and two circular embryonic leaves (cotyledons) emerge in search of light and the remains of the seed shell are pushed away. This marks the beginning of the seedling stage.

Germination is initiated by soaking seeds either between wet paper towels, in a cup of water at room temperature, in wet peat pellets, or directly in potting soil. Peat pellets are often used as a germinating medium because the saturated pellets with their seedlings can be planted directly into the intended growing medium with a minimum of shock to the plant.

Seedling phase

The seedling stage begins when the seed coat splits open and exposes the root and round “seed leaves” or cotyledons. It lasts from 1 to 4 weeks and is the period of greatest vulnerability in the life cycle of the plant, requiring moderate humidity levels, medium to high light intensity, and adequate but not excessive soil moisture. Most indoor growers use compact fluorescent or T5 fluorescent lights during this stage as they produce little heat. HPS and MH lights produce large amounts of radiant heat and increase the rate of transpiration in the plant which can quickly dry out seedlings with their small root systems.

The plant will naturally begin to develop identifiable sex characteristics in this stage after 4 to 6 weeks, but some growers will hasten this by switching to a 12/12 hour light period to induce flowering and, once sex is determined and the male plants are removed, returning the plants to the vegetation stage with 6 hour light period. However, forcing a plant to flower and then return to the vegetative stage adds significant extra growing time and has been shown to produce lower yields and reduce potencies by a factor of 2 to 3, so growers who wish to sex the plants as early as possible without these drawbacks take a clone of each seedling, root it, and force flowering in a separate growing area.

Vegetative phase

Duration: 1–2 months indoors. In this stage the plant needs a significant amount of light and nutrients, depending on the genetics of the particular plant. It continues to grow vertically and produce new leaves. The sex is starting to reveal itself, which is a sign that the next stage begins. Concurrently the root system expands downwards in search of more water and food. Some newly developed strains (auto flowering hybrids) omit the vegetative stage and pass directly from seedling to pre-flowering.

When the plant possesses seven sets of true leaves and the 8th is barely visible in the center of the growth tip, or shoot apical meristem (SAM), the plant has entered the vegetative phase of growth. During the vegetative phase, the plant directs its energy resources primarily to the growth of leaves, stems, and roots. A strong root system is required for strong floral development. A plant needs 1 or 2 months to mature before blooming. The plant is ready when it has revealed its sex. Plant size is a good indicator of sex. Females tend to be shorter and branchier due to their raceme type inflorescence than males, whose flowers grow in panicles. The males tend to have fewer leaves near the top and females have more leaves surrounding the flowers.^[10] The males are then usually culled when they are identified, so that the females will not be pollinated, thus producing "sin semilla" ("without seed") buds.

During the vegetative phase, cultivators generally employ an 18- to 24-hour photoperiod because the plants grow more quickly if they receive more light, although a warmer and cooler period are required for optimal health. Although no dark period is required, there is debate among cultivators as to whether a dark period is beneficial, and many continue to employ a dark period. Energy savings often support using a dark period, as plants undergo late day decline and therefore lighting during the late night hours is less effective.

The amount of time to grow a cannabis plant indoors in the vegetative stage depends on the size of the flower, the light used, the size of the space, and how many plants are intended to flower at once, and how big the strain gets in "the stretch" (i.e., the first two weeks of flowering).

Cannabis cultivators employ fertilizers high in N (nitrogen) and K (potassium) during the vegetative stage, as well as a complete micro nutrient fertilizer. The strength of the fertilizer is gradually increased as the plants grow and become more hardy.

The modification of a plant's growth habit is called training. Indoor cultivators employ many training techniques to encourage shorter plants and denser canopy growth. For example, unless the crop is too large to be extensively pruned, cultivators remove adventitious growth shoots, often called suckers, that are near the bottom of the plant and/or receive little light and will produce poor quality buds.

Many cultivators also employ other techniques:

Topping

Topping is the removal of the top of the apical meristem (dominant central stem), called the apex or terminal bud, to transfer apical dominance (the tendency for the apex to grow more rapidly than the rest of the plant) to the shoots emanating from the two nodes immediately beneath the pruning cut. This process can be repeated on one or both of the two new meristems, when they become apically dominant, with the same results. This process can actually be repeated nigh infinitely, but over-diffusion of apical dominance produces smaller, lower quality buds, so it is usually done no more than a few times. Topping also causes more rapid growth of all of the branches below the cut while the plant heals.

Pinching

Pinching (also called super cropping) is similar to topping in that it causes lower branches to grow more rapidly, but the apical meristem maintain apical dominance, which is especially useful if the plant has already been topped. Pinching is performed by firmly pinching the apical meristem(s) so as to substantially damage vascular and structural cells but without totally breaking the stem. This causes lower limbs to grow more rapidly while the pinched tissue heals, after which time the stem resumes apical dominance.

LST'ing

LST stands for Low Stress Training and is another form of supercropping, many times referred to as LST super-cropping. This technique involves bending and tying the plants branches to manipulate the plant into a more preferred growth shape. This method of training works very well for indoor growers who need to illuminate their plants using overhead lights. Since light intensity greatly diminishes with increased distance (Inverse-square law), LST'ing can be used to keep all growth tips (meristems) at the same distance from the light and can achieve optimal light exposure. LST can be used in conjunction with topping, since topping increases axial growth (side shoots). Topping is often done a few weeks before beginning LST'ing. The training works by changing the distribution of hormones—more specifically Auxins—in the plant. LST'ing resembles the training of grape vines into their support lattices. Outdoor gardeners also employ training techniques to keep their plants from becoming too vertical.

Pre-flowering phase

Also called the stretch, this takes one day to two weeks. Most plants spend 10–14 days in this period after switching the light cycle to 12 hours of darkness. Plant development increases dramatically, with the plant doubling or more in size. (See reproductive development below.) Production of more branches and nodes occurs during this stage, as the structure for flowering grows. The plant starts to develop bracts/bracteoles where the branches meet the stem (nodes). Pre-flowering indicates the plant is ready to flower.

Flowering phase

The flowering phase varies from about 6 to 22 weeks for pure indicas with their shorter flowering time than pure sativas. Mixed indica/sativa strains have an intermediate flowering time.The sex is clearly revealed in the first the flowering phase. Males produce little ball-like flowers clustered together like grapes called panicles. Most plants (except auto flowering strains that flower independently of photoperiod) begin to flower under diminishing light. In nature, cannabis plants sense the forthcoming winter as the Earth revolves about the Sun and daylight reduces in duration (see also season). If females are not pollinated (fertilized by male pollen) they start to produce buds that contain sticky white resin glands or trichomes in a final attempt for pollination by windborne male pollen. The trichomes produce resins that contain the largest amounts of THC and CBD, the two main psychoactive substances. Fertilized females continue to produce resinous trichomes but more plant energy is consumed by the production of seeds, which can be half the mass of a fertilized bract; thus, to maximize resin per gram, infertile cultivation is preferred.

Inflorescence that produce no seeds are called sin semilla (which translates to "without seeds" in Spanish, and is often misspelled as one word). Potent sin semilla is especially important to medical users, to minimize the amount of cannabis they must consume to be afforded relief. Cannabis with seeds is generally considered to be of inferior quality and/or grown with inferior technique.

Cannabis grown is induced into flowering by decreasing its photoperiod to at least 10 hours of darkness per day. In order to initiate a flowering response, the number of hours of darkness must exceed a critical point. Generally the more hours of darkness each day, the shorter the overall flowering period but the lower the yield. Conversely, the fewer hours of darkness each day, the longer the overall flowering period and the higher the yield. Traditionally, most growers change their plants lighting cycle to 12 hours on and 12 hours off since this works as a happy medium to which most strains respond well. This change in photoperiod mimics the plant's natural outdoor cycle, with up to 18 hours of light per day in the summer and down to less than 12 hours of light in fall and winter. Some 'semi-autoflowering' strains that have been bred exclusively for outdoor use, particularly in outdoor climates such as that of the UK, will start flowering with as much as 16–17 hours of light per day. Usually they can start flowering in July and finish far earlier than other strains, particularly those that haven't been bred as outdoor strains. Semi-autoflowering strains can be harvested before the weather in northern latitudes becomes very wet and cold (generally October), whereas other strains are just finishing flowering, and may suffer from botrytis (grey mold) caused by wet weather. Alternatively growers may artificially induce the flowering period during the warmer months by blacking out the plants for 12 hours a day i.e. by covering the plants with black plastic for example, which excludes all light during this period so the plant can flower even during long days.

Although the flowering hormone in most plants (including cannabis) is present during all phases of growth, it is inhibited by exposure to light. To induce flowering, the plant must be subject to at least 8 hours of darkness per day; this number is very strain-specific and most growers use 12 hours of darkness.

Flowers from certain plants (e.g. cannabis) are called bract/bracteole, and are (with cannabis) the most prized part of the plant. During the late period, the bract/bracteole are easily visible to the naked eye. Bract/bracteole development begins approximately 1–2 weeks after the photoperiod is reduced. In the first weeks of flowering a plant usually doubles in size and can triple. Bract/bracteole development ends around 5 weeks into flowering and is followed by a period of bract/bracteole “swelling”. During this time the buds greatly increase in weight and size.^[6]

Cannabis plants contain a number of different specific compounds at various ratios. Cannabis contains more than 460 compounds;^[11] at least 80 of these are cannabinoids,^[12][13]chemical compounds that interact with cannabinoid receptors in the brain.^[14]

The most psychoactive cannabinoid found in the cannabis plant is tetrahydrocannabinol (or delta-9-tetrahydrocannabinol), commonly known as THC. C. sativa and C. indica generally grow tall (some varieties reach 4 m) and their females start the production of flowers rich in tetrahydrocannabinol (THC) (up to 29% by weight) as the season changes from summer to autumn.

• Cannabis (Wikipedia)