by Robert Connell Clarke
Pruning
Pruning techniques are commonly used by Cannabis cultivators to limit the size of their plants and promote branching. Several techniques are available, and each has its advantages and drawbacks. The most common method is meristem pruning or stem tip removal. In this case the growing tip of the main stalk or a limb is removed at approximately the final length desired for the stalk or limb. Below the point of removal, the next pair of axial growing tips begins to elongate and form two new limbs. The growth energy of one stem is now divided into two, and the diffusion of growth energy results in a shorter plant which spreads horizontally.
Auxin produced in the tip meristem travels down the stem and inhibits branching. When the meristem is removed, the auxin is no longer produced and branching may proceed uninhibited. Plants that are normally very tall and stringy can be kept short and bushy by meristem pruning. Removing meristems also removes the newly formed tissues near the meristem that react to changing environmental stimuli and induce flowering. Pruning during the early part of the growth cycle will have little effect on flowering, but plants that are pruned late in life, supposedly to promote branching and floral growth, will often flower late or fail to flower at all. This happens because the meristemic tissue responsible for sensing change has been removed and the plant does not measure that it is the time of the year to flower. Plants will usually mature fastest if they a allowed to grow and develop without interference from pruning. If late maturation of Cannabis is desired, then extensive pruning may work to delay flowering. This is particularly applicable if a staminate plant from an early maturing strain is needed to pollinate a late-maturing pistillate plant. The staminate plant is kept immature until the pistillate plant is mature and ready to be pollinated. When the pistillate plant is receptive, the staminate plant is allowed to develop flowers and release pollen. Other techniques are available for limiting the size and shape of a developing Cannabis plant without removing meristematic tissues. Trellising is a common form of modification and is achieved in several ways. In many cases space is available only along a fence or garden row. Posts 1 to 2 meters (3 to 6 feet) long may be driven into the ground 1 to 3 meters (3 to 10 feet) apart and wires stretched between them at 30 to 45 centimeters (12 to 18 inches) intervals, much like a wire fence or grape trellis. Trellises are ideally oriented on an east-west axis for maxi mum sun exposure. Seedlings or pistillate clones are placed between the posts, and as they grow they are gradually bent and attached to the wire. The plant continues to grow upward at the stem tips, but the limbs are trained to grow horizontally. They are spaced evenly along the wires by hooking the upturned tips under the wire when they are 15 to 30 centimeters (6 to 12 inches) long. The plant grows and spreads for some distance, but it is never allowed to grow higher than the top row of wire. When the plant be gins to flower, the floral clusters are allowed to grow upward in a row from the wire where they receive maximum sun exposure. The floral clusters are supported by the wire above them, and they are resistant to weather damage. Many cultivators feel that trellised plants, with increased sun exposure and meristems intact, produce a higher yield than freestanding unpruned or pruned plants. Other growers feel that any interference with natural growth patterns limits the ultimate size and yield of the plant.
Another method of trellising is used when light exposure is especially crucial, as with artificial lighting systems. Plants are placed under a horizontal or slightly slanted flat sheet of 2 to 5 centimeters (1 to 2 inches) poultry netting which is suspended on a frame 30 to 60 centimeters (12 to 24 inches) from the soil surface perpendicular to the direc tion of incoming light or to the lowest path of the sun. The seedlings or clones begin to grow through the netting al most immediately, and the meristems are pushed back down under the netting, forcing them to grow horizontally outward. Limbs are trained so that the mature plant will cover the entire frame evenly. Once again, when the plant begins to flower, the floral clusters are allowed to grow upward through the wire as they reach for the light. This might prove to be a feasible commercial cultivation technique, since the flat beds of floral clusters could be mechanically harvested. Since no meristem tissues are removed, growth and maturation should proceed on schedule. This system also provides maximum light exposure for all the floral clusters, since they are growing from a plane perpendicular to the direction of light.
Sometimes limbs are also tied down, or crimped and bent to limit height and promote axial growth without meristem removal. This is a particularly useful technique for greenhouse cultivation, where plants often reach the roof or walls and burn or rot from the intense heat and condensation of water on the inside of the greenhouse. To prevent rotting and burning while leaving enough room for floral clusters to form, the limbs are bent at least 60 centimeters (24 inches) beneath the roof of the green house. Tying plants over allows more light to strike the plant, promoting axial growth. Crimping stems and bending them over results in more light exposure as well as inhibiting the flow of auxin down the stem from the tip. Once again, as with meristem removal, this promotes axial growth.
Limbing is another common method of pruning Cannabis plants. Many small limbs will usually grow from the bottom portions of the plant, and due to shading they remain small and fail to develop large floral clusters. If these atrophied lower limbs are removed, the plant can devote more of its floral energies to the top parts of the plant with the most sun exposure and the greatest chance of pollination. The question arises of whether removing entire limbs constitutes a shock to the growing plant, possibly limiting its ultimate size. It seems in this case that shock is minimized by removing entire limbs, including proportional amounts of stems, leaves, meristems, and flowers; this probably results in less metabolic imbalance than if only flowers, leaves, or meristems were removed. Also, the lower limbs are usually very small and seem of little significance in the metabolism of the total plant. In large plants, many limbs near the central stalk also become shaded and atrophied and these are also sometimes removed in an effort to increase the yield of large floral clusters on the sunny exterior margins.
Leafing is one of the most misunderstood techniques of Cannabis cultivation. In the mind of the cultivator, several reasons exist for removing leaves. Many feel that large shade leaves draw energy from the flowering plant, and therefore the flowering clusters will be smaller. It is felt that by removing the leaves, surplus energy will be available, and large floral clusters will be formed. Also, some feel that inhibitors of flowering, synthesized in the leaves during the long noninductive days of summer, may be stored in the older leaves that were formed during the noninductive photoperiod. Possibly, if these inhibitor-laden leaves are removed, the plant will proceed to flower, and maturation will be accelerated. Large leaves shade the inner portions of the plant, and small atrophied floral clusters may begin to develop if they receive more light.
In actuality, few if any of the theories behind leafing give any indication of validity. Indeed, leafing possibly serves to defeat its original purpose. Large leaves have a definite function in the growth and development of Cannabis. Large leaves serve as photosynthetic factories for the production of sugars and other necessary growth substances. They also create shade, but at the same time they are collecting valuable solar energy and producing foods that will be used during the floral development of the plant. Premature removal of leaves may cause stunting, because the potential for photosynthesis is reduced. As these leaves age and lose their ability to carry on photo synthesis they turn chloro tie (yellow) and fall to the ground. In humid areas care is taken to remove the yellow or brown leaves, because they might invite attack by fungus. During chlorosis the plant breaks down substances, such as chlorophylls, and translocates the molecular components to a new growing part of the plant, such as the flowers. Most Cannabis plants begin to lose their larger leaves when they enter the flowering stage, and this trend continues until senescence. It is more efficient for the plant to reuse the energy and various molecular components of existing chlorophyll than to synthesize new chlorophyll at the time of flowering. During flowering this energy is needed to form floral clusters and ripen seeds.
Removing large amounts of leaves may interfere with the metabolic balance of the plant. If this metabolic change occurs too late in the season it could interfere with floral development and delay maturation. If any floral inhibitors are removed, the intended effect of accelerating flowering will probably be counteracted by metabolic upset in the plant. Removal of shade leaves does facilitate more light reaching the center of the plant, but if there is not enough food energy produced in the leaves, the small internal floral clusters will probably not grow any larger. Leaf removal may also cause sex reversal resulting from a metabolic change.
If leaves must be removed, the petiole is cut so that at least an inch remains attached to the stalk. Weaknesses in the limb axis at the node result if the leaves are pulled off at the abscission layer while they are still green. Care is taken to see that the shriveling petiole does not invite fungus attack.
It should be remembered that, regardless of strain or environmental conditions, the plant strives to reproduce, and reproduction is favored by early maturation. This produces a situation where plants are trying to mature and reproduce as fast as possible. Although the purpose of leafing is to speed maturation, disturbing the natural progressive growth of a plant probably interferes with its rapid development.