There are many reasons to clone your outdoor marijuana plants, but nearly all of them involve wanting to continue a special plant from a special strain. The other big reason, which amounts to the same thing, is for seed production. After a successful harvest a grower might fall in love with a particular strain and want to always have some more of it in successive harvests.
Though at first blush it sounds difficult, and maybe even a little bit unethical, rest assured it's easier than you think and as old as cultivation itself. Many people have cloned already: by taking cuttings from non-marijuana plants, letting them root, and reporting them. The principle is the same with marijuana, but there are a few extra steps you may want to try to ensure success, since this is not just an ordinary houseplant.
Though almost any mature marijuana plant can be cloned, it is probably best to choose a younger one that is about two to three months old. lf it is unavoidable, and you have to use an older plant, then the process doesn't change, but be more attentive to the needs of the plant, especially its watering needs, in the first few weeks. Before you clip a branch from the parent marijuana plant, heavily water the plant and check the PH of the soil. Know what is within good range for your climate and soil type. If necessary, you may need to make adjustments before proceeding. Heavily water the plant for about three days.
When clipping the branch, try to choose one of the larger ones near the bottom with good, full leaves. Cut at an angle, as you would flowers before placing them in a vase. As soon as you've cut the branch place it in some lukewarm water. After transporting the cutting back to your nursery (or greenhouse) prepare it for rooting in small peat pots or rooting cubes, which are available at any plant store. They will also offer soilless mixtures for rooting cuttings. These are usually gel mixtures or liquids that are diluted in mineral water or other not-tap water. Some marijuana growers prefer rolling the branch in a rooting powder or gel. Each method has its adherents and detractors. Decide for yourself over time.
Keep the marijuana cuttings moist by periodically spraying them with water and place them in an enclosure that allows airflow but keeps them from getting too dry. One idea a cannabis grower in San Francisco uses is a tent made out of resealable freezer bags. This allows him to leave a small portion open to allow air to circulate throughout the day; easy access for the multiple water mistings the clones receive per day; and a tent to trap the humidity inside. At the same timed expose the marijuana plants to near constant tight- much more than 12 hours a day - unless you are cloning for sex. In a few weeks, the clones should develop roots. Replant carefully.
Cloning For Sex
When cloning for females however, there is one extra step you should take and that is to sex the little cuttings. Instead of exposing the marijuana plants to near constant light you have to actually convince them that they are flowering by depriving them of light. The cuttings must also be mature enough to flower and thus taken from marijuana plants that have about three to four reeks vegetative growth in them which is distinguished by calyx development. For about two weeks make sure the marijuana plants receive complete darkness for at least 12 hours straight. If you make a mistake and some light, even the smallest amount gets to them, then you can't count the days leading up to that and you have to start over.
If you've done your job correctly, after two weeds tiny blossoms will have appeared in the node region of your cuttings. Keep flowering them and eventually it will be clear which are female and which are male. Transplant the females and get rid of the males unless you want them for seeding or something. Your marijuana plants will return to the vegetative state in a little less than a week provided that they receive at least 13 hours of sunlight but more light is better. If this does not work then bathe them in light for 24 hours for at least 2 days before letting them re-sync with the seasonal light pattern.