Marijuana cloning is popular usually with indoor growers or gardeners. The main reasons people will choose to clone marijuana is the ability to achieve an all female garden. If you clone an already existing female plant, then the clone will be guaranteed to be female itself. When starting from seeds, unless feminized marijjuana seeds are used, then the plant will either turn out female or male. The other main advantage of cloning is to speed up the whole growing process. Growers can start one set of clones ready for the next cycle while the current cycle is finishing.
The process of cloning marijuana involves taking a cutting of the original plant and inserting it into a piece of rockwool. The plant is then placed under a fluorescent light and within 3-5 weeks if successful, roots will be begin to emerge from the bottom of the rockwool. This is then transplanted into a larger pot. Growers will often use a rooting medium to increase the likelihood of the clone's success.
Below are a set of cloning pics.