It is legal for adults over 21 years of age to grow limited quantities of marijuana in states such as Colorado, Alaska, Washington DC, Oregon (after July 15, 2015) and Washington state (medical marijuana only).
It’s a complex task and one that requires step- by-step guidance if good quality is expected. One of the most important ingredients for a successful grow is good quality seeds. That said, here is an overview of the basics:
Soil is required with sufficient nutrients. Commercial potting soils usually indicate 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. The pH should be between 5.9 and 6.5. This pH can be adjusted and commercial fertilizers (even organic) tend to make the soil more acidic (ie decreasing its pH).
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) decrease THC potency and slow growth. At 13 °C the plant undergoes a mild shock, though some strains withstand frost temporarily.
Light can be natural for outdoor growing or artificial for indoors. 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 shown that, when subjected to a regimen of constant light without a dark period, most types of flora, including cannabis, will begin to show signs of decreased photosynthetic response, lack of vigor, and an overall decrease in vascular development. There is no defined flowering “stage”, unless you are discussing an auto-flowering variety (Cannabis Ruderalis). Typically, flowering is induced by changing the light schedule to 12 hours of light and 12 hours of 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 (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.
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. 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 are taken up from the soil by plants. 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.
*The above text is compliments of Wikipedia