Light and Dark
Plant leaves capture light energy and turn it into the sugars that power their growth. This process is called photosynthesis. Up to a point the more light available to a plant the more it can grow. Darkness is also important for plants however. During dark periods plants transfer excess energy they have made in their leaves down into their roots. This promotes better nutrient uptake which in turn allows for better photosynthesis. With plants such as cannabis darkness also promotes the creation of flowering hormones. Giving cannabis the right amount of light at the right time is therefore the key to successful growing and flowering.

Photoperiod
The photoperiod is the number of light hours verses dark hours in one day or 24 hour period. Cannabis uses the photoperiod to tell it when to grow and when to flower. In nature the photoperiod is controlled by the seasons. The long summer days promote growth in cannabis but as the autumn draws in the days shorten. The increased daily darkness gradually triggers them into flowering ready to reproduce for the next year. When growing indoors you control the photoperiod so you can change from growing to flowering whenever you like.

Photoperiods Indoors
Most cannabis plants will grow happily when receiving around 18 hours of light a day. If you want them to stop growing and start flowering instead however the hours of light need to be reduced to around 12 hours a day. You don`t need to do this gradually, just switch from 18 hours to 12 when you are ready to flower. Once the light cycle has been changed it will take about 2 or 3 weeks for for plants to turn completely from growing to flowering. During this time you should gradually see a slowing in vertical growth and the appearance of more and more flowers as flowering hormones build up.

Photoperiods Outdoors
Even though the seasons control the photoperiod outdoors you can force outdoor plants to flower early by covering them or placing them in a light free place for 12 hours a day. Depending on the variety grown and the time of year you may only have to do this for a month or so. If day length is decreasing and the plants are well into flower a few hours extra light each day will not usually be enough to revert them back to vegetative growth.

The Colour of Light
Daylight is made up of all the colours of the rainbow. Some of these colours are more important to cannabis plants than others. Blue light helps with growing and leaf production while red light promotes flower formation. Most artificial lamps can`t provide a balanced spectrum and are usually 'strong' in one particular colour only or are very inefficient to run. The choice for indoor growers is to either balance out the spectrum with different types of lamps, or choose a lamp with a spectrum that favours the type of growth they want to encourage the most.

Types of Artificial Lighting
There are really only two choices of lamp types for indoor growers, high intensity discharge lamps (HID lamps) and florescent lamps. Other lamps such as standard household bulbs or halogen spotlights are just not up to the job of growing cannabis. They convert most of the power they consume into heat not light or produce a light spectrum that won`t support good plant growth.

Lumens and Watts
Lumens are a measure of light intensity. The more lumens per watt the more light a lamp produces.

Lamp	Lumens per Watt (approx.)
High Pressure Sodium (HPS)	130
Metal Halide (MH)	100
Fluorescent (Standard)	83
Mercury-vapor	50
Quartz-halogen	21
Incandescent (Regular Bulb)	15

HID Lamps
There are two basic types used for growing, both available in various wattage's. High pressure sodium (HPS) and Metal Halide (MH). HPS lamps are strong in the red spectrum of light and are good for flowering. MH are strong in the blue spectrum of light and are good for growth. Although they favour different growth stages you can use either sort for growing or flowering cannabis and still get good results. If you only want one lamp however choose a HPS, they give more light per watt at a wave length more usable by light hungry flowering plants. Growers using more lights for flowering often use 2 HPS to 1 MH to provide a better balanced spectrum of light, many others just use HPS and are more than happy with the results.

Fluorescent Lamps
Fluorescent lamps are fine for growing, seedlings or cuttings but flowering plants really need more intense HID light. If fluorescents are your only choice for flowering choose alternate rows of "Cool White" (blue) and "Warm White" (red) bulbs placing them as close together as possible. As fluorescent light is weak aim to keep plants short and check them daily to make sure the lights are only a few cm`s away from the tops. You should get reasonable results but don`t expect monster buds.

Distance from Plants to Lamps
Unlike florescent lamps placing HID lamps too near to plants can cause burning of the tops and even running or stretching of the buds. However, the closer the lamp to the plants the more intense the light they receive. (Light intensity from lamps drops by 1/4 every foot it travels). In general keep 400 watt lamps 20 - 45 cm above plants and 1000 watt lamps 35 - 75 cm above plants. As a rough guide put the back of your hand under the lamps at the same height as the plant tops. If you can hold it there comfortably for 30 seconds they should be OK. Light Distance Calculator

How Much Light
When using HID or fluorescent lamps indoors plan on giving 30 to 60 watts of light for each square foot of growing space. At a minimum outdoor or window grown plants will require at least 4 hours of direct midday sun to grow well. More is of course better. Window or greenhouse growers can always supplement poor light conditions with lamps if practical. The lamps don`t have to be on all day and as they only supplement the light they can be more thinly spread.
Light Area Calculator

Lamp Reflectors (Shades)
High intensity lights also come with different reflectors. All combinations work OK but a well designed reflector can increase available light by up to 30%. Basic principles are horizontal reflectors will direct more of a bulbs light at the plants than a standard vertical fitting, and a hammered, matt or dull finish will reflect light more evenly than a mirrored or gloss finish.

Light Movers and Balancers
These motorised movers draw very little power (5-15 watts) yet increase a lamps coverage buy about 1/3. They benefit overall growth by lighting the plants from all sides equally, also because the lamps are moving they can be placed nearer to plants without fear of burning them.

Darkness Means Darkness
When flowering cannabis the dark periods for your plants must be total and undisturbed. Even short flashes of light will stress them and slow or stop their flowering response. Remember it`s the length and quality of the darkness makes your plants flower. Make sure your grow space is light tight by getting in it and turning off the lights, wait for a few minutes for your eyes to adjust and then check for light leaks. Don`t be tempted to peak at your plants when they should be left in the dark. In an emergency one disturbance won`t do that much but continued breaks in the dark cycle will.

Timers and Contactors
A timer switch for lamps is needed to ensure regular light cycles. However due to their high start up charge HID lamps will often burn out standard timers. Contactors or relay switches are used to counter this. These are really just high powered switches that lamps are plugged into. Timers are still used but now they turn on the current carrying contactor not the lamp.

Bulbs
Bulbs need to be handled carefully, dirt or greasy finger marks on a bulb will damage it when it heats up. Use a bulbs packing sleeve to hold it or screw it into its fitting to keep it clean. Light output from a bulb drops as soon as its switched on. If you don`t have a light meter change your flowering bulbs every crop or two to be sure they are producing enough light.

Flowering and Strains
As a rule of thumb stick to 12 hours light, 12 hours dark when flowering but remember that sometimes this can be adjusted a little. Mainly indica varieties may flower happily getting only 10 or 11 hours darkness each day. Mainly sativa varieties on the other hand may sometimes require up to 13 or 14 hour darkness each day to flower satisfactorily.

Cost of Running Lights
It costs surprisingly little to power even a 400 watt HID lamp and it`s easy to work out the cost. Light Cost Calculator

Altering Light Cycle Times
When flowering the most important thing is the 12 hours of darkness that the plants are receiving every day. If you need to alter the on/off times of your grow room lights just add the extra hours to the end of a dark period and turn on at the new time instead.