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Grow Lighting

why is lighting important?

Light is vital for plants to grow and you will want to ensure you buy good quality grow lights. Having a good light source will allow your plants to grow well and develop quality buds, and the more light the better. The sun and natural light is obviously the best light and has the perfect spectrum of UV rays and light that plants require. 

However, if growing indoors, there are a variety of lighting equipment that will work just as well. You will need to consider the type of light used (colour spectrum), the strength of the light, the distance between your plants and the light source, as well as keeping to a schedule of when your lights are on or off. 

grow lights for cannabis

It is important that you use the correct lighting when growing cannabis indoors. It has to be a light that has been specially designed for growing cannabis, any random light just will not do. During the seedling and vegetative stage, cannabis plants prefer a blue, whitish type of light. This reflects the type of light that would be produced by the sun during spring and summer.

Fluorescent bulbs and metal halides (MH) lights work best for this purpose. During the flowering stage, your plants will need a more red/orange light, which reflects the type of light in the Autumn months. You can create this type of light by using high-pressure sodium (HPS) lighting. 

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strength of your lighting

Now that we have found the right light to grow cannabis, lets look at the intensity of that light. You will need to be careful not to use lights at that are too intense as this could cause light burn. On the other hand, using a light that is too weak will mean your plant will have to stretch up the reach the light or even not grow well as all. To avoid both these instances, we can measure the strength of light using two methods

The first method you can use to measure the strength of your lighting is by lux and this is the preferred way by many cannabis growers. Lux measures the strength of the light as it touches a surface. During the vegetative stage, the lux rating should be around 40,000lux to 70,000lux. Through the flowering stage this should change to 60,000lux to 85,000lux. Lumen is another way to measure intensity of lighting, by calculating the flow of the light (the more the lumen the brighter the light). 

lighting to plant distance

It is important to place your plants at a certain distance to the light to avoid the light being to intense or the plant not receiving enough light. Also remember that as your plant grows, you may need to adjust the distance between the top of your plant in the lighting source.

Distance between plant and light using HID lighting (MH and HPS):

150W (0.6m x 0.6m) area space: 20cm to 30cm

250W (0.6m x 0.6m) to (0.8m x 0.8m) area space: 25cm to 35cm

400W (0.9m x 0.9m) to (1m x 1m) area space: 30cm to 48cm

600W (1m x 1m) to (1.2m x 1.2m) area space: 35cm to 63cm

1000W (1.2m x 1.2m) to (1.5m x 1.5m) area space: 40cm to 79cm

Distance between plant and light using LED lighting:

240-400W: 41cm to 76cm

450-550W: 51cm to 76cm

600-850W: 61cm to 66cm

900+W: 66cm to 107cm 

watch our for light burn

Too much or to little light can cause stress to your cannabis plant. If the leaves are turning yellow, it could be a sign of light burn, i.e. They are receiving too much light. Another sign too much light is that the leaves will start pointing up and the edges may become crispy. If you spot any of these signs, act quickly and move the lighting up higher. 

cannabis light burn
Yellowing leaves could be a sign of light burn - (CC BY 2.0)

grow light schedule

Growing quality cannabis requires a light schedule to predict maximum growth and the best quality buds. During the first few week’s cannabis plants need a lot of light. Most growers will stick to a schedule of 18 hours of light, 6 hours of dark (18/6) or even a full 24 hours of light (24/0) during the seedling stage. If you want your plant to flower however, you will need to reduce the light once the vegetative stage is over.

Once the plant is receiving 12 hours of darkness each day, it is sort of ‘tricked’ into thinking that the cooler Autumn/Winter weather is arriving and this will indicate to the plant that it is time to start flowering. You will need to ensure that when it is dark, it is completely dark, as even the smallest amount of light creeping through can alter plant growth. 

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making the most of light

Using reflective surfaces in your grow space, such as reflective material or paint, can really help boost the amount of light your plants recover, even up to 30%! By increasing the light this way, you will ensure that there is less waste, and it will save you money. Grow tents will come with this reflective material inside but if creating your own grow space, there are materials that you can use.

You can paint the walls, using a dull white or a reflective paint. You could cover the space polyester film such as mylar or foylon. Foylon is slightly more durable than mylar and easier to clean but is more expensive. Many growers use mylar just fine though and this is still a great option. 

types of grow light

Types of grow light

HID (high-intensity discharge)

HID lights include MH and HPS lights and are a popular choice for both beginner and experience growers. HID lights are very efficient and your set up should include the light bulbs, a ballast (which connects the light with a normal light bulb plug) and a reflector hood (to divert light directly onto your plants).

You will probably want to buy both MH and HPS lights. MH lights emit a blue/white light perfect for the seedling and vegetative stage but when you reach the flowering stage you need to change to HPD lights, which emit a deep red glow.

It is possible to only use HPS lights although during the vegetative stage growth may not be as optimal. MH lights wont work during the flowering stage, so if you only want to buy one light then go for HPS. Also, if choosing both sets of light, remember to buy a ballast that adapts to both type of light.

LED Lights

LED lights have become increasing popular with indoor cannabis growers due to their durability and cheaper energy conversions (up to 40% energy consumption compared to HID lighting). They are however a LOT more expensive to buy in the first place. They tend to emit less heat, last longer and take up less space. 

Led_digital_visuals-3_grow_light
LED lighting – (CC BY-SA 3.0)

light costings

To measure how much your grow lights will cost you first need to find out how much your energy provider is charging per kilowatt hour (kW/h). You will likely find this on your electricity bill.

1 kilowatt = 1000 watts

Example: If you pay 25 cents per kWh, a 1000w bulb will cost 25 cents per hour. If using a 600w bulb, it will be 60% of that cost (15 cents per kWh), if using a 400w bulb it will use 40% of the same cost (10 cents per kWh) and so on. This calculation is for each bulb.

You can then work out how many hours your lights will be on and times that by the cost of electricity per kWh. Remember to add in the cost of the ballast wattage. If your bulb is 600w per hour and the ballast is 100w, then you will be using 700w per hour. 

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About the Author

Our deep love of plants and fascination with Cannabis has enabled over 25 years of successful small scale Marijuana cultivation from indoor hydroponics, greenhouses and outdoor growing set-ups.

As Cannabis laws around the world change, *we support the movement toward freedom of choice for responsible, consenting adults who wish to experience the joy and wonder of growing a Cannabis plant.

*All info is for entertainment purposes only.  We do not condone illegal growing of Cannabis.   Consult your state laws accordingly. 

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