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Lighting Cannabis Plants

Lighting cannabis

Lighting your cannabis plants is one of the most crucial aspects of growing cannabis.

Light is weight

White light promotes healthy growth, yellow light is important for the bloom phase. Fluorescent lights can be used as a source of white light, while for yellow light there are the High Pressure Sodium (HPS) lamps. Using Fluorescents during bloom though is not recommended; it works (with the right colour code) but the yields are lower. So for growth: sodium or fluorescent, and for bloom: sodium.

How many hours of light does a plant need?

  • 18 hours light per 24 hours during the grow phase.
  • 12 hours light per 24 hours in de bloom phase.

NB!: these periods must be consecutive!

How long should clones be kept in the growth phase?

The most usual is 1 week on 18 hours of light, this can be shorter if you want your plants to be smaller, but a minimum of 3 days (the plant needs to ‘settle’). Longer can also be fine, then the plant will be bigger and fuller, but you do need to have the space for them (example is a mother plant, which stays under 18 hour light).

 How many lumen (light strength) does an HPS give off

  • 250 Watt -> 31000 Lumen, effective surface area 0.5 m2
  • 400 Watt -> 55000 Lumen, effective surface area 0.7 m2
  • 600 Watt -> 98000 Lumen, effective surface area 1.2 m2

Distance between lamp and plant

  • 250Watt: minimum 20cm
  • 400 Watt: minimum 40 cm
  • 600 Watt: minimum 50 cm
  • 1000 Watt: minimum 90 cm
  • Fluor. between 5-10 cm.

If you use extra cooling (air cooled hoods, water cooled lamps, extra pc-fans under your bulb) then you can hang them lower.

Can you grow under fluorescents?

Yes you can, but best only in the growth, not bloom period. Light colours 11 (daylight), 21 (cool white), 31, 33 (cool white), 82 (warm white, bloom) are the most used.

A few more codes and valuations

33          Neutral                          Fair

82(7)      Extra warm white            Good

83(0)      Warm wit                       Good

84(0)      Cool white                      Good

865         Daylight                         Very Good

93(0)      Warm white                     Very Good

94(0)      Cool white                       Very Good

95(0)      Daylight                           Very Good

96(5)      Daylight                           Very Good

There are fluorescent bulbs in various sizes

  • 59cm  => 18W (ca. 1300-1600 lumen)
  • 90cm  => 30W (ca. 2500 lumen)
  • 120cm => 36W (ca. 3200 lumen)
  • 150cm => 58W (ca. 5200 lumen)

(combinations 1:2 warm / cool white seems to work better) With fluorescents advised distance from plant 5, max 10 cm. In other words, as close as possible, it can even not do any harm if the leaf tips touch the bulb, since it is not hot enough to burn them.

An Example

Using a lamp is on 18 hours per day, for 31 days.

  • Lamp = 600 watt = 0.6 kWh
  • Cost price per kWh = let’s say 0.20€, though this can vary. Check on your bill. Also day and night tariffs vary (if you have a D/N meter)
  • 0.6 (how many kWh) x 18 (number of hours per dag) = 10.8
  • 10.8 x 31 (number of days) = 334.8
  • 334.8 x 0.20 (cost price per kWh) = € 66.96 use

You will pay € 66.96 per month. Your lamp is on during the day for 18 hours and the cost price is 0.20 € per kWh. This is just an example, the price per kWh varies (and at night you pay a lot less).


If you are considering starting up, I suggest you also think about investing in simple two auto 20 A fuses and laying a direct cable between the counter and the grower’s corner.

Then take a type XVB 5G2, 5mm² cable and use blue and brown for one group of plus sockets and grey and black for supplying another group. No risk of socket mistakes and over-heating of old,  , worn out or heavily overloaded wires. No worry about unexpected tripping of the contacts and putting your cycle under stress. No hassle with extension leads and domino effects.

Buy two double building boxes from your local hardware store (Niko Hydro 55) and four plug sockets, for a lover and enjoyer certainly enough to supply your “tanner” and ventilator through a single 400 with enough juice.

There is often voltage loss due to poor insulation resulting in lower final voltage in existing old plug sockets, warming of the mains itself, and rapid wear at the end appliance can also be connected.

For techies and detail freaks who increase the resistance of the conduit, stray currents moving through insulation to wet places can soon lead to nominal voltage losses of 10 to 15 Volts, which in terms of percentage is not so serious, but if we then apply the laws of electricity, since voltage and current are inversely proportional to each other via resistance. In normal language, with the same load (resistance), the decrease of the voltage leads to an increase in the current, and that’s not good...


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