Lights & Timing: Lumen Output and Your Grow
Lumen Output
As you may be aware, when you’re lighting your grow there are several options available for you. From compact fluorescent lights (CFLs) to high intensity discharge (HID) lights such as the metal halide (MH) or high pressure sodium (HPS) and even LED setups, you have a lot of room for choice when it comes to your indoor marijuana growing. Some bulbs have significantly different lumen output.
Whatever light, or combination of lights, you choose to use, it’s good to remember that standard incandescent light bulbs (the kind you use around the house) are absolutely awful for use as grow lights. Only slightly better than incandescent bulbs are mercury vapor lights, the kind frequently used for the floodlight over your driveway at night.
Since you’ll most likely be choosing between CFLs or HID lights like the MH and HPS, those are the two options that we’ll examine here. As a general rule of thumb, HID lights are far superior for plant growth but not everyone can afford them. Here are some of the other pros and cons of HID lights vs. CFLs:
High Intensity Discharge Lights
- produce far more lumens per bulb
- also produce far more heat; you absolutely will need cooling with these lights
- light bulbs (the MH and HPS) can be expensive to replace ($60. – $100.+)
- on the other hand, HID lights last a long time, too
- provide excellent canopy penetration
- cost more to operate
Compact Fluorescent Lights
- are much cheaper than HID bulbs (at least for small grows)
- produce far less heat per bulb (don’t be fooled, though, you’ll get heat if you set up a large grow with these)
- provide poorer canopy penetration
- cost less to operate
- grow time takes longer
- ideal for use in micro-grows or for use on a sea-of-green (SoG)
Various grow lights
By now you’ve also heard the term lumens, and you may be wondering what all the fuss is about, as well as how to determine the lumen output of your lights. Lumens are the measurement for the amount of usable light your lighting system produces. Basically, you can see lumens as the amount of energy your lights provide for your plants, so the more lumens you have the better.
While the exact lumen output will vary from bulb to bulb, and from manufacturer to manufacturer, here are some general guidelines for establishing the lumen output of your light:
- an incandescent light (common light bulbs) only produces 10 – 17 lumens per watt
- a mercury vapor (or flood light) will put out 45 – 50 lumens per watt
- CFLs can put out anywhere from 50 – 70 lumens per watt
- metal halide lights will produce 75 – 100 lumens per watt
- high pressure sodium lights put out 85 – 150 lumens per watt
As a particularly light-loving plant, marijuana flourishes best when it receives a minimum of 3,000 lumens per square foot. Of course, marijuana plants love it even more if you have 6,000 lumens per square foot, and if you want to fully emulate nature consider this: on a bright day the sun can put out 10,000 lumens per square foot or more.
So to decide how much light you need, you’ll need to first establish the square footage of your grow space. Here are a few basic grow room sizes you might be dealing with, and their corresponding total light needs:
- 2’ x 2’ = 4 sq. ft.; 12,000 – 36,000 lumens
- 4’ x 3’ = 12 sq. ft.; 36,000 – 100,000 lumens
- 4’ x 4’ = 16 sq. ft.; 48,000 – 140,000 lumens
- 4’ x 5’ = 20 sq. ft.; 60,000 – 180,000 lumens
- 5’ x 5’ = 25 sq. ft.; 75,000 – 220,000 lumens
When you’re establishing the light lumen needs of your grow space, remember to consider the distance your light will be from your plants. The further your lights are from the plants, the less canopy penetration you will have and the more your plants will grow tall in an effort to reach the light; as a general rule of thumb, you don’t want tall, spindly plants, you want little, bushy plants.
Ultimately the light source you choose will be highly dependent on your budget and your intended grow space. Most seasoned growers would opt for the HID lights such as the MH and the HPS, but there’s a world of variety out there and if you are limited by grow space or budget, CFLs can be a viable alternative to more expensive, higher heat-producing HID lights.
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