Indoor Marijuana Lighting Guidelines for Heavier Yields

Growing indoor marijuana is still the preferred method for most growers because it offers security, protection from pests and diseases, and total control over environmental conditions.

Outdoors in the right geographic location at the right times of year, you get free sun, and intense direct sunlight is better for your marijuana plants than any indoor lighting could ever be.

Indoors, you have to use electricity to generate light energy.

Cannabis growers have the following generic lighting options: fluorescent lighting, metal halide, LED, plasma lighting, and high pressure sodium.

Of these, metal halide and high pressure sodium are two types of high intensity discharge (HID) lighting, and therefore in the same category.

People who write about cannabis lighting often confuse us with talk of light intensity, wavelength, and lumens.

I hope I don’t add to the confusion, but it’s good to talk about this briefly.

Light intensity is measured in lumens. Each type of lighting has different light intensity amounts per watt of power, and in different light spectrums.

High lumens doesn’t necessarily mean you get a high amount of the wavelengths that best power your marijuana plants.

How much intensity you get from a hydroponics grow light, and what spectrum the grow light gives you, are two very important considerations for your marijauna grow op.

By “spectrum” I mean the wavelengths of light produced. These wavelenghts often but not always correspond to the color your eyes see when the light is on.

Here are some generic descriptions of lighting choices and their intensity/lumens data:

A fluorescent bulb at 40 watts will put out 3000 lumens. The light is almost pure white, with very little heat generated.

A metal halide bulb at 400 watts puts out 36,000 lumens. The light has a lot of blue to it.

high pressure sodium bulb at 400 watts will put out 45,000 lumens. The light has a visibly orange color to it. The high pressure sodium bulb puts out about the same amount of heat as the metal halide. Heat from HID bulbs is a problem because it can make the grow room too hot, and because it can burn your cannabis plants if your plants are too close to the bulb.

Now let’s take a closer look…

Fluorescent lighting is only useful for growing cannabis clones and seedlings. Fluorescent lighting isn’t the optimal choice for later stages of marijuana growth.

Your clones/seedlings will thrive under fluorescent bulbs, but only for the first 2-3 weeks of growth, at the most.

If you leave young plants under fluorescent bulbs for too long, they stretch to get light, and this makes them too weak and too vertical.

A metal halide bulb is the way to go after your clones have taken root and until you flip them into flowering.

When your plant starts flowering, what you want is a light source with more orange to it. The high pressure sodium bulb is your preferred choice in bloom phase.

Some marijuana growers argue about whether to use high pressure sodium for the entire season. Some believe using HPS in grow phase instead of just in bloom phase shortens internodes and makes plants thicker and more dense.

I talked to the other professional marijuana growers on staff here, and we all agreed that the best thing to do is “ratio lighting.” Here’s the ticket:

In grow phase, use 70% MH and 30% HPS. Reverse that ratio in bloom phase.

An added benefit of using 30% MH in bloom phase is that MH generates UVB, which is said to spur resin production.

Regarding plasma lighting, the unit made by Chameleon Plasma has an ideal spectrum for grow and bloom phase is nearly twice as efficient at converting watts into the usable light spectrum called photosynthetically active radiation that your marijuana plants need.

Plasma lighting generates about half the heat of a similar-wattage HID unit, and the unit will last with strong light output for 25,000 hours.

We have another article you should read, one that’s about LED lighting.

We understand that HID lighting generates a lot of heat and sucks down a lot of watts, and that the LED, induction, and CFL manufacturers try to convince us that you can grow full-size plants with LED or CFL, all the way to harvest.

HID lighting costs you extra electricity because you have to cool your grow room, and if your marijuana plants are too close to the lights, they can get burned.

But unless you want to grow small autoflowering marijuana plants or “sea of green” clone gardens, LED, induction lighting, and CFL are not the indoor marijuana lighting that you need.

To get your best yields in marijuana gardens with dense, thick, light-hungry marijuana plants that are more than two feet tall, go for metal halide and high pressure sodium, and do what you have to do to reduce the heat so your grow room stays around 74 degrees Fahrenheit.

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