The distance from the top of your cannabis plants to the bottom of your grow lights is one of the most crucial factors to control in your indoor marijuana garden.
You want your plants far enough away so they aren’t burned by the bulbs or the heat that comes off them, especially growing with high intensity discharge (HID), light-emitting ceramic, double-ended HID, or LED hydroponics indoor marijuana lighting.
If you’re growing with fluorescent (lighting we recommend only for clones, seedlings, or in the first couple of weeks of grow phase), you don’t worry much about heat or about too much light intensity.
With fluorescent indoor grow lighting, you worry if your plants are getting enough PAR (photosynthetically active radiation) and how many lumens of light are reaching the surface of the plants.
Some people buy expensive PAR and lumens meters to measure how much light and how much PAR light are hitting your canopy, but a much easier strategy for giving your plants enough light is to use approximately 46-52 watts of lighting power per square foot of garden space.
This is a generic wattage, and has to be adjusted depending on the kind of grow lights you use, how efficient your grow op climate control is, the growth phase and health of your marijuana plants, etc.
The advice in this article is for marijuana seedlings or clones that have been growing for at least three weeks and have established root systems.
If you put young clones or seedlings under HID or high-powered LED lighting too early, you drown them in light, resulting in stunting and nutrients deficiencies.
A 1000-watt HID light with a superior reflector and a high-end bulb such as Hortilux engineered for growing marijuana covers approximately 17-23 square feet of grow space.
You can use this formula with HID, LED, and light-emitting ceramic (LEC).
Efficient LEDs such as Lush Lighting hydroponics grow light equipment provide more PAR per watt than HID and LEC, so you could go to as low as 40 watts per square feet using a professional LED.
In that case, the Lush Lighting Dominator XL draws 600 watts of power but covers about 12-18 square feet.
If you’re using double-ended HID HPS lighting, a 1000-watt bulb delivers 22-30 square feet of coverage, especially if you have an effective reflector.
With double-ended HID lighting, you might use as little as 35 watts of lighting power per square foot, and still get fantastic results.
Because most high-quality HID HPS double-ended bulbs can’t be air-cooled, but are very intense in light output and heat, growers keep their plants 25-36 inches away to prevent burning.
If you want to maximize the yield per watt of your double-ended HID lighting, increase your indoor marijuana grow room air conditioning.
The cooler your room, the closer you can get your lights to your plants.
You must also adjust the distance from lights to plants as appropriate for the phase of growth your indoor marijuana plants are in.
Seed-grown marijuana plants and marijuana clones are harmed by too much light.
It slows their growth and can permanently stunt them.
Growers who give metal halide or powerful LED lighting to plants that haven’t been alive long enough to develop at least five sets of leaves and a sufficient root system are harming their plants and wasting their electricity.
That’s why many growers start their indoor marijuana clones or seedlings under a high-output T-5 fluorescent light such as Hortilux Power-Veg.
Using the Lush Dominator XL, start with the light 23-32 inches above new clones or seedlings.
After the clones or seedlings have 2-4 new leaf sets, you lower the lights 3-4 inches.
When the plants are at least one foot tall, with established roots and at least 8-9 leaf sets, you move the lights to within 18-24 inches of the plants.
As the plants get taller, the closer you can get the lights without creating a temperature of 80F or higher at the canopy level, the better.
The closer the lights are to the plants, the more light penetration there is, which translates into higher yield and faster growth.
But you don’t want your plants to get light-burned.
Measure the temperature at the canopy, and also do the “hand test.”
The hand test is when you hold the back of your hand at canopy level facing your light.
If the light feels too warm for your skin, warmer than the sun at mid-day in summer, it’s too warm for your plants.
You use trimming to easily keep your plants at the same canopy level.
When you have diverse indoor marijuana strains growing, which means one strain of plants grows taller or shorter than the other strains in the grow room, you may want to consider using different height for pots.
Or putting plants on bricks or other supports, to ensure that the top of all plants is the same distance from the bottom of the indoor grow lights.
There are obvious things you’ll realize as you make indoor marijuana lighting choices.
For example, if you have air-cooled lights and/or lights with a piece of glass isolating the bulb from open air at the bottom of a reflector, that makes a difference in how much heat and light intensity gets to your plant canopy directly.
Remember when your plants are about two weeks before flipping into flowering to trim away excess sucker branches and large leaves to increase light penetration and air movement through your indoor marijuana plants, and to ensure that all plants get even amounts of lighting from the start of their life until harvest day.