How To Plan Indoor Lighting

How To Plan A Small Grow Room

Planning a small grow room is a challenging and fun project.  Whether you are growing vegetables in the winter months, flowers in December, or Cannabis for CBD oil (where legal and licensed), a little planning up front provides significant benefit during your growing seasons.

COB LED Cannabis grow light full spectrum high intensity Cree

Having a guard cat is optional for a grow room.

Not all grow rooms need be the same.  Some growers may just be interested in growing several plants to maturity, while other growers may be developing mother plants and are planning a cloning system to supply plants for an outdoor garden.

However, the basic starting point questions are the same for all:

  • How much light do my plants need at each growth stage?
  • What are the physical dimensions of my plants?
  • How much work space do I need in addition to the grow area?

We will walk through the planning steps in this article by answering these questions in more detail.

What type of plants?

This question is important for both the amount of light and for the ultimate spacing requirements.

For example, growing greens (lettuce, spinach, etc.) takes up space in width and length, but does not grow very tall.  On the other hand, tomatoes may grow up to four feet high and bush out to a diameter of three feet.  Cannabis cultivars grown for CBD oil will require about three to four feet in height (depending on trimming practices) and may require lateral spacing of two to three feet between stalks. (Again, depending on trimming).

COB LED CREE grow light horticulture indoor high intensity

Plan your grow room for your type of plants. While many rooms will be fixed in width and length due to the availability of space, the vertical dimension will be important relative to the installation height of the indoor grow light.

What are the specific lighting requirements?

First, a brief science lesson!

Light may be measured in a number of ways, all of which are scientifically accurate.  But while some measurements are accurate, they may not be important for your plants!

Wattage is accurate but not important to the plants. 

It is important to the grower because wattage is the measurement of electricity used to light the lamp.

It is not a measurement of the light produced. 

For example, a 32 watt fluorescent bulb and a 40 watt incandescent bulb consume about the same amount of electricity in an hour, but the amount and type of light produced is very different. This chart is a comparison of regular, everyday light bulbs.  As you can see, it takes 100 watts of electricity to produce 1,600 lumens in an incandescent bulb.  It only takes 22 watts of electricity to product exactly the same brightness of light using an LED.  Using watts to compare grow lights is apples and oranges!

So how do you measure the brightness of the light?  The unit of measurement is either candlepower or lumens.  Both describe the brightness or intensity of light at the source of the light.  A lamp that produces 1,600 lumens is a bright light, but only as measured at the light source.

The plant is not right next to the lamp. 

Often, the lamp is suspended 36 inches from the tops of the plants. This means that we need to measure the amount of light that actually falls on the plant.  Typical measurements for this include LUX (Lumens per square meter) and foot candle meters squared. Again, this is measured from the starting point of lumens, not watts.

It turns out that there is yet another problem with using lumens for plant lighting.  The light range used to measure lumens corresponds to the range of light that is most important to humans.  This range, 500 to 650 nanometers wavelength or dark green to medium red, is not the same as the optimal range of light for plants.  The optimal range for plants is from around 400 nanomenters (deep purple) to 700 nanometers (far red).  This range is called Photosynthetic Active Radiation (PAR).  PAR describes the quality (color requirements) of the light, not the quantity of light that actually shines on the plant.

The next question is how much light does a plant actually receive?  Although it sounds light a term from the movie “Back to the Future”, this measurement is called PPFD or Photosynthetic Photon Flux Density.

This is the measurement of the amount of PAR light that falls on a specific area. The scientific measure for this is micromoles per square meter per second.

Cree full spectrum light PAR PPFD high intensity grow light

This is the spectrum that is produced by the Broomstraw Farm Chip On Board LED grow lights.  This light is very close to natural sunlight across the PAR spectrum.

OK, now that we have the science out of the way, how do we use this information in the best way?  The lighting manufacturer should provide a PPFD value for the light as measured over a square meter at a specified distance from the light. If that is not available, then a measurement of Lumens or LUX at a known distance can be approximately converted to PPFD. 

Use this conversion calculator to calculate Lumens to PFF or LUX to PPFD. 

As a grower, you should determine the amount of PAR light required for your plants. 

There are many sources on the Web for this information.  Some sources recommend around 150 Micromoles / sq meter / second for clones, 350 micromoles / sq meter / second for vegetative stage, and 500 to 600 micromoles / sq meter / second for the flower stage.

For example, the Broomstraw Farm Grow Light BF3590.6 produces 312 micromoles per square meter per second at a distance of 59 inches from the light.

A typical CBD oil producing cannabis plant requires 380 micromoles/meter squared/second.  The BF3590.6 hung at a distance of 45 to 50 inches from the top of the plant will produce sufficient light to grow cannabis plants in a 54 square foot area, around 7 feet by 7 feet square.

In the flower state, the same plants would require 600 micromoles/meter squared / second, or a BF3590.6 hung at 18 inches above the canopy. This would cover around 27 square feet, or approximately 5 feet by 5 feet.

Rule of thumb:  Spectrum is critical and light must be measured at the plant, not at the light!  And, light diminishes inversely with the square of the distance between the light source and the point of use.

How many plants?

This is often dependent on available space.  Estimate the area required by one plant at maturity (or at the stage of terminal growth) to see how many plants will fit into your future grow room. Typically, the available space will be the limiting factor.

If you have the luxury of a large space, then design your grow area for the number of plants.

Will you grow to flower or grow to clone?

This is important in cannabis production - growing only in the vegetative growth state will require 300 to 600 micromoles/meter squared / second (PPFD), but growing to flower and full maturity will require 600 micromoles/meter squared / second (PPFD). Flowering or fruiting for most plants requires more light than the seedling or clone stage. 

Often, clones in the first two or three weeks require much less intense light than they need sometimes only three weeks later in the process.

Having an intensity control on the light (usually not practical for fluorescent or High Intensity Discharge (HID) makes managing the transition from clone to vegetative state to maturity much easier for the small grower.

For example, by using a variable length hanging fixture and the built-in dimmer switch, a grower could raise six Cannabis CBD plants in 14 square feet using the BF3590.4 from clones under the reduced intensity allowed by the dimmer switch to mature plants in bud stage at full light intensity.

Save room for a work area

There are many gardening tasks that will be required in the grow room: planting, trimming, cloning, watering, fertilizing, and harvesting are just a few.  Be sure to allow sufficient room to work.  Make sure that you check your light source for UV and Infrared radiation.  Some grow lights will require eye protection (sunglasses). The Cree Chip On Board LED used in the Broomstraw Farm Grow Lights is rated as RG-1 Low Risk in Cree Technical Article CLD-AP34 REV 23, Page 9.

Example Grow Rooms:

Bill has a small closet in his house that he thinks is perfect for a grow room.  It is three feet wide and three feet deep.  9 square feet or about 1 square meter can easily be lighted by a single BF3590.2 or a single BF3590.4 depending on the PPFD requirements of the plants. This closet is located close to a bathroom in Bill’s house, so a water supply and clean-up area is near.

Fred, on the other hand, is able to use part of his laundry room.  In this house, the builder included a large walk-in closet in this area.  This closet is 5 feet deep and eight feet wide. Fred will use the BF3590.6 to grow 12 cannabis plants to maturity on one side of the closet, and will use the BF3590.2 to light a cloning area on the other side.  He will be able to include a small table in the middle for a work station.


  • Understand your goals and desires. Know your plants!
  • Make sure that you design a lighting system that is plant-friendly using proven scientific measurements. Humans can use “watts” as shorthand to buy incandescent light bulbs designed by Edison in 1879. In the 21st Century, we know that plants need light that meets their PAR requirements, and light that is sufficiently intense to drive growth to maturity as measured by PPFD and NOT by watts.
  • Remember that the advice “Uncle Charlie” is giving you may be based on what worked in the good old days. Don’t fall for the claim that the new light is just as good as an old High Intensity Discharge Light that consumes 400 watts.  There is no reason to replicate ‘bad’ light with new technology.
  • Be sure to include work areas.

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