Higher Learning: Mastering Marijuana Grow Room Mathematics

I flunked math in the fifth grade and forever after feared numbers. Sure, we have technology to do simple calculations for us, but to make that technology work, we must first understand what we’re trying to calculate and then supply the raw data. Technology can’t do all our thinking for us, after all.

The cool and unexpected thing about growing cannabis is that it helped cure my math phobia somewhat, because analytic skills are essential to successfully growing weed. In many ways, mathematical reasoning is the basis for much of what we do in our grow ops. And I’m about to show you why…

The Mathematics Of Cannabis Grow Op Planning

As you plan your cannabis grow op, you must first perform a series of mathematical calculations and analyses. These will include:

  • How many square feet do you have for your grow space?
  • How much cannabis do you want to harvest?
  • And how often?

From these and other initial calculations, you must first create the blueprint for your grow-room structure and equipment purchases.

Following are some of the relatively straightforward math problems you must first before embarking on your cannabis growing season.

How Much Grow-Light Capacity Should You Install?

My simple baseline is to use 50 watts of high intensity discharge (metal halide and high-pressure sodium) lighting per square foot of garden space. And for professional LED grow lights, I want a baseline of 55 watts per square foot.

This lighting wattage baseline comes from watching how plants respond to varying grow-light intensities, measuring light drop-off and grow-light footprint, and assessing effects of grow lights on growth rate and harvest weight.

My mathematical calculation starts with having one 1000-watt HID per 20 square feet of grow space. This is just the initial calculation. When you get the grow light into your space and switch it on, you see the actual spatial footprint the light creates, and you might need to adjust your grow-light capacity based on what you see.

Depending on variables such as the reflective material you’re using in your grow op, your growing style, and your choice of grow lights and grow bulbs, you might use slightly less or more than 50 watts of HID light per square foot, but not by much.

How Much Coco Coir, Soilless Mix Or Soil Do You Need?

How much material do you need for your cannabis root zone? It somewhat depends on the material you’re using, but it mostly depends on how many plants you’re growing and what size containers you grow them in.

You could do the figuring out in your head, but it’s a lot easier using this online calculator provided by Smart Pots. In a cannabis grow op where you’re using a 1000-watt HID grow light and placing under that light four large photoperiod full-size plants in 10-gallon pots filled with soilless mix, the calculator will show you need 5.35 cubic feet of root-zone material. You could pick up a couple of 3.8 cubic foot bales of Pro-Mix soilless mix and have plenty left over for the next grow.

This grow-room calculation is best figured out early in your planning. Many a grower has gotten to transplanting time, only to realize they don’t have enough root-zone media. By doing the math before you embark on the grow, and as a consequence doing the shopping ahead of time, you’re unlikely to find yourself in the situation of having to correct errors along the way.

Calculating Bloom-Phase Plant Stretch

Anticipating bloom-phase plant stretch is a crucial math problem that requires the wisdom of experienced growing.

Your plants will approximately double in height after bloom phase starts. Sometimes they may more than double in height. You want to carefully research your strains before you start growing them, so you can determine plant stretch characteristics.

I’ve had indica, Kush and Afghanica plants that started bloom phase at 25 inches tall and ended this phase at only 40 inches tall. On the other hand, I’ve had Kali Mist, Sour Diesel and sativa-dominant strains start bloom phase at 22 inches tall before ending up at 55–61 inches tall.

Less experienced growers may ask why it’s so important to know how tall your crop will get. Well, cannabis plants grow best when they’re 2–3 feet away from professional grow lights. If you’re using double-ended bulbs, it could be as much as 3–4 feet.

If plants are too close to the light source, they can get burned and their buds will be thin. If they’re too far away, they grow poorly and produce low yields.

These calculations, along with knowing the ceiling height of your grow room and what kind of grow lights you’re using, will determine how tall you can let your plants grow in grow phase.

Here’s a sample scenario: A grow room has eight-feet-tall ceilings and seven-feet-tall light stands. The grower is using 1000-watt HID grow lights that need to be at least two feet above the plant canopy, and the grow light reflector is seven inches tall. Right there, the grower has already lost 31 inches out of the total possible 84 inches of vertical height, leaving only 53 inches of space.

With that intel, the grower knows the plants can be no taller than approximately 53 inches at full height in bloom phase. If the plants are taller than that, they’ll be too close to the grow lights.

The plants are housed in individual pots that are 11 inches tall. This decreases the total vertical height of the crop to 42 inches. To ensure your crop is fewer than 42 inches at its tallest, the grow-phase height of the plants should be no taller than 21 inches. That way, when they double in height during bloom phase, they’ll top out at 42 inches.

To summarize: The plants are expected to double in height during bloom phase, growing to a max of 42 inches. The pot height adds 11 inches, so the plants top out at 53 inches, leaving two feet between the plant canopy and the bottom of the grow-light reflector.

Note that this scenario assumes a doubled plant stretch. If you’re growing sativa that might stretch more than double its starting height, you may have to end grow phase when the plants are fewer than 20 inches tall.

If the grower is using LED grow lights, then the calculation will be different, because these lights can be closer to the plant canopy. I’ve run professional LEDs at 12–14 inches from the bloom-phase canopy instead of the 24 inches required for the HID grow lights, and the LED lights were only three inches tall. This gives at least a foot of extra plant height to work with, so the grow-phase plants will be allowed to grow taller than if the grower is using HID grow lights.

The Mathematics Of Cannabis Grow-Room Electricity

Simple addition skills, along with knowledge of electrical circuit-carrying capacity, can prevent a grow-room fire and other electricity-related dangers.

Your baseline analysis should include knowing how many amps your grow-room appliances draw and how many amps your electrical outlets are wired and paneled for.

A typical bedroom grow-room electrical circuit has 15-amp capacity and electricians recommend not running a circuit above 85 percent of its capacity, especially not with old wiring and for sustained lengths of time.

So, let’s crunch the numbers.

A 1000-watt HID grow light is a 9-amp draw on a 120-volt circuit. An exhaust fan is a 1-amp draw, as is a powerful aeration fan. A professional dehumidifier can draw 6–9 amps.

This is way more than a standard 15-amp circuit should handle. And yet, some cannabis growers will plug all those appliances and more into a power strip plugged into one 15-amp circuit, creating overload conditions that can lead to circuit breaker shutdowns, overheated wiring — and even fires.

Doing the grow-room electricity math before you start plugging things in is an essential safety action. I always install two 30-amp circuits and make accompanying electrical panel upgrades for any room where I intend to run a grow op.

I could go on and on about all the different ways mathematics helps you pre-plan and implement your grow-op protocols and procedures. Just being able to correctly interpret dosage instructions on hydroponics nutrients containers so you provide the right amounts of nutrients elements to your crop is a math exercise. When I’m delightfully stoned out of my mind, I find it particularly difficult trying to convert liters into gallons and vice versa, even though it’s a simple thing to do.

You can easily see that cannabis growing and mathematics go together like hand in glove. So even if you’re afraid of numbers like I used to be, I recommend embracing the benefits of utilizing mathematical analysis as you plan and run your next cannabis grow op.


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