I want to grow my cannabis outdoors, I really do. I see tantalizing videos and photos online of huge outdoor marijuana plants in Northern California, plants that look like trees, each yielding two or more pounds of dry-weight buds, and I drool with desire.
I realize enviously that those growers didn’t pay for grow lights, electricity, dehumidifiers, fans, light stands, light movers, deep water culture systems, Mylar, and all the other things you need for an indoor grow room.
But where I live, it’s just not worth it to grow outdoors. Grown outside, my plants become a buffet for aphids, thrips, mealybugs, whiteflies, fungus gnats, gray mold and powdery mildew. I’ve tried every foliar spray, every systemic plant protector. Nothing worked well enough to prevent severe crop losses.
And then there was that constant daily fear of knowing my plants were visible from the air or to anyone who trespassed on my property.
These problems are some of the main reasons why most growers opt to use an indoor grow room. When I made the firm decision that an indoor garden was the best option for me, I set out to create the perfect space inside my house.
So, let’s take a look at the factors one must consider before choosing how to create and run an indoor grow op.
Grow-Room Size, Space, Lights & Heat
Most of us who grow cannabis indoors utilize a spare room for our inside gardens. Bedrooms in most apartments and condos are between 100–160 square feet, while bedrooms in most detached houses are at least 160 square feet.
A basic starting place for any indoor grow room is your choice of grow lights. A 1,000-watt professional HID grow light provides adequate average coverage for 16–20 square feet of floor space. You can place 6–12 full-size photoperiod cannabis plants in that space and harvest an average of 1.5 pounds per light every four months if you’re growing from seed, and a couple of weeks faster if you’re growing from clones or using autoflowering cannabis.
If you’re a skilled grower using high-yielding cannabis genetics and a feed program made specifically for cannabis, you could harvest three-plus pounds per 1,000-watt light, especially if you’re using a double-ended high intensity discharge (HID) grow light.
Aside from which lights to choose, another important factor to consider is how much weed you want to grow. Your answer will help you determine how many square feet of grow space you’ll require, and what your lighting options will be as a consequence.
When working out floor space, always factor in an additional 12–15 square feet for each 1,000-watt light. This add-on space is needed to accommodate the light stand and freestanding grow-room equipment, and to give you room to maneuver as you inspect, trim, water and tend to your plants.
In my grow-room planning, I allot 32 square feet of total floor space for every thousand watts of grow lights. If you’re thinking of a smaller grow light (250, 400 and 600 watts are popular grow-light configurations), you simply adjust your math calculations accordingly. For example, allot 18 square feet of floor space for a 600-watt grow light.
Doing the math involves knowing how much bud you want and how big your accessible room size is, but that’s just the beginning of your grow-room planning.
Check out the below YouTube clip from Monster Gardens about indoor garden setup:
Climate Control, Venting, Aerating & Exhausting Your Grow Op
If you want to harvest substantial amounts of bud and be able to experience the full joy of an indoor grow op, you’ll be using professional HID lights or LED grow lights, not fluorescents or low-watt lighting. Professional grow lights generate a lot of heat in an enclosed space, and you have to find a way to get rid of that heat.
You could choose to run a grow room that doesn’t have its own dedicated air conditioner and exhaust fan system. But if you make that choice, it will be much more difficult to have fully effective temperature and humidity control, especially if you’re running much more than a 400-watt professional HID grow light.
Even one 600-watt HID bulb in a bedroom grow room can generate enough heat to make the grow room too hot for plants (especially in warmer months), unless the heat is removed via ducting or mitigated via air conditioning.
That’s why it’s best to go through the hassle and expense of installing a room-mounted ductless mini-split air conditioner, as well as a ventilation fan and duct, and to do that during the grow-room build-out, if possible.
Some growers opt for air-cooled grow lights, which involves exhaust fans, ducting, and holes in the wall. But just know that when your grow room has its own dedicated air conditioner, your plants are a lot healthier and more productive, and you do a lot less work to keep them happy.
Calculating Air Conditioner Load For Your Cannabis Grow
If you determine a mini-split air conditioner is what’s needed for your grow room, then you must calculate your climate control needs and select adequate air-conditioner capacity.
To calculate how much air-conditioning capacity you’ll need for your grow room, first ascertain the total cubic feet of the room (not just square feet of floor space), and the type of grow-op appliances you’ll have in the space.
I recommend using the online BTU calculator on the Sunlight Supply website. This tool gives you the option of inputting the total wattage or number of grow lights, dehumidifiers, hydroponics ballasts, and general grow gear, as well as the total room size. Then it automatically calculates how many BTUs of cooling power your air conditioner should have.
This calculation will reveal a climate-control limiting factor that restricts how many grow lights you should put into a room. The limiting factor involves how much cooling capacity you need versus how much you can install.
For example, a 204-square-foot grow op with a 1,000-watt grow light, digital ballast, dehumidifier, filtration scrubber and exhaust fan requires approximately 14,000 BTUs of cooling power.
The Ideal Air range of mini-split air conditioners are made specifically for grow rooms, and they also offer an online BTU calculator. Ideal Air sells a 12,000 BTU model for $1,500, a 24,000 BTU for $2,700, and a 36,000 BTU for $3,100. If you went with the lowest-priced model, you’d have to rely on your whole-house split unit (or exhaust fan venting) to make up the 2000 BTU deficit.
I always opt for a 36,000 BTU model. It’s far better to have excess capacity than not enough. If you try to rely on your whole-house air conditioner to do all or part of your grow-room climate control, you create other problems. But even with a 36,000 BTU model, you’d still only have enough cooling capacity to run two 1,000-watt lights in that room, unless you had vented grow lights to remove the heat.
Efficiently getting rid of grow-light heat is especially difficult if your grow room has no dedicated air conditioner; you have to leave the grow-room door open and/or use exhaust fans in an attempt to manage that grow-room heat. What that means is compromised security, a grow room that’s not sealed, a vector for pests and diseases, and potentially higher electricity costs. All this spells more work for you.
Cannabis Construction Project Vs. Grow Tent
You have another big choice to make when deciding how to create your indoor grow op: Should you build a grow room inside an existing space in your home, or use a grow tent?
I’ve seen handy DIY growers take an existing bedroom, basement, garage, trailer, even a railroad car, and in a few weekends convert it into a marijuana grow room, without hiring contractors. They build a room within a room, adding interior walls and electrical outlets to create a controllable grow space.
To begin with, these growers have skills in framing, carpentry, drywall, insulation, electrician tasks, HVAC, plumbing, door hanging and general construction. I admire their self-sufficient acumen, because building a grow room yourself means that nobody else has to know about it.
Problem is, building a grow room involves installing new electrical wiring, adding electrical panel capacity, and doing other activities that, if done wrong or without adherence to building code, can lead to safety issues, structural failures, or bureaucratic hassles from building inspectors.
If you don’t have a mastery of DIY construction skills, you have to call in the building professionals. Be aware that requesting extra air conditioning capacity, a vent hole, and high-capacity electrical circuits will cause some licensed contractors to figure out what you’re planning to do, which creates additional security risk.
Check out the below YouTube clip from the Grateful Grower, who discusses what he’s learned from his years spent setting up his own grow rooms:
The Grow Tent Option
Instead of framing a room within a room — and conducting full-on grow room planning, design and build-outs — some marijuana growers prefer to use dedicated grow tents.
The best quality grow tents offer light-tight enclosures, highly reflective interiors, built-in ports and hangers, fast setup, portability, and convenience. They range in size from just big enough for a 250-watt grow light, to four-feet deep, eight-feet wide, and 8–9 feet high. The price for the biggest grow tent can be as high as $950.
A grow tent eliminates the need for framing new walls, installing reflective material, and figuring out what to hang your lights from. But the grow tent is going to be in a room, and that room should have a dedicated mini-split air conditioner (unless you’re using 600 watts or less of grow-light power). If you’re running a couple of professional grow lights along with fans and other hardware, the room the grow tent sits in needs a 30-amp circuit and outlet box.
These realities force some growers to go with a small grow tent lit by a 250-, 400-, or 600-watt light, including the option of using fluorescent or low-power LED grow lights.
For climate control in those low-watt situations, the grower uses a small mini-split or relies on whole-house air conditioning. The grower using HID grow lights of 600 watts or less might yield a pound every harvest. If the grower has opted for non-HID bulbs and is using T5 fluorescent or LED, the yield will be significantly less than that.
Before You Make a Grow-Room Build-Out Decision…
Decisions about building a grow room, how many grow lights to use, and whether to convert an existing room into an indoor garden space or use a grow tent involve complex, interrelated calculations and considerations only you can make.
Here’s a brief list of guidelines for the marijuana grower contemplating a grow-room build-out.
How Much Money Do You Want To Spend On Your Cannabis Grow?
How much total money and time can you invest in lights, vent fan, air conditioning, construction supplies, and other materials to build a grow room that provides the yield you seek? Do you have the money and time for this project?
Here’s a case study to help you with that analysis.
Building a 210-square-foot grow room with a mini-split air conditioner, vent fan and duct, carbon filtration, 30-amp electrical load and circuit panel changes, two professional 1,000-watt HID grow lights, and reflective wall coverings can cost at least $4,500 — and that’s if you do the labor yourself.
Note: This figure doesn’t include cost of a hydroponics system, light stands, fans, pumps, hydroponics nutrients, and other grow supplies — it’s just the cost of obtaining the basics of an indoor grow space and grow lights.
On the other hand, if you run a small grow tent with a 250- or 400-watt grow light, you might get away with using your whole-house air conditioning and avoid the big costs of building a full-size grow room, so you spend fewer than a thousand dollars.
The salient difference is that the full-size grow room can produce 3–6 pounds of dried and cured bud every crop cycle, but a 250-, 400- or 600-watt HID grow tent will produce much less than that every crop cycle.
What Are The Risks And Rewards Of Building Your Own Grow Room?
Only you can determine the balance of risks and rewards of hiring honest, skilled, licensed contractors to do your grow-room build-out, or doing it yourself. Of course, choosing the former will ensure quality workmanship and materials (which is especially necessary when it comes to electrical wiring and air-conditioner installation). As you contemplate DIY, know ahead of time that you’ll need many hours and a set of construction skills for the project.
How Will Building A Grow Room Affect The Structure And Value Of Your Home?
If you build a grow room instead of using a grow tent, how will that affect the structural integrity and market value of your dwelling? Most of the time, it kind of wrecks the room you’re modifying, and you have to dismantle the grow op before selling the house. If you rent a dwelling and start putting up new walls, cutting holes in old ones, and fooling around with electrical panelling and wiring, your landlord might tag you for seriously damaging the property, make you pay for it, and evict you.
How Much Cured And Dried Cannabis Do You Want To Produce?
Analyze how much cannabis you personally consume. If you provide medicine to others, tally that up, too. If you only need to grow a couple of pounds per year, then you only need a grow tent and a 400-watt light, along with cannabis-specific hydroponics nutrients and all the other gear that goes with cannabis cultivation.
On the other hand, if you want to harvest a couple of pounds or more every 3–4 months, you need a professional grow room with at least 1,000 watts of grow lights. Your grow room does best with its own air conditioner, and an enhanced electrical panel and outlet box. It will cost you more to create this larger grow room, but you’ll grow more weed.
Building a professional cannabis grow room is a major project that will require a big monetary investment, and requires skill and time. If it’s done right, you have a customized enclosed space that can provide a near-perfect environment for your plants, and can be used long term. Plus, you’ll more than recoup the money you spend, with the build swiftly paying for itself in increased yields, grower security, and ease of growing.