Chapter 5 – Growing Top Shelf Buds

This is chapter 5 of a 14 part weekly series where we talk about grow room environmental control. Follow step by step to grow top shelf harvests like a pro. The information and instructions are highly scalable. These proven and well developed methods and technologies will work for anyone. Whether you want to grow some personal crops of buds at home or if you intend to fill a warehouse.

After plant genetics, this the most important area to pay attention to.

Cannabis grow room lightingWhile we cannot go so far to say good crop lighting is “easy”, we can say it is not hard when set up properly. There are good quality LED grow lights from out of the box. A well-tuned grow room environment does not come tidily packed in a carton. It must be built with some careful considerations that relate to your situation. If your grow room stays hot and dry, the best lighting and crop genetics in the world will never reach their full potential. Conversely, if the grow room air is too cool and humid, plant growth slows to a snail crawl and invites problems like root rot or bud rot.

The right growing environment with good lighting on superior plant genetics will turn crops of buds into yields of dreams.

What is the right growing environment?

This will depend on the cropping phase, just like nutrients, as discussed in crop fertilizers. In most general terms, cannabis thrives in a temperature range of 72 to 85 degrees Fahrenheit with a humidity range of 45 to 75%. Carbon Dioxide (CO2) from the growing atmosphere is an important “nutrient” for crop growth. CO2 is one of the main ingredients in the photosynthetic reaction plants have. Ambient natural CO2 levels typically range from 350 to 500 PPMs (parts per million). Plants will quickly use this up in a closed environment when growing, so it must be replenished.

Cannabis plants do not just breathe in, they also need to breathe out. This “exhale” is typically water vapor, oxygen, and other metabolites (like ethylene) that need to be managed. These need to be removed from the growing atmosphere just like CO2 needs to be replenished. If waste air builds up in a closed environment it can hurt plant growth.

Air Movement

Air movement in the plant canopy is vital, it helps keep grow room air mixed. For healthy growth plants should have even temperature and humidity levels surrounding them and to blow away the plants waste gases (breathing out) while replenishing CO2 (vital for growth). Air movement also strengthens stems and branches so they can support heavy weight buds.

So, let us look at the basic environmental conditions for each growth and bloom phase and then have a look at what it will take to achieve these values indoors. We will also talk about how your situation is relevant to the choices you will need to make to get the right gear to maintain a healthy growing environment.

Propagation: starting seeds or cuttings

Plants will like it warm and humid here. Warm temperatures keep metabolic rates high, stimulating early growth and development. Higher humidity levels help protect tender young plants from overworking themselves to the point of stress or even plant death in severe cases. Humidity domes with adjustable vents can help keep temperatures and humidity levels higher than the surrounding air when plants are small and starting out.

Temperature: 80 to 85 degrees Fahrenheit

Humidity: 75%+

CO2: 350 to 600 PPM

Air Movement: none to minimal

Early growth: Rooted cuttings and young seedlings

At this point plants will have small root systems and just a few leaves. Here we want to stimulate development by gradually increasing the demands on the plant. You want to slowly increase air movement, keeping temperatures steady and gradually decreasing humidity levels. It is also vital not to work plants too strongly too early. Think of yourself as a plant coach, you need to train your athletes, starting out gradually. Eventually they will be capable of great feats in terms of development, but it does not come overnight. Push too hard and you will damage your plants coming out of the gate. They may never fully recover and you are inviting problems like crops pests and diseases.

Temperature: 75 to 80 degrees Fahrenheit

Humidity: 65 to 70% humidity

CO2: 350 to 600 PPM

Air Movement: slow and gentle

Established Plants, Vegetative Growth

Here plants are anywhere from six to twelve inches tall, near as big as we will want to get them for the budding planting densities we discussed earlier, i.e., 5 to 9 plants per meter square for flowering.

Following this guide, plants will be in their final container size for at least three to four days after transplant before starting the bloom phase. It is important that some new roots are established after transplant before switching the crop to bud, otherwise there is too much going on for the plants to respond to if switching to 12/12 right at transplant.

At this phase air movement is further increased while temperatures remain steady with a slight decrease in humidity levels. Carbon Dioxide levels can be supplemented if you desire faster growth rates or opt to add CO2 in the budding phase (you need to train plants gradually for higher levels of CO2).

Temperature: 75 to 80 degrees Fahrenheit

Humidity: 55 to 65% humidity

CO2: 350 to 800 PPM

Air Movement: stronger (do not overdo it)

Flowering Plants, Early to Peak Budding

Plants are from twelve to eighteen inches tall and well branched at this point, in a planting density of 5 to 9 plants per square meter, following our efficient cropping model.

If we have carefully followed this guide, plants are healthy and free of problems; amped up pro athletes, ready to perform great feats in a short time span.

If we set up our environmental controls and equipment well, we can influence crop development from how we set the growing atmosphere. For example, if we want taller plants with greater spacing between flowering sites, we can widen the difference day and night temperatures. Conversely, if we want to pack buds tighter up and down. More compact plants we can keep tighter numbers between our dark and light cycles for temperature and humidity.

Temperature: 75 to 85 degrees Fahrenheit DAY, 70 to 75 degrees Fahrenheit NIGHT

Humidity: 55 to 60% humidity DAY, 45 to 55% humidity NIGHT

CO2: 350 to 1200 PPM

Air Movement: strong (do not overdo it)

Budding Plants, Mid to Flower to Ripening

Cannabis BudsHealthy well developed cannabis buds are large and dense. Air movement and the right humidity levels become sensitive issues in hydroponic gardens supporting rapidly expanding buds in high yielding densities. At this point plants are relying much less on the foliage and begin transferring stored nutrients and sugars into a final push where buds swell up, ooze resin, and gain considerable dry weight potential.

Running a gradually drier and cooler environment from late flowering through ripening to harvest makes for superior flavors, aromas, density, and weight. It makes for a difference you can weigh AND taste!

A dense cannabis bud, and even more so a garden full of them, holds a tremendous amount of water that is constantly evaporating from the bud and getting replenished by the hydroponic root system. This puts a lot of water vapor into the air that must be managed. You do not want to stifle water loss from live buds. This helps drive their development and is how water and nutrients travel from the roots to where they are needed in the plant. If you have been adding supplemental Carbon Dioxide levels in the growing environment, now is the time to cut. If CO2 is high during ripening, flavors and aromas are diminished. In extreme instances contributing to “hay” like aromas, i.e., flat tasting, bland buds with a hint of green in the bouquet.

Temperature: 70 to 80 degrees Fahrenheit DAY, 65 to 70 degrees Fahrenheit NIGHT

Humidity: 45 to 50% humidity DAY, 35 to 45% humidity NIGHT

CO2: 350 to 600 PPM

Air Movement: strong (do not overdo it)

What Environmental Conditions We Need to Monitor and Control in a Hydroponic Garden

  • Day/Night Temperatures
  • Day/Night Humidity Levels
  • Air Purification
  • Root or Reservoir Temperatures
  • Air Exchange (fresh air in, old air out)
  • Carbon Dioxide*
  • Air Movement

How to Monitor and Control Different Environmental Factors, Equipment & Considerations


Consistent and accurate temperature control can take some resources. We recommend LED grow lights because they help keep the growing environment easier to manage than hotter HID lighting like HPS (high pressure sodium).

Regardless, it is rare to never that a grow room does not require careful consideration and a respectable outlay for cooling equipment. This may also incorporate ventilation (air exchanges of fresh air to replace old air). Most used are Air Conditioning (ACs), exhaust and intake fans. Use a combination of both for best results. A “sealed grow room” or CEA (controlled environment ag) grow room will give you the best results and fewer potential problems. The higher startup cost being the tradeoff.

Fan Cooling Basics

For cooling with exhaust and intake fans only (no AC), the outside air will need to be at least 15 degree Fahrenheit cooler than your target air temperature. This is because of the heat added from high intensity grow lighting. That can be challenging or near impossible in summer months. However, if you have a large volume of air in your grow room and draw outside air from cool spots and you are not in the middle of a heat wave you might be able to get by without harvest quality or yields suffering too greatly. If you can pull it off, this will save you a LOT of power versus running air conditioning (AC).

For every 1kW (1000 watts) of lighting, you will typically need 800 CFM (cubic feet per minute) of exhaust fan output and around 400 CFM of intake fan performance.

Small to Mid-Scale Grow Rooms

Inline centrifugal fans are great for small to mid-scale grow rooms. They run forever, are quiet and perform well even with lots of ducting. Additionally, you can add more inline fans to your duct runs at various points. You can double or triple the CFM without adding more duct work. However, the more fans you add, the louder it gets. Try and avoid too many bends or turns in ductwork. This makes a really big difference in how much air can move via fans within a minute.

Most indoor grow rooms will run duct work that is from 6 to 14 inches in diameter. Insulated ducting can be a little quieter and avoids condensation or drip when drawing cold air through a warm building or room.

Of further benefit with centrifugal fans (versus shaded pole blower, furnace type fans) is that they work well on a fan speed controller. You can dial up or down both intake and exhaust rates to create the right amount of air exchange and cooling power. Specialized temperature controllers that automatically ramp up or slow down fan speed based on your set temperature are available and very economical. They tend to be quite reliable too. The TV-2 from GroZone controls is a good example, while there are many brands to choose from. Otherwise, a line voltage cooling thermostat will do the job to control cooling fans, but may not give you much, if any, control between day and night temperatures. This will not keep fresh air flowing at a lowered rate when grow room temperatures are below set cooling points.

Air Conditioning Basics

If you are serious about growing high yielding crops indoors year round, in most places you will want or need an air conditioning system. For every 1kW (1000 watts) of lighting you will need a minimum of 3500BTUs of cooling power. Since no appliance is 100% efficient, it is better to use 4500BTUs as your benchmark. It is better to have your cooling system run less often than it is to have it run full time and be inadequate.

ACs that are well suited for grow rooms usually are not cheap. A window type AC can be made to work, but they tend to send grow room smells outdoors. However, when combined with a good fan system for grow room cooling, it may be enough to get you through those extra hot few weeks of summertime in the case of smaller hobby sized growing endeavors.

Best Choice

In most cases a split unit AC or Heat Pump (cools and heats) is the best choice. They are typically rated for higher BTU cooling capacities and isolate grow room air from outside air. Good AC or Heat Pump Split units are energy efficient (SEER rating) and run very quietly indoors (air handling unit) and outdoors (connected heat exchanger). They can be purchased with precharged gas lines, making it a fairly DIY affair versus professional installation. One of the drawbacks with the precharged DIY lines is that that typically by law they can only be supplied half charged. Because of this you will not realize the full cooling potential.

An installation from a trusted professional, while more money, will give you maximum cooling power and added peace of mind. Make sure to buy a condensate return pump for your split AC system. The condensate captured (substantial volumes) from the AC system is very pure and clean water once the appliance has been used for a month. This is high quality water that can be stored and treated for reuse on crops. It also allows you to measure how much water your plants are transpiring, so it is a strong crop diagnostic tool too.

Make sure to get a good quality thermostat for your split AC. Preferred is one with a remote temperature sensing device and that allows different day and night temperatures.


Cannabis Growing Room GeneratorIn most instances a heater will not be needed because dehumidification equipment that runs often during the dark cycle to keep humidity optimal usually adds enough heat. However, if fan cooling only or using fans only for dehumidification, a heater may be needed for the dark cycle and even the light cycle in more rare instances. You do not want grow room temperatures to ever dip below 65 degree Fahrenheit during dark cycles or below 72 degrees Fahrenheit during light cycles. Make sure the heater you choose will not pose any hazards and is controllable by thermostat. Gas fired heaters are not recommended. They can produce waste gases that are toxic to plants and may be potentially dangerous to people and structures. Electric is clean, safe, and reliable for indoor cultivation purposes.

Root & Reservoir Temperatures

If reservoir or hydroponic nutrient solution temperatures exceed 75 degrees Fahrenheit for prolonged periods, you may require a reservoir chiller or cooling coils. If you set up your horticultural trays and reservoirs as we have recommended, it is unlikely you will need to cool the reservoir in most instances. During dark cycles in cooler climates, heating the nutrient solution to keep it above 65 degrees Fahrenheit may be necessary. This is easy to accomplish with a grounded submersible aquarium heater. 250 watts or greater is recommended for every 35 gallons of nutrient solution to be warmed up. Horticultural heat mats with thermostatic controls (Super Sprouter) can be placed underneath horticultural trays. This provides gentle, even, and accurate bottom heat for sensitive roots in climates that tend to stay cooler.

Humidity Control (Vapor Pressure Deficit Management)

Humidity levels play a critical role in how hard your plants work in the growing environment. Adequately established plants can bloom and ripen at lowered humidity levels. Plants and dense buds are full of moisture and plants possess large root systems to support and keep up with the demand. During propagation and vegetative growth, humidity levels may need to be elevated. Cooling systems and air movement can quickly dehydrate and stress plants with less developed foliage and root systems. Plants create their own humidity levels once big enough in each sized area. This is when dehumidification becomes necessary. During dark cycles when cooling equipment cycles infrequently this becomes important.

Even experienced growers often overlook or underestimate the value of good humidity control, both for increasing and decreasing Relative humidity levels in the growing environment. Do not make this mistake. Your grow room will only operate as best as your most limiting factor allows.


A misting system capable of delivering a fine mist or fogging system is recommended for humidification. Residential type humidification systems were not designed for keeping up with the intensive environments typical of a modern grow room. On the plus, using LED grow lights reduces the cooling load requirement compared to HID lighting, so the air tends to hold humidity a little more readily.

Always use pure water such as reclaimed condensate or RO filtered water in misting systems or foggers. If you do not things get plugged up and grow lights and other surfaces develop a mineral layer from water impurities. Be especially careful with water condensing any potentially contacting electrical equipment. Quality grow gear is CSA certified for use in damp locations.

Hydro Fogger

Cannabis Grow Room hydro foggerA Hydro fogger is reliable and works well for raising humidity. They can set you back upwards of 400-500 dollars but think of it as a vital piece of growing equipment, just like your grow lights.

Commercial high pressure misting systems are better for larger scale applications. Resourceful smaller scale growers can also be successful with smaller scale DIY misting systems.

Avoid spraying or fogging moisture directly onto plants wherever possible during humidification. If you use pure water as recommended, even if foliage gets wet under bright light it will not cause sunscald.

A humidistat (controls increasing humidity) will help you control the amount of humidity in the growing environment with a level of precision when strategically placed in the grow room; this is best to compare against a relative humidity monitor as some are more accurate than others.


The need to lower humidity levels is typically only required for well-established plants or occasionally during the dark cycle in earlier development. Dehumidifiers are easy to find and can often be found on sale. For an 8’ X 12’ grow room a 35 to 50 pint per day output capacity is usually ample. Avoid blowing the warm air from your dehumidifier directly onto foliage, or it may cause excessive dry spots that are more prone to insect or foliar disease issues.

A de-humidistat (controls lowering humidity levels) helps keep crop humidity levels from rising past optimal in the grow room. It is usually best to keep a buffer between your dehumidification and humidification set points, it is rare you need both on the ready (unplug the one that is not needed). Otherwise, they will try to beat each other, and both run constantly.

Carbon Dioxide CO2)

Carbon dioxide gas from the atmosphere is quickly depleted by fast growing hydroponic cannabis crops in a tightly closed growing environment. Without adequate CO2, growth slows or stops. Outside air is usually 350 to 500 PPMs in CO2 which is enough. In areas occupied by people, like office buildings, it can be over 1200 PPM! At a minimum, a slow bleed of fresh air come into and out of the growing area will provide enough CO2 to grow healthy crops (while getting rid of stale grow room

air). This is accomplished with the intake and exhaust fan system with ducting to and from outside or cooler adjoining areas.

Supplemental CO2

The next step is supplemental CO2. Typically adding extra CO2 is reserved for growers who have the rest of the grow room and hydroponic system dialed into perfection. In a finely tuned grow room, supplemental CO2 can shorten cropping time and add 25% harvest weight. Some people do not like the quality of CO2 supplemented buds as much as natural CO2 levels. This suggests that while bigger, they lack the density, aroma, and flavor of their natural CO2 level counterparts.

If you choose to supplement CO2, do the necessary research. We also recommend buying your supplemental CO2 system a complete package from a reputable supplier with experience. A poor quality CO2 system is a waste of money and can potentially cause harm to you or crops. Bottled food grade carbon dioxide is preferred to gas fired CO2 generators and monitors.

Gas fired CO2 generators are potential hazards as they use combustion. Additionally, if they do not fire right or get dirty, they can contaminate the air with ethylene (harmful to plants) or carbon monoxide (harmful or fatal to people).

Air Movement

Intake and exhaust fans add a level of air movement, as we discussed in the Grow Room Cooling section above. Additionally, the grow room will require a good air circulation system. Air movement serves several purposes.

Firstly, mechanical fans, oscillating fans and orbiting fans “stir” the air in the grow room. This keeps temperature, humidity, and carbon dioxide levels even from top to bottom of the grow room. Stratified air will create problems for crops and potentially lead to damage of grow room ceilings, walls, or other surfaces.

Secondly, air circulation in the grow room moves fresh air over the surface of plant foliage and removes stale and humidified air from around plant leaves. Plants breathe in and out through tiny openings in the leaves called stomata or stomates. CO2 goes in, water vapor and oxygen go out. The humidity level and amount of air movement around the leaf have a strong influence on driving this process. This in turn stimulates plants to take up more water and minerals from the roots to replace water vapor exiting the plant through the leaves.

Wall Mount Oscillating Fans

Cannabis Growing Room fansWall mount oscillating fans are fine for smaller grow rooms, usually one or two wall mounts per light is plenty. Quality varies with wall mounts; some do not last very long before the sweep motion quits. Stationary floor fans are always a good idea to help keep temperatures in the grow room even and to keep fresh air moving through dense planting canopies.

HAF (horizontal air flow) and V-Flow (vertical air flow) fans are an excellent choice for larger volumes of grow room air, especially where ceiling heights above twelve feet may occur. These are industrial grade fans and will have a long service life if used and maintained to the manufacturer’s directions. With industrial fans, you may spend more while needing fewer fans for the same sized area versus traditional wall mount oscillating fans.

Air Purification

Just like you, plants prefer to breathe fresh and clean air. Contaminants in the air like dust and spores can plug up the tiny openings on plant leaves or contribute to foliar diseases. You do not want anything dirty from the air sticking to your buds. You want the cleanest smoking buds possible. Air purification also reduces and removes odors from the air. It is fine for your plants to smell dank, but not the surrounding area.

Activated carbon filter properly sized for your grow room is a good place to start. Equip it with the correct sized inline centrifugal fan set to scrub and recirculate grow room air 24/7. If you keep the pre-filter clean and replace it as necessary, a good quality activated carbon can have a service life of many years. High humidity levels will diminish the ability for the filter to trap contaminates. Water vapors will fill the tiny pore spaces in the carbon bed that are intended to trap pollutants. A Can 50 filter with an 8 inch inline fan on a speed controller is typically adequate for an 8’ X 12’ grow room.

Professional Ozone and UV air cleaners may be used in larger facilities or very carefully by experienced smaller scale growers. Too much ozone (O3) or UV in an enclosed space will not only hurt plants but it can also harm or even be lethal to people and animals. It is critical that ozone or uv devices be sized correctly for the air volume and workload they are intended to treat. It is best that they are not able to operate when the area is to be occupied by people or pets.

Other Important Considerations for a Healthy and Easy to Manage Growing Environment


If the growing area is poorly insulated, not only will it be loud from the outside, but it will take more energy to keep a healthy grow environment as equipment may need to cycle more frequently.


Cannabis grow room seal cracksVolatile organic compounds are impurities that may off gas from things like plastics or chemicals. These chemicals can be in  paints or epoxies that can poison your plants from the air. These may be difficult to detect and can be a costly source of frustration, i.e., “why do my plants keep turning yellow with claw like foliage when everything in the growing environment reads perfect”? Not using epoxy floor coatings and avoiding PVC and Vinyl coverings is a good idea to help avoid potential problems.

Sealing cracks

Seal cracks around doors or framing in the grow room. When combined with negative air pressure cracks can suck all kinds of problems into your healthy growing environment, things like spider mites or powdery mildew spores. They may also create or allow light leaks to occur that can interrupt the plants critical dark period. The dark cycle should remain dark. Green LED work lights should be used when you absolutely must check or work on something in the grow room after the lights go out during the budding cycle.

Links to prior chapter and scheduled release dates for upcoming chapters