Choosing a grow tent dehumidifier is essential for maintaining proper VPD in your grow. Unless you live in the desert your grow will likely require a dehumidifier to deal with the massive amounts of water that plants transpire into their environment during late veg and flower. In the graph below you can see my SensorPush VPD reading before and after adding a de-humidifier to the room. With VPD that is out of range, as shown below on the left, you will eventually run into issues with mold, powdery mildew and stunted growth.
Overview
Why do you Need a Grow Room Dehumidifier?
Dehumidifiers are typically used in late veg and throughout flower once the plants are established and their leaves take up a large portion of the room or tent. Plants increase humidity in the air through a process called evapotranspiration. Water from the DWC reservoir (or soil) makes its way up through the roots of the plant, through the stems, and up to the leaves (transpiration), where it’s evaporated into the air through pores on the leaves, called stomata.
In order for transpiration to occur efficiently your plants require the vapor pressure deficit (VPD) to be within an acceptable range. For Cannabis I have found this range to be:
- Clones / Seedlings 0.6 – 0.9 kPa
- Vegetative Stage 0.9 – 1.2 kPa
- Flower 1.2 – 1.6 kPa
What Happens when VPD is Too High or Too Low?
When VPD is too high the plants will transpire too quickly. This will lead to dry leaves and nutrient burn. As a natural defence mechanism the plants will close stomatal pores, which will in turn slow growth and stunt the plants.
When VPD is too low the plants will struggle to transpire and risk of mold and mildew increases exponentially.
How Big of a Dehumidifier do you need for a Grow Tent/Room?
Humidifier ratings are typically expressed in pints. A dehumidifier’s pint capacity is the amount of moisture (in pints) that it is capable of removing from an area in a 24-hour period. Most humidifiers also give a square footage rating but this should not be used by growers who deal with plants that are constantly transpiring moisture. In my experience you should spec your dehumidifiers out as follows:
| Footprint | Humidifier Capacity |
| 3’x3′ – 4’x4′ | 20 – 35 pint |
| 5’x5′ – 8’x8′ | 30 – 50 pint |
| 9’x9’+ | 50 – 70+ pint |
If you live in a dry climate go for the lower end of the range, and if you live in a particularly humid area coastal area go for a dehumidifier on the high end of the range. You can always over-spec the dehumidifier helping prolong it’s lifetime.
Dehumidifiers: Reservoir Vs Hose Connection Vs Pump
All of the dehumidifiers I’ve selected below have garden hose attachments for draining the water that accumulates. Unless you don’t have access to a drain I recommend running the hose instead of the built in reservoir… lest you want to be emptying the reservoir multiple times a day. Most Dehumidifiers have a reservoir that is a fraction of their max daily output.
The difference between a hose connection and pump is a pump does not require a downward grade, while just a hose connection does. This means that without a pump you need to rely on gravity, while with a pump you can run the hose however you like.
I suggest getting one of these Garden Hose to 1/4″ Barb Adapters and a length of 1/4″ ID hose to run the water line. It is much cleaner than running a garden hose for the water line.
20 Pint Dehumidifiers
30 – 35 Pint Dehumidifiers
