One of the more annoying problems we deal with as hydroponic growers is keeping an oxygenated root zone at the perfect water temperature. It’s crucial to squeeze as much yield out of our plants as possible and minimize the potential of disease and infection. However, keeping your hydroponic reservoir cool can be a challenge.

Why Do Roots Need Cool Temperatures?

Most indoor hydroponic growers that use a deep water culture system will run into the reservoir temperature issue at some point in a grow. DWC systems tend to heat up quick when under high wattage HID lighting.

This bumps up the temperature of the nutrient reservoir, which increases the temperature around the root zone. Because of this, the amount of dissolved oxygen in the root zone is lowered. When you combine these factors with plants growing faster in a hydroponic environment, you’ve created the perfect storm for oxygen deprivation. And when that happens…you open yourself and your grow up to pathogens like pythium.

For best results, you want your air temperature to be higher than your water temperature. Typical ranges are 75-80F (23.9-26.7C) for air temperature, and 68F (20C) or lower for water temperature.

Even though roots grow better at the slightly higher temperatures of 70-75F (21.1-23.9C), you would be inviting in pathogens and root diseases that thrive at these temperatures as well. Keeping yourself and your roots safe means operating under 68F.​

Listen to this post on the Epic Gardening Podcast

Subscribe to the Epic Gardening Podcast on iTunes or Spotify

​So, How Do I Cool My Hydroponic Reservoir?

There are a variety of good ways to cool down your reservoir, ranging both in price and how annoying they are to deal with on a daily basis.

Regardless of the method you choose, you have to ramp up your cooling efforts in the warm summer months – especially if you use a deep water culture or small-reservoir system (the small reservoir size heats up faster).

Buy a Chiller

The most effective (and most expensive) method is to buy a water chiller. These are electric units similar to air conditioning units that are made for operating underwater. They’re basically composed of fans, compressor coils, and a refrigeration line.

All you do is plug and play with these bad boys. To cool water even quicker, make sure you’re circulating your nutrient mixture.

Most commercial growers will use a 1.5-2 horsepower chiller to handle most of the cooling responsibilities and then manage the rest on a case by case basis.​

Recommended Water Chillers

Click here for the in-depth review of hydroponic water chillers.

Paint Your Reservoir

Darker colors absorb more heat. If your reservoir is made of a dark plastic, consider painting it a lighter color – preferably white – to reflect some heat. Using something as simple as white spraypaint can do the job and knock a couple degrees off of your reservoir temperatures.

Keep It In The Shade

This is a really obvious tip, but I feel like I had to mention it. If you can minimize the amount of light hitting the reservoir itself, you’ll be minimizing the amount of heat that is transferred to your nutrient solution. Keep it in a shady area of block the surface with cardboard or aluminum foil.​

Increase The Size Of Your Reservoir

Many hobby hydroponic growers use smaller reservoirs which are exceptionally prone to temperature fluctuation. By building or buying a larger nutrient reservoir like this 75-gallon monster, you can add some stability to your temperature without building or buying any other cooling materials.

As an added bonus, your pH and ppm will remain more stable as well due to the larger volume of water.​

Top Off Your Solution

An elegant solution is to simply add cooler nutrient solution to balance out the temperature in the reservoir. This isn’t a solution you can use all of the time, because chances are high that if you have a temperature issue, it’s a persistent one. Just use this as a spot-fix if you have a one-time temperature fluctuation.

Note: If you add cooler nutrient solution quickly, the drastic temperature change could shock your roots.

Bury Your Reservoir in The Ground

If you’re growing outside, you can dig out the ground and bury your reservoir. The cool, dark environment below the surface of the soil will keep the reservoir exceptionally cool. You’ll almost never go into danger zone temperatures in the root zone if you use this method – but it does require a lot of effort.​

Make Your Own Cooling Coil

If you don’t want to go for one of the more expensive water chillers you can try to make your own cooling coil. I’m not going to go into the details of how to do that in this post, but there’s a great article here about it: http://www.instructables.com/id/How-to-make-a-wort-chiller-for-homebrewing/​

Make a Swamp Cooler

A swamp cooler is a pretty ingenious cooling method – I didn’t know about it until I ddi some research. If you take a simple clip-on fan and blow it across the top of the reservoir, you will see incredible temperature drops. You can expect a 5-10F decrease in temperature…but at a cost.

Usually you’ll need to top up your reservoir more often, because swamp coolers are utilizing evaporative cooling (meaning you’re losing water to the air). If you think about this deeper, this also means that your ppm will increase due to evaporation…so be careful if you use this technique.​

Do you have any other methods you’ve used to cool your hydroponic nutrient reservoir? Let me know below!

The Green Thumbs Behind This Article:

Kevin Espiritu

Founder