Water temperature and effects on hydroponics

What is the best water temperature for plants?

In a natural environment cannabis does not thrive submerged in water: it prefers alternate wet/dry climate (which we replicate with the wet dry cycle when growing in soil or coco-perlite) in order to drag oxygen into its root system and thereby into the plant.

However when we start to adjust the natural order with an indoor grow room, and especially when using the highly artificial hydroponic method of growing cannabis, the plants’ root system needs careful manangement. If you plop a cannabis plant into a bucket of water it will die if left for a few days. This is because the root system will be unable to perform osmosis, and, deprived of oxygen the plant will be unable to photosythesise and eventually it will wither and die.

(so why do cannabis growers use hydroponic systems? Quite simply because given the right conditions, hydroponics enable bigger, quicker results than any others. Hydroponic grows “mainline” the nutrients, water and oxygen direct to where the plants can make use of them: the root system).

When growing with hydroponic systems there are a number of different ways to introduce oxygen into the system. The main examples are:

  1. DWC (Deep water culture)With DWC the oxygen in the system is introduced mechanically through an air pump and air stone underneath the root system.
  2. EBB & FLOW.The tray is flooded periodically, submerging pots filled with ceramic pebbles. As the water drains through, it drags all the old oxygen out of the sytem and pulls in oxygen rich enhanced new nutrient.
  3. NFT (NUTRIENT FILM TECHNQUE)This is a system where a film of water is passed over a tray where the roots sit on a spreader mat. As the roots grow some sit submerged and others arch out of the nutrient film to reach air in a highly humid, light sealed enviroment. Some growers will also add an air stone to enhance the oxygen content within the main reservoir and thereby the film of water. Dependent on the height of the trays in relation to the reservoir tank, the fall of water from the tray back to the tank can also introduce oxygen.
  4. DUTCH POT / BATO BUCKET SYSTEM – read this article here and follow WildWW’s grow here




What is the ideal hydroponic reservoir temperature?

So why am I talking about oxygen when the subject matter is water temperature?

The temperature of water and its oxygen content are closely related, and the temperature of the solution needs to be fairly closely monitored in order to maintain dissolved oxygen levels. At low water temperatures, the plants go into shock, but as the temperature rises in the tank the solution loses the percentage ratio of its oxygen content. Here are some figures:


tank temperature & % of oxygen in the solution (ppm) 

10ºc (50ºf) 13 ppm

20ºc (68ºf) 9-10 ppm

30º (86ºf) 7 ppm

As you can see the oxygen content in the resolution approximately halves for every 10ºc rise in solution temperatures.

ok got that! :)

In easy terms, ideal temps are between 18 and 20 ºc and if your water temps are anywhere near 25 -30ºc then the oxygen levels can be as much as half of the desired levels.

In easy terms, ideal temps are between 18 and 20 ºc and if your water temps are anywhere near 25 -30ºc then the oxygen levels can be as much as half of the desired levels.


As a direct result of the water temperature being at higher temps the plants will need more oxygen at the roots so the problem is almost squared and there is 25-40 % less oxygen in the solution and the plants will need double the amount than normal.

In real terms the effect is that the oxygen level in the solution is only at 25 % of the desired levels and you can chuck as much light or feeds at them as you like but if they are only running at 25% of the required dissolved oxygen levels then this is hardly conducive to happy healthy growth is it?



Some of the results of low oxygen levels in the rez tank are:

  1. roots are unable to work effectively, leading to:
  2. a build up of toxins meaning that the plant will be unable to take up the water and feeds needed for healthy growth
  3. The whole plant begins to deteriorate as photosynthesis and carbohydrate rates slow leading to wilting.
  4. Leaf damage and root die-back. Ethylene is released and causes a toxic overdose and the roots fail.


And all of the above cause ideal conditions (especially in warm water) for every hydroponic growers worst enemy: 

water temperature hydroponicsAqua Euro USA Max Chill Aquarium Chiller $395.95


So of course the real solution to high water temperatures in a hydroponic reservoir is a water chiller. We tried inserting bottles of ice water into the res tank but despite dedicating a small chest freezer to the production of ice, found it almost impossible to sufficiently and consistently cool our water (warmed by hot sunshine on the pipes outside). Its yet another expensive piece of kit to add to the grow room, but if you are growing hydroponically in an environment where water temperature rises either due to external environment (like ours) or internal environment (the tank is warmed by heat thrown out by your HPS), then a water chiller IS be the only solution.

This great chiller features:

  • digital display
  • titanium heat exchanger
  • super quiet operation
  • powerful compressor


water temps hydroponicsAqua Heat Titanium Heater 300 Watt $31.00
And of course if the problem is that your water temps are too low, then its worth investing in a water heater to maintain a healthy temperature:


10 thoughts on “Water temperature ranges in hydroponic systems

  1. asdasd says:

    Worst fucking site on the internet. There’s no fucking need for all the annoying as fuck popups. I was interested in buying seeds but i’m going elsewhere due to this annoying, terrible fucking site.

    • weed widow says:

      wow – what a response. there should be only 2 popups – one a FB like one – one for seeds. There’s a “never see this message again” link on this one, once you press that you wont see the ad unless you come back to the site 2 days later.
      I’d be interested to know what other popups you’re hitting, maybe there’s an old one still running loose that I missed deactivating… maybe next time you leave a negative review, use a real email address so people can respond to your complaints. … unless of course you;re just venting a bad day in a troll-style comment

  2. gent says:

    What other damage am I likely to incur due to water temperature rising due to my lamps. I’m currently placing shields on the pots. Got a chiller but don’t work.
    Freezing bottles at the moment. ….and yeah its a never ending ball ache with no real result

    • weed widow says:

      Yeh to be honest I think the freezing bottles is the act of a man/woman in desperation lol. I had high water temps and 3 grows in a row failed thanks to a pythium infection which destroyed healthy cuttings once they went into the hydroponic system (but my water supply probably had pythium in it, but this multiplied hugely thanks to the high temps). certainly your plants wont do as well in higher end temp ranges. Whats the issue with the chiller? or can you move the water reservoir out of the grow room into another place and cool it there?

  3. gent says:

    The reservoir is already out the room. The water coming out the chiller is 4.8….New chiller. ….now its still not cooling enough through the system. My line temp is around the 22 mark.
    Pulling my hair out lol

  4. Janman says:

    Is there any instructions on making a water chiller,using a de-humidifier. Or a maybe a window air conditioner. Hey.. Few dollars , little work.tadaa..

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