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Grow Q&A: Why Are My Cannabis Clones Moldy?

Dear Dan,
I’m having a hard time with my clones.  I am using two gallons of distilled water with a little bit of Superthrive, and after 8 to 10 days I get this white mucus on the cut ends. The water temperature is 75 to 78 degrees, and I’m running the misting pump 24 hours a day. I need help! This is my second try and I lost them all on the first due to this ugly fungus. — Tom S. 

Dear Tom,

The water temperature in your cloner is too high. You need to bring the water temperature down to between 65 and 70 degrees with 65 as the ideal. If the temperature is too low, it will shock the plants. Too high and the dissolved oxygen levels decrease dramatically—and Pythium (mold) can and will become a serious issue.

Lowering the temperature of your nutrient solution can be accomplished in a number of ways. Some cloners come with the pump on the outside and this can reduce heat as well. Sometimes, moving a cloner out of a warm growroom under HID (High Intensity Discharge) lighting and into a cooler space with a fluorescent or LED light system can cool the water as well. Painting a dark-colored reservoir with a lighter color on the outside also decreases the amount of heat absorbed.

You may have to bite the bullet and invest some money into a water chiller. They come in a variety of prices and sizes to handle different reservoirs so you have to find the right one for you to dial in the temperature to exactly where you want it on the digital display.

A cheap but temporary alternative is to freeze a 2-liter bottle of water and chuck it into your reservoir. Bear in mind that this is not a long term solution and is a relatively crude and inaccurate way to accomplish cooler nutrient solution temperatures with consistency.

Don’t miss our previous Grow Q&A: My Pot Seeds Sprouted! Now What?



  1. Avatar

    Kenshin Himura

    September 28, 2016 at 8:06 am

    You can’t do hydro without a water chiller bro. I dare ya to try.

  2. Avatar

    marco esquandolis

    September 29, 2016 at 11:18 am

    I disagree. I’ve seen deep water culture running through the summer in the 80F temperature range with no problems, certainly no mold. Sure, high temps are not desirable, and you’ll want to add more bubble stones, but high temps do not mean mold. Mold loves temperatures in the 60s…just look around in damp basements.
    My advice would be to stop using Superthrive in the cloning stage. The only time I’ve seen mold is when folks have put something they shouldn’t into the hydro bucket, like House and Garden’s Roots Excellerator or molasses. I suspect superthrive has some sugar in it and the mold is eating that. Otherwise, if you’re just using water (or water with salt fertilizer) you shouldn’t see mold no matter the temperature.
    If you see mold again try a tablespoon of hydrogen peroxide or a few drops of chlorine bleach.

    • Avatar


      October 5, 2016 at 5:39 pm

      Do you have any spliff man?

  3. Avatar


    October 5, 2016 at 5:36 pm

    Cloning machines are a mess and humans took cuttings for over a hundred years without resorting to gimmicks. The only tools necessary are a humidome, a scalpel, a heating mat and thermostat, and rooting hormone with indole butyric acid AND napthalene acidic acid, they are synergistic, meaning they work better together than each does alone, separately. Make sure before you take clone one you have everything at hand and the final destination properly set up. Clones require very little light so they can sit on a heating mat, off to the side of a vegetating area. Adjust the temperature on the thermostat so it’s at 76-78 degrees. Most humidomes have ventilation holes in the top ensure they are closed until the roots start growing. You’ll want to take clones from the healthiest mothers, obviously. I have never had mold and my success rate is is usually 95% or better.

  4. Avatar

    Aqua Logic Inc.

    October 17, 2016 at 12:16 pm

    For hydro we recommend premium titanium water chillers from Aqua Logic. Check them out at Made in the USA and built to last, Aqua Logic Chillers are the best solution for hobbyists and commercial operations alike. Aqua Logic has builds water chillers of all sizes to handle any application. Give them a call for sizing assistance or to learn more.

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