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Adjusting duckweed pH

November 19, 2012

Attention urban duckweed growers!  To get the best bang for your buck growing duckweed indoors  keep an eye on the pH of your water column.  Duckweed prefers a neutral pH of 6.8 to 7.1. Adding fertilizers can sometimes push this  to the high side. For small batches indoors under grow lights, I usually rummage through my kitchen looking for something to lower my pH back down. One part clear vinegar diluted to 100 parts water. is a handy solution. I keep a glass jar of it nearby and dispense by eye dropper to my gallon-sized trays of duckweed/water. I use a digital electrode pH reader. It’s saved me lots of expensive pool pH sticks and is a lot faster. If I go overboard on the vinegar and end up lower than I want, I use a faint pinch of baking soda to bring it back up again to my pH range, taking care not to add too much soda.

Happy growing!

3 Comments leave one →
  1. Susan Munroe permalink
    November 20, 2012 11:31 pm

    “Cement will always keep your pH high.”

    When the cement is fresh, yes; new concrete has a pH of 12 to 13. But you can bring it down by repeatedly filling it with water, letting it sit, draining it, and repeating several times. If you don’t have freezing weather in winter, you can let Mother Nature take care of it. Otherwise, let the fresh water sit in it after several drainings, and then check the pH of the water. Eventually, as the concrete carbonates, the pH should drop to about 7.0.

    Adding some distilled white vinegar and letting it soak may speed things up, but after draining and refilling with plain water, let it sit for several days and check the pH again.

    I have some homemade concrete spheres that have been sitting out in the weather for a few years and are growing moss. Moss won’t grow on high-pH surfaces.



  2. hamdi shaar permalink
    November 20, 2012 12:54 am

    In a duckweed fountain, if your pH is typically too high, then most likely you have a carbonate in your system buffering your pH and keeping it within a certain range. Usually this is cement. If this is the case then your options are limited since the cement will always keep your pH high.

    To lower, you can try an acid, but I would not recommend it because your pH will just rise again. For pH raising I recommend hydrated lime- which you can get at most gardening stores.

    You will be wasting your money adding acid, if there is cement incorporated into the fountain- if you have cement in the fountain, then you will have to keep adding acid until the fountain dissolves. If you really want to control pH and bring it down you may have to seal the fountain first and then add acid. Otherwise, you will have to keep adding acid and you will get a very brief response in pH- pH will drop immediately and then raise within the course of 2-3 hours back to original levels (if there is cement in the statue). If you still want to try it, you can use nitric, phosphoric, formic or humic acid. I would recommend either formic or humic acid, in very small quantities. There are also commercial pond pH adjusters that I would look at if I were you. A simple Google search should turn up a number of them.


    • November 20, 2012 8:03 am

      Hamdi, excellent advice. If an outdoor cement-lined pond, this is a good route to go. My indoor containers are glass and stainless steel, so the fertilizer itself was the variable. I was looking for off-the-kitchen shelf ways to raise and lower pH as an ad hock method. Will keep your advice in mind as next summer I was seriously considering building a cement-lined pond. Perhaps I should go with a plastic lined one instead.


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