Disinfecting Pots and Trays

Question:  I recently bought bleach to use to disinfect pots and trays.  I brought Ultra Clorox bleach and used it at a one to nine ration as I had done with another bleach.  When I called the Clorox service number, I was told not to use the Ultra product for this purpose.  I wonder what is best to use now.  I also know some people use vinegar.  I have not been able to establish the measurements of a pH level to aim for when using vinegar for disinfecting.  Any comments?

Answer:  At the Violet Barn, we have long used both bleach and vinigar to clean and disinfect pots and trays, and often for leaves and cuttings as well.  We’d always used the “one to nine” ratio, but this question forced us to do a bit more research.  Here’s what we found.

First, “bleach is bleach”.  The active ingredient (sodium hypochlorite) is the same in all bleaches.  However, not all of what youcan buy is of the same strength–not all, or every brand of bleach, is the same.  Ultra Clorox, for example, is simply a more concentrated form of “regular” Clorox.  And your “generic” or “store brand” labels may be even less concentrated.  Look on the label to compare the concentrations of the active ingredient.  This will determine how much bleach you will need to use to effectively disinfect your pots.

Second, what is the recommended dilution?  According to the EPA service bulletin for Clorox, the proper dilution rate for “plant containers in nurseries” (among other things), for the purpose of “plant parasitic nematodes, plant disease-causing fungi and general surface disinfection” is an approximately .85% active ingredient diultion rate.  What does this mean?  Ultra Clorox, for example, contains 6% active ingredient.  When 1 part Ultra Clorox is mixed with 6 parts water, the dilution will be .85% (i.e. 6% divided by 7 total parts).  If your bleach is more dilute, mis with less water.  For example, our store brand bleach has about 3% active ingredient.  We would mix 1 part of this store brand with 2 parts of wter to achieve the proper dilution (i.e. 3% divided by 3.5 total parts).  If your bleach is more concentrated, dilute it more.

What about vinegar?  Vinegar does two things.  First, because it is acidic, and much of the “crust” or residue you find on old, used, pots is alkaline, it works very well to dissolve and clean away these.  We’ll often soak really hard-to-clean pots in a pure (or nearly so) solution of white vinegar to make old pots look new again.  Second, vinegar works well as a disinfectant, especially when used in combination with bleach–it makes the bleach even more effective as a disinfectant!  It’s been found that bleach (which is slightly alkaline) is a much more effective disinfectant when it is acidic (pH 6 to 6.8).  A recommened solution for household use, for example, would be a follows: 1 oz. bleach (6% dilution) added to one gallon water, then add 1 oz. white (5% distilled) vinegar.

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