Pre-Composting Then Vermicomposting

Here is a really good question from Bobby:

I really enjoy your website and have learned a lot. I have
2 old weber bar b que grills with attached tables and wheels works
great. I am thinking of putting all “food” in a compost pile for a
week or two and then feed that to my red worms. It seems it would be
easier to control quality and quantity of food. What would be the
advantages and or disadvantages of this method?

Hi Bobby,
Not 100% sure I follow how you are using those grills for a vermicomposting system – but it sounds interesting nevertheless!

You are absolutely right – composting food waste for a short period of time (often referred to as ‘pre-composting’) before feeding it to your worms is an excellent strategy, but of course there are a few disadvantages as well. Anyway, as per your request, here is a breakdown of the pros and cons of this approach:

    Advantages of Pre-Composting

  • Partially breaks down materials, so faster vermicomposting
  • Microbial colonization of wastes, so lots of food for worms (and less lag time before worms start processing it)
  • Allows you to deal with excess amounts of waste and control amount given to worms
  • Helps to avoid overheating in vermicomposting system
  • Can kill weed seeds and pathogens (when present in your waste materials) if large enough volumes are composted
  • Lets you create the ‘ultimate’ worm food mix before it goes in the worm bin
    Disadvantages of Pre-Composting

  • Requires that you have a yard and space for composting
  • Can lead to infestations of outdoor pests (house flies, fruit flies etc etc) in your bin if enough there isn’t enough heat generated during the composting stage
  • If not properly handled, materials can go anaerobic and be unpalatable for your worms
  • Takes extra effort

As you can see, if you have the space and are willing to put in the effort needed to compost the materials properly, pre-composting can be a great strategy.

Hope this helps, Bobby!

[tags]composting, pre-composting, hot composting, vermicomposting, thermophilic, worm composting[/tags]

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    • John Augenstein
    • August 14, 2008

    To set the stage, I’m raising eisenia fetida in my garage in three 30 gallon plastic tubs. I harvest the compost at a rate of one tub each month.
    I’ve been experimenting with pre-composting worm food for several months now. I started by using material directly out of my compost tumbler which consisted of about 50% completed compost plus material in all stages of breakdown since I add garden trimmings, grass clippings and overripe vegetables on an ongoing basis. The worms do just fine with it and I don’t have an excess water problem in the worm tubs but it’s a much denser food and I had to cut my quantities at feeding time about in half. Overfeeding this stuff doesn’t bother the worms but it does reduce the quality of the vermicompost somewhat.
    About a month ago I was given a half watermelon that was past it’s prime. It was’t a good time to feed, so I chunked it up and layered it into a 24 qt plastic pail with a pile of 1″ squares of cardboard from a cardboard box. The mix was about 25% cardbord and the balance watermelon. I laid a piece of plexiglass over the top of the pail to keep the flies out and promptly forgot about it. About a week later I remembered it and found the bucket was only about half full. The watermellon was reducing and giving up water. I poured of the water, which was cloudy and smelled like vinegar, then dumped the contents back and forth between two pails to areate it and put the plexiglass back over it for another week. It further reduced to a little less than a third of the pail. I poured off the water again and fed the remains to my three tubs by laying it on the top of the bed. When I pulled the covers off the next day to check the worms, I almost couldn’t see the food for the mass of worms all over the top of it. This was true of all three tubs.
    I have just set up four more pails as follows:
    1. Watermellon and cardboard (to duplicate the test)
    2. Watermellon and chopped wheat hay
    3. Squash/cucumber and cardbord
    4. Squash/cucumber and grass clippings
    I’ll get back to you when I have more results.
    I hope there’s something helpful here.

    • Bentley
    • August 14, 2008

    Awesome, John!
    You sound like true experimental vermicomposter. I love it!
    I too am always playing around with various food mixes, and letting things sit before feeding them to the worms.

    You made a really good point – something I should have added to the “pros” of pre-composting. When you mix materials and let them pre-compost for a decent amount of time, the chances of you “overfeeding” with this material is far less than if you simply throw things directly in the bin, because some stabilization, water-release etc etc is going to occur thus rendering the materials a lot more worm friendly.

    Anyway, do keep us posted on your experiments!



    • Sherry
    • August 16, 2008

    A few weeks ago I had a lot of watermelon rinds. I kept them in a container in frig and fed them as needed to the worms. They couldn’t keep up, so the watermelon was in the frig for quite a while. Just last week I checked the bowl and there was a lot of cloudy liquid in the bowl, and a lot of white slime on the melons. I poured it down the drain, and put one piece of melon in each bin. I was concerned that it might be starting to ferment, so didn’t want to overfeed.

    The next morning I checked and that was the busiest spot in the bin. Hordes of worms under the melon loving it. They promptly got the rest of the melons. I think they were liking the fermentation, alochol!

    • Bentley
    • August 20, 2008

    Hi Sherry – I’ve seen the exact same thing. Worms absolutely go crazy for rotting melon even if it’s going a bit sour from fermentation. You are wise to not overfeed though – excess quantities of various anaerobic bi-products (such as alcohols) can kill your worms pretty quickly.


  1. With respect to large-scale vermicomposting – we also pre-compost to limit the further composting potential of the feedstock material once fed to the worms. The last thing we want is for our worm beds to start heating up due to a composting action. Weed seed destruction and pathogen destruction are also big factors for us (from a product liability standpoint). Many of our large greenhouse clients would not be happy if weed seeds were passed along to them (neither would I). All in all – I think it really helps in providing a very stable material for the worms.

    • Bentley
    • August 27, 2008

    Hi Shawn,
    Thanks for sharing your thoughts (apologies for the delay replying). You’ve highlighted some really key points there!


    • Ashley
    • July 17, 2009

    We are just starting to composting worm bin system and have no idea what we are doing?? Anyway you could give us some good tips. I have been collecting food and storing it in a plastic container for a week on the counter. Will it still be good for the worms??

    • Milton
    • October 29, 2013

    Does pre-composting reduces the quality of the vermicompost?

    • Amrita
    • October 10, 2015

    I’d like to chime in on the whole precomposting issue. Just to give you a little background. I’ve been vermicomposting in my garden house for the last two years in styrofoam coolers. It has worked very well. Just recently we purchased three 4’x8′ professional fiberglass worm bins. I’m in “worm heaven”.

    So far I have only started one half of one bin. My husband made some great removable dividers for me. I want to go slow so that I can see what the potential is for the bins and deal with any “problems” moving into a bigger system. Right off the bat I’m dealing with temperature issues. It’s hot here in Southern Oregon. I have the bins in a large attached garage that stays between 65 and 85. There is sufficient airflow from a good sized window and cracking the garage doors.

    I did not realize that heat could be an issue when I set up the first side and put 20 pounds of worms in the next morning the temp was up to 89. I ran went to the freezer in the garage where I keep the bags of freezing food for the worms and buried the bags all through the bin. It quickly brought the temperature down into a safe range. However after two weeks I’m still dealing with temperatures that fluctuate from 79 to 83. It seems that the worms are tolerating it OK now and I never did see evidence of distress. I measure the temp at least a couple of times a day in different areas of the bins. I do see some variations of temps through the big bin, so I’m pretty sure the worms find what is comfortable for them. But, I measure because I don’t want it to get out of hand.

    What I have decided to do as a regular practice now is use those old styrofoam coolers–probably about 9 of them (which are still in great shape after two years serving as worm bins) as my pre-composting bins. I set them up the other day with what I’m using as my starter bedding (shredded newspaper and cardboard that have been soaked in worm tea, a little bunny poo, composted leaves, a little azomite, coffee grounds and ground egg shells). In addition (and just to see what would happen)I also added in a small amount of alfalfa meal and some fresh bokashi which I haven’t used before. Wow, what a mix. It heated up to 110 overnight and has maintained that temp for days. Now that’s a way to make some manageable compost that hopefully will kill off any weed seeds.

    I’m going to give it a couple of weeks to finish and cool down before I start the second side of the bin with another 20 pounds of worms. Fingers crossed.

    Any comments or suggestions are totally welcomed and appreciated.

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