Culturing Grindal Worms

How I Culture Grindal Worms


(click here to listen to the podcast on culturing Grindal Worms)

You will need:


A container with lid (a plastic margarine, yogurt, cottage cheese, or sour cream tub works well for small numbers of worms. You can use a plastic shoebox –the type that sells for about a dollar—for larger numbers of worms.


If you are using a soil-type culture substrate, you’ll need an “interior lid”:An extra container lid of the same type, or a sheet of glass or rigid plastic that will cover most of the surface of the container.


Originally, I used a large paperclip or similar object (to perforate the container for ventilation) Lately I have been cutting small holes in the lid with a knife and stuffing them them with filter floss or similar material


Optional, but recommended: a plastic bag large enough to completely enclose the culture container (this helps keep fungus gnats, fruit flies, and the like from contaminating your cultures)


Bedding material

1. Coconut fiber method Coconut fiber, also called coir, is sold as compressed, dried bricks for reptile bedding; there are a couple of different brands. One block will last a long time.


2. Soilless method These types of bedding include open cell foam blocks, such as that sold for use with aquarium filters, and green scrubber pads, or even filter floss. I have recently begun an open cell foam culture and a green scrubber pad culture. Both are proceeding nicely.


Food (I used to offer a variety. I have used cereals such as enriched oatmeal, cat food kibble, and fish food pellets, as well as some vegetable scraps such as cooked broccoli, thawed frozen peas, and slices of zucchini and cucumber. HOWEVER, I NOW USE KITTEN KIBBLE EXCLUSIVELY WITH EXCELLENT RESULTS. I moisten dry food before I add it to the container, but in soiless cultures I don’t bother.)


Freshly dechlorinated water


Your starter culture


METHOD 1: Coconut Fiber Method


  1. Modify the lid of your container as above to allow for ventilation. If you do not cover the container, it will dry out and/or attract pests.


  1. You will need to break off a chunk of the coconut fiber and soak it in water for a few minutes to allow it to expand. I have found that a saw works better than a kitchen knife for this purpose. The coconut fiber expands considerably, so it is better to break off a smaller piece than you think you will need. You will want a layer between 1 and 2 inches of expanded coconut fiber in your culture container.


  1. If needed, cut your “interior lid” so that it covers most of the coconut fiber, leaving a bit of space around the edges.


  1. Add your starter culture, which may be in the form of pure worms, or a portion of medium from an existing culture. You don’t need many worms to get started, so don’t be alarmed if you don’t see too many at first. These worms take a few weeks to get fully established, but once they get going, they reproduce QUICKLY.
  2. Put 1-3 small portions of moistened food on the surface of the coir.
  3. Put the interior lid on top of the food and coir.
  4. Put the lid on the container.
  5. You will need to wait a couple of weeks for the worms to reach a density sufficient to enable harvesting. There are myriad methods of harvesting available on the internet, but I prefer to use a plastic basting brush or cotton swab to scrape them off of the lower surface of the interior lid when they form clumps near the food. I then dump them into a small glass of clean water and allow them to sink to the bottom. I leave them there for a few minutes, which appears to allow them to separate from particles of food and coir. I then suck them up with an eyedropper and give them to the fish.


  1. Although grindal worm cultures last longer than microworm or walterworm cultures, they should be sub-cultured. It will probably be several months before you need to do this, but it is always a good idea to have more than one culture on hand in case of a crash. You may either add a quantity of worms from your old culture to the new one, or simply remove half of the culture medium from the old container and add it to the new one. Freshly hydrated coir may then be added to both cultures to replace what you removed. This latter method is probably quicker, as both cultures will have large numbers of reproducing worms.


METHOD 2: Soilless Method


Some excellent websites that describe soilless grindal culturing, and upon which I base my own methods,  can be found here: (coming soon)


  1. Modify the lid of your container as above to allow for ventilation. If you do not cover the container, it will dry out and/or attract pests. Especially with the soilless method, it is important to maintain a very humid environment in the culture container. Oxygen is important, but especially since you’ll be opening the culture often, don’t overdo it on ventilation. Some people  use a large plastic bag  in which to enclose the  culture. You can cut a ventilation area out of the bag a couple of inches long and an inch or so wide and cover it with a strip of coffee filter. Tape it on. This will provide sufficient air exchange without allowing flying insects in. For my cultures, Icurrently cut four 1/4 inch ventilation holes in the plastic lid, and stuff them with bonded aquarium filter pad. I have found this method satisfactory.


  1. Whether you are using open-cell foam, scrubber pads, or filter floss, give your substrate a good rinse to remove fine particles or any contaminants. If the material you are using is thin, as in  the case of scrubber pads, you will want to stack several layers of pads on top of each other. (I use about 3 to six layers, depending on the depth of the culture container.) Immerse the bottom layer or two in dechlorinated water. Before you add your worms, immerse them for an hour or so in dechlorinated water to help remove any hitchhiking pests that might be clinging to the worms or the remains of the substrate of the original culture. Add a small amount of moist food near the worms, put on the lid, and insert the culture container into the bag.


  1. Watch the culture carefully, checking on it at least once a day. Keep in mind that these cultures cycle just like an aquarium does, so start slowly and be patient. As soon as you notice that the worms have finished the food, add more. Gradually increase the amount of food as the worm population grows. If you feed too much, the food will be eaten by fungus instead of worms. If this happens, remove any large patches of fungus and wait a day or so before feeding again. The goal is to feed enough that the worms will finish most of their rations within 24 hours, without ever going hungry. The worms reproduce quickly…within a week you should notice some reproduction, and after several weeks, if all goes well you will have thousands of worms.

  2. Change the water frequently—every other day is good. You can hold the substrate back with one hand and pour off the waste water with the other, replacing it with fresh dechlorinated water, or use a turkey baster and a cup to remove the water.


  1. When the worms have covered the surface of the substrate, you are ready to harvest. You can often use a moist cotton swab to harvest a small number of worms fom the sides of the container. You can also  remove clumps of worms from the surface of the substrate, or you can remove a layer of culture material and immerse it in a container of clean, dechlorinated water. Many of the worms will swim out into the water and sink to the bottom. You can pour off and replace the water to rinse the worms, and then collect them with an eyedropper or turkey baster to feed to your fish.


  1. Besides water changes, the pads will need to be periodically replaced, or at least thoroughly rinsed out. Don’t replace all of the pads at once, as apparently the grindal ecosystem you have established depends on beneficial bacteria, much as a cycled aquarium does. Removal of all of the pads could cause your culture to crash.


  1. Maintain more than one culture; this way you can rotate feeding from various cultures, and it is good to have a backup, as unforeseen crashes do occur. Three is a nice manageable number.







Try to adjust the quantity of food so that the worms finish what you give them in about 24 hours. Do not overfeed them, but do not allow them to go without food for long, either, as this will probably cause your culture to be less prolific.



Mites are probably the most notorious pest of grindal worm cultures. They seem to appear out of nowhere, but they have a hard time surviving in soiless cultures.  They do not appear to harm the worms directly, but they do compete for food with the worms and are especially annoying when starting a new culture. The mites seem to find the food before the worms do. I have found that adding several pieces of food and then removing them (and the attached mites) after an hour or two can reduce their numbers. This does not eliminate them, but can give new cultures of worms the chance they need to get started.



It is a good idea, when starting a new culture, to purge the starter worms of pests by immersing them in clean, dechlorinated water for an hour or so. Pour off and replace the water several times, paying particular attention to pour off any floating particles that may contain tiny juvenile mites or the like.  Watch the worms, though—If they start to stiffen and straighten out, they are running low on oxygen.  


The Web can provide other suggestions on how to reduce or eliminate mites—do a search for “grindal worms mites” and you will be rewarded. As others report, as as I have mentioned before I have found far fewer, if any, mite problems in soiless cultures.

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