Should I Put Coffee Grounds in a Worm Bin?

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First off, in my post on using coffee grounds in your garden I linked to the wrong article. The correct, and very useful publication by Linda Chalker-Scott, “Using Coffee Grounds in Gardens and Landscapes” can be found here.

There were a number of questions and emails about the pamphlet’s recommendation not to add coffee grounds to your worm bin. Why might coffee grounds not be good for worms? Chalker-Scott cites a study, “Evaluation of three composting systems for the management of spent coffee grounds” that looked at using worms to compost coffee waste. The study showed high worm fatality in spent coffee grounds due, the authors speculate, to the acidic pH of coffee and harmful organic compounds. The addition of cardboard reduced fatality. They go on to suggest pre-composting coffee grounds for three weeks before adding to a vermicomposting bin.

It should be noted that the study was looking at worm bins where the feedstock was entirely made up of spent coffee grounds. Adding a few coffee grounds to a home bin made up of a diversity of feedstocks is probably not going to kill the worms.

But, in a discussion thread on the Garden Professor’s Facebook group speculating about what percentage of coffee grounds would be safe to use, I found myself agreeing with Raymond Eckhart who says,

In the absence of peer reviewed literature as to what percentage is acceptable, the cautious approach is to avoid it altogether, is my takeaway. If and when someone studies the issue to determine a safe percentage, it would be unwise to recommend the practice, given the results of the referenced paper.

Coffee grounds also form large anaerobic clumps worms don’t like. Clearly, they prefer vegetable scraps and large amounts of fluffy carbon material like cardboard and wood shavings.

Now wouldn’t it be great if Elon Musk would fund local Extension Service home gardening research rather than trying to figure out ways to blast rich people into space? We need definitive worm bin advice!

Coffee Grounds in the Garden

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According to a handy fact sheet from Washington State University, Coffee grounds will buzz your garden. Coffee grounds build humus, boost nitrogen, phosphorus and zinc, bind pesticides and toxins, prevent bacterial and fungal infections and feed earthworms. Authored by Dr. Linda Chalker-Scott, Extension Urban Horticulturist and Associate Professor, this peer-reviewed pamphlet also provides a set of suggestions for using coffee grounds in the garden:

  • Coffee grounds should be composted before used as a soil amendment but can be used fresh as a mulch.
  • Fresh grounds are phytotoxic, so keep them away from direct contact with roots.
  • Coffee grounds will not necessarily make your soil more acidic.
  • Don’t use coffee grounds where you are starting seeds.
  • Despite rumors, coffee grounds do not repel pests.
  • Let coffee grounds cool before adding to compost bin so you don’t kill beneficial microbes. And don’t let coffee grounds amount to more than 20% of your compost pile.
  • Don’t add coffee grounds to vermicomposting bins.

If you’re using coffee grounds as a mulch Chalker-Scott has two suggestions:

  • Apply a thin layer (no more than half an inch) of coffee grounds. Cover with a thicker layer (four inches) of coarse organic mulch like wood chips (Chalker-Scott 2015). This will protect the coffee grounds from compaction.
  • Don’t apply thick layers of coffee grounds as a standalone mulch. Because they are finely textured and easily compacted, coffee grounds can interfere with moisture and air movement in soils.

The Wonder of Worms

saint worm

[Another entry in the Back to the Garden series, which you can access by clicking the tag of the same name to the left.]

As I’ve been saying for the last couple of weeks, the key characteristic of the loving landscape is healthy, living soils which foster plant and animal health without artificial inputs. Compost, mulch and worms form the holy trinity of organic soil health.

Compost and mulch we’ve covered. Today I want to talk about worms, both worms in the wild and worms in your house.

Odd facts: Did you know there are about 4,300 species of earthworms world-wide? Did you know that the Australian Giant Gippsland earthworm can grow to be 3 meters (9.8 feet) in length? Shai-Hulud! I’ve also seen references to a 22 foot (6.7 meter) long worm discovered in South Africa, but can find nothing substantial to back it up, and have decided that it’s an Internet myth. What I do know, though, is that I’m glad I don’t live under water with the sea worms.

But I digress. The real wonders of this world are invisible, or so humble as not to be noticed. Like saints of the soil, garden variety worms pass through the world quietly, leaving miracles in their wake.

Continue reading…

015 Worm Composting and Skunks

Our worm bin.

Our worm bin.

On the fifteenth episode of the Root Simple Podcast Kelly and Erik discuss how our cleaning project is going, worm composting, the ongoing skunk menace in our garden and we review two books. Apologies for some clipping in the audio and the cat interruptions.

During the worm composting segment we cover:


What are we reading

Worms Eat My Garbage by Mary Appelhof.

Bread: A Global History By William Rubel.

Kelly mentions Werner Herzog’s Happy People: A Year in the Taiga.

If you want to leave a question for the Root Simple Podcast please call (213) 537-2591 or send an email to [email protected]. You can subscribe to our podcast in the iTunes store and on Stitcher. The theme music is by Dr. Frankenstein. Additional music by Rho. A downloadable version of this podcast is here.

Worm Compost Leachate, Good or Bad?

Image: Permaculturewiki.

Image: PermaWiki.

In the course of preparing for our worm composting demo last week Kelly and I came across a lot of conflicting information. One of the most contentious issues in worm composting is what to do with the liquid that comes off the worm bin, called leachate.

The controversy stems, in large part, from the debate over aerated compost tea (ACT) vs. non-aerated compost tea. Fans of ACT do not like the fact that worm bin leachate is anaerobic, which they believe encourages the growth of microorganisms unfavorable to plants. They like to point out that worm bin leachate is not ACT.

The ACT debate needs a much longer post, but I did find two peer reviewed studies showing the benefits of un-aerated worm compost leachate: “Vermicomposting Leachate (Worm Tea) as Liquid Fertilizer for Maize and “Vermicompost Leachate Alleviates Deficiency of Phosphorus and Potassium in Tomato Seedlings.” I also found several Extension Service publications touting the use of worm bin leachate.

There are some caveats, however. First, it needs to be diluted–at least 1:1 and maybe, according to some sources, as much as 1:10. And you should probably test it out on a few plants before applying it to your whole garden.

And, from a food safety perspective, I’d avoid applying it to leafy greens and lettuces. I’d also point out that if you have a lot of leachate it might mean that your worm bin has too much moisture in it.

What do you think? Have you used worm bin leachate successfully? What side of the aerated vs. non-aerated debate are you on?