I recently have read some articles that have made me think twice before putting coffee grounds into my compost or spreading in the garden as mulch. The articles that concern me make the case that the caffeine in coffee grounds is bad for plants because caffeine is the coffee plant's chemical used for allelopathy. The author of one article did a small test of his own showing bad results when adding coffee grounds, and another more scientific test showed equally poor results hinting at possible phytotoxic tendencies in coffee grounds.
So my question for all of you is, can anyone confirm these types of results?
And, if these results do rest on the presence of caffeine are there easy methods for ridding caffeine from used coffee grounds?
I have thought about soaking them in water and filtering it out (essentially brewing coffee on a bigger scale) a couple of times to get more caffeine out of the grounds, but then I worried about also leaching out nutrients.
From what I understand, most (all?, a significant enough amount?, I dunno.) of the caffeine in coffee grounds is removed by the brewing process.
Further, I would argue that the fertility benefits likely outweigh any allelopathy caused by residual caffeine.
Do these articles give any numbers for the trace amounts involved?
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I often see questions of allelopathy on this forum, but few instances where alleged allelopathy actually affects anyone whether it's about cedar wood mulch or now caffeine. I would submit that at 8.09 mg/g it would take a significant amount of coffee to overpower your plants, and that this effect could even be used to beat out competition among "weeds". At this point all I have is anecdotes to offer, but considering how ubiquitous the use of coffee grounds in compost and mulch is in this forum you'd think we'd have a horror story or two if it was that bad.
I haven't seen any adverse effects using coffee grounds. Between crops, I normally till in 1/4" to 1/2" layer of grounds along with an inch or two of compost, plus any residual mulch. That means that I have coffee grounds being tilled in every 30 to 120 days for most crops. I don't always have enough coffee grounds to add to all garden beds, therefore I have the opportunity to observe any differences between beds getting grounds and those that don't. As I said, I haven't been seeing any negative effects. Right now I'm using a bit more than a 5 gallon bucket of grounds a week. I'd love to have more!
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If you play with the search function (trying both the search button and the Google search button) you'll find lots of discussions about coffee grounds. This winter I'm trying tons of them in conjunction with chickenpoop and leaves but I don't have finished compost to play with yet so I can't say it's a winner or not yet.
Speaking only from personal observation in my garden, I've never found coffee grounds to create any problems. However, I compost 90% of the coffee grounds I collect, as they are a wonderful green that heats up the pile. But I will sometimes side-dress established plants with a couple of handfuls of coffee grounds and it's never seemed to hurt anything.
My hunch is that if your soil is healthy and full of microbes and fungi, that soil life will help "digest" the coffee grounds quickly and make the nutrients available to the plants nearby.
When in doubt, just compost the coffee and then spread the finished compost once it's done cooking.
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Most of my compost contributions consist of coffee grounds and used raw paper rabbit bedding. The worms just love it.
I have found that I can encourage worms to move around my garden beds just by adding coffee grounds to where they aren't.
I think the caffeine thing is much like the acidity thing, in that most of what's most volatile has already been leached away in the water used to make the coffee. I would guess that the caffeine left in the spent grounds aren't bioavailable enough to have any allelopathic effect.
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In my two potted lemon trees, whenever they start yellowing a bit, I tuck one serving of used coffee grounds under the mulch on the pot and they green right up. It never causes fertilizer burn (as happens with even the most diluted urine I've tried, or with small chunks of cow manure I've stuffed under there).
I don't remember the details, but I think Dale Hodgins on this forum had a whole long thread about the (great) progress of a new garden that he and his friends fertilized exclusively with used coffee grounds, last summer or the year before.
In practice, used coffee grounds don't seem to harm plants and do seem to help. Of course excessive amounts might be harmful, but that's true of everything!
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