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Will Seeds Germinate in Used Coffee Grounds? Test Results Are In.

Timothy Boyer Ph.D.'s picture
Used coffee grounds for gardening.

There’s a lot of conflicting information when it comes to coffee grounds and gardening. Here’s what one simple test showed when asked if pepper seeds will germinate in used coffee grounds.

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The Genesis of the Test

COVID-19 social distancing/isolation came at a bad time for me…as if there’s ever a good time—but you know what I mean. It was that time of the year when I typically make several trips per week just trying to get my gardening started and always finding myself in need of “just one more thing.”

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When we started social isolating in March, we stocked up on food and necessities with the goal to limit our trips out of the house to just once in every three weeks. “Impossible!” you say? “What about veggies?” We managed to cover our dietary vegetable needs with large pots of vegetable soups made at the beginning of each 3-week period, supplemented with fermenting Kimchi stored in the fridge in large jars. Oh…and lots of beer.

Anyway, one item I found myself running out of was vermiculite and seedling soil (peat) for some additional seed-starting. With images of food shortages, anarchy and “another four years” plaguing my thoughts, I found myself a bit more ambitious in my produce canning goals for the coming summer.

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Scrounging around through old planters filled with last year’s potting soil, scraping out the bottom of a compost barrel, and tearing up used biodegradable seed pots, I managed to concoct a suitable medium for my next batch of seedlings to last me until I could no longer hold off my next trip out of the house for supplies.

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All of this end-of-life-as-we-knew-it based activity and an article about using cardboard toilet paper rolls as repurposed seed pots from a library copy of “Survival Guide” magazine got me thinking about what would a survivalist do if there was no seedling starter soil available? No, I am not a survivalist. But I am a big believer in repurposing anything and everything, and that magazine has some excellent repurposing tips.

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Since, I had repurposing in mind, I began to think about my daily coffee grounds that I add to my compost pile. Perhaps, coffee grounds would make a suitable seed starter. The research was on.

Unfortunately, after a few hours of internet surfing and reading articles, there was a lot of conflicting information about the use of coffee grounds in gardening: it should be used/it shouldn’t be used; it’s too acidic/it’s too alkaline; it kills off plants/it nourishes plants; it does more good in the garden soil/it does more good in the compost pile; it needs to washed first/it needs to be used straight from the pot, etc.

Put It To The Test

That’s when I decided to “put it to the test.” I had some left-over vermiculite and pepper seeds, a lot of saved used coffee grounds, and nothing else better to do at the moment.

To be clear this is not a strong use of the scientific method; Not enough samples were tested to make this statistically significant; nor was this experimenter double-blinded—although I might have been wearing my beer goggles at the time.

The goal of this simple test was just to see if seeds from either sweet and hot pepper plants would germinate (or not) in a seed starter soil consisting of 50% vermiculite and 50% freshly used coffee grounds.

Vermiculite was allowed because I knew that the compaction of used coffee grounds would most likely be detrimental to seedling growth.

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As a control, a typical seed starter mix of 50% vermiculite and 50% commercial seedling soil (peat) was used in comparison, being treated identically regarding watering and temperature conditions. Approximately 3 weeks later I had my answer.

Images of the Test and Control Seedling Pots

With and Without Coffee Grounds

As you can see in the image I took today, there appears to be no difference in the growth or appearance of seedlings emerging from the coffee ground test samples shown on the left, and in the non-coffee ground controls shown on the right. The images are of sweet pepper seedlings; however, the hot pepper seedlings not shown were identical in both growth and development.

In Conclusion

Hence, the results suggest (“suggest” is a commonly used word when scientists do not want to be too committed to anything that will likely be proven false or flawed in a paper the following week) that at least for common sweet pepper and hot pepper seeds, a 50:50 seed starter mix using coffee grounds and vermiculite will support germination and growth/development up to at least the cotyledons (seed leaves) stage.

To be clear: A 50:50 mix of vermiculite with almost anything else might yield the same results—even with a 100% media of vermiculite. Regarding germination and nutrients, the seed most likely provides all the nutrients it needs to support germination and growth up to the cotyledon stage. There is no implication that used coffee grounds nourished the seedlings; only that it does not appear to prevent or suppress germination. What would happen afterward if the transplanted seedlings were exposed to a high concentration of used coffee grounds remains to be tested further.

However, since the question was about the potential repurposing uses of used coffee grounds for seed germination, at least for sweet and hot pepper plants, it appears to work fine.

The purpose of this article was to encourage you to consider your own gardening experimentations rather than relying solely on what someone else says or does. There’s a lot of misinformation out there and sometimes you have to be willing to get your hands dirty to uncover the truth…and learn a little something more about nature.

For more details about used coffee grounds and its uses in gardening, please refer to the references below—they are the most informative ones I have found thus far.

If you have found any practical uses for used coffee grounds in your gardening, please tell us about it in the comments section below.

Timothy Boyer has a Ph.D. in Molecular and Cellular Biology from the University of Arizona. For 20+ years he has been employed as a freelance health and science writer. Today, with a background in farming and an avid home gardener, Timothy continues writing about science with a focus on the connection between plant biology and gardening for healthy living. For continual updates about plants and health, you can also follow Timothy on Twitter at TimBoyerWrites.

Images courtesy of Pixabay and the author

References:

Coffee grounds— will they perk up plants?” by Linda Chalker-Scott, Ph.D. Master Gardener WSU editor, Extension Urban Horticulturist and Associate Professor, Puyallup Research and Extension Center, Washington State University.

Coffee Grounds and Composting” Oregon State University Extension Service

Add Used Coffee Grounds to Your Garden Soil and Get Amazing ResultsSUNSET magazine June 26, 2006 | Updated April 20, 2020.

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