4 minute read
Utah Associated Garden Clubs
The Coffee Effect, Beneficial or Not?
By Geoff Ellis, Past President, Utah Associated Garden Clubs
Common garden folklore has it that coffee grounds are a good fertilizer, especially for plants such as azaleas and blueberries that need acid soil. But are coffee grounds really beneficial for our garden plants? Caffeine is a poison, which the coffee plant produces to prevent its seed from being eaten by insects and other pests (it is actually lethal to humans in excessive amounts, but you would have to drink about 30 cups of coffee in one sitting to consume a fatal dose something that no one, to my knowledge, has ever done).
I decided to test the effects of coffee grounds on plant growth, using “Champion” radishes and “Pot and Patio Blend” lettuce, both chosen for their ease of growth and short time to harvest. For each test, I filled three pots with an identical soil mix in which 20 seeds per pot would be sown and the seedlings later thinned to the best 10 plants. For the first two radish tests, one pot received no coffee, one had a cup of coffee grounds sprinkled on top, and one had a cup of coffee grounds mixed into the soil.
The radishes grown in the spring (A) all grew very poorly, with many weeds showing up, but there was no difference between pots. The radishes grown in the fall (B) did much better, but there was a dramatic difference in growth. Germination was equal in all three pots, but the seedlings in the “mixed-in” pot soon became stunted in comparison to the “no coffee” and “coffee on top” pots, and they never caught up. The other two pots produced edible radishes, while the “mixed-in” pot produced nothing but seedling-sized runts.
The lettuce test (C) also produced dramatic results, but in a different way: seedlings in the “no coffee” and “mixed in” pots grew equally well, but germination in the “coffee on top” pot was greatly reduced, and the few plants that did grow never filled the pot with an abundance of leafy greens like the other two pots.
I wondered if the freshness of the coffee grounds might have something to do with the variety of these results, so I did two more radish tests: one with fresh coffee grounds, old coffee grounds, and none (D); the other with twice-brewed coffee grounds (which should have had more of the caffeine leached out), once-brewed coffee grounds, and none (E). I was hoping to see some dramatic results again, but in both of these trials there was no significant difference between pots.
What can I conclude? The coffee grounds did not produce noticeably better growth in any of these tests, and they produced significantly worse growth in two of the tests, so I can’t recommend them as a super fertilizer, at least not for radishes or lettuce. Someday if the Garden Clubs acquire a large budget and ample growing space, it would be interesting to do a more extensive experiment, including acid-loving plants. In the meantime, I would suggest putting your coffee grounds on the compost pile to break down, but not putting them on your young plants.
