By Jeff Rugg- Q: My local coffee shop offers five-pound bags of used coffee grounds for free. According to the signs, we can take them home and use them in compost piles, for mulch in our flowerbeds and to prevent insects on the soil of indoor flowerpots.
So far, I haven’t seen anyone take any, but it seems like a better thing to do than throw them in the trash. If I take some home, how much should I use, and will it be any good for my plants?
A: As with most things in life, moderation is key. You can use coffee grounds in your landscape, but don’t overdo it. Coffee grounds are dead organic matter. Dead stuff decays. Decaying coffee grounds are not much different than any other decaying organic matter.
Compost piles produce the best compost when they are made from a variety of components. If possible, it is best to add no more than about 20 percent of an ingredient to the compost pile at a time. In the fall, when there are huge quantities of tree leaves and not much else, we don’t have much choice.
But, if you could get coffee grounds at that time of year and mix them into the leaves, you could use a lot more than at other times.
It would take a huge quantity of coffee grounds to use them as mulch. They might smell for a while, too, and I am not sure that would be pleasant. I know that people think coffee grounds will acidify the soil, but again, it would take a large amount to even begin to change the acidity. The existing soil components would cause the soil pH to revert back to its original state, as soils do with any other soil-acidifying product.
A half-inch layer on the flowerpots might prevent some insects that feed on fungi in the soil. It would be worth a try, and if it doesn’t work, I don’t think it will harm the plants.