If you read "coffee in the garden" and visualize relaxing in your backyard enjoying a steaming cup of joe, you are in for a surprise. It turns out that used coffee grounds have some worthwhile uses in the garden. A substance that you ordinarily throw in the trash can actually be reused to good purpose. Waste not, want not!
Used coffee grounds are high in nitrogen, and along with the residual oils, lignin and fatty acids can give your plants a nutritional boost. The texture improves soil structure, making it a good ingredient in compost and mulch. Earthworms will ingest coffee grounds, and it is their castings that help enrich the soil.
It's even possible that coffee grounds can prevent mold and bacterial growth on plants. Tests have shown that coffee compost can suppress the growth of certain funguses, such as Fusarium and Pythium, and bacteria, including Staphylococcus. This has not been tested in gardens, however.
There's been little research done on the effect used coffee grounds have on plants, and some of the touted advantages – such as making soil more acidic – don't hold up under scrutiny. Probably the safest advice when using coffee grounds in the garden is: don't overdo it.
How to use coffee grounds
Mix them into a cooking compost pile. This is the safest way to reap the benefits while avoiding the problems pure grounds can create. Just keep them to 20% or less of your compost ingredients.
In a thin layer, topped with mulch. Too thick a layer of solid coffee grounds won't allow water through to plant roots. A half-inch of grounds with a few inches of mulch on top gives those earthworms a nice caffeine buzz while improving soil texture at the same time.
In place of deicer. One quirky use for coffee dregs: For those who don't like to use salt-based deicers in the winter, spread spent coffee grounds on paths and driveways instead. They don't melt the ice, though the dark color does absorb solar radiation rather than reflect it. A thick cover of dry spent grounds improves traction better than salt, and won't harm plants or soil the way salt can.
How not to use coffee grounds
On seedlings. Coffee beans have allelopathic qualities, meaning that they contain chemicals that inhibit the growth of other plants. It's a survival mechanism. Black walnut trees are the best-known example, but other plants do, too. Seedlings grown in or mulched with coffee grounds may die.
As a single source of mulch. The grounds are fine and compact easily. Using only coffee grounds as a mulch could create a barrier that keeps water from the root zones of your plants. Plus, compacted grounds grow mold.
To acidify the soil. As for using coffee grounds on acid loving plants such as rhododendrons or blueberries? Not all that helpful. The pH of used coffee grounds is, surprisingly, not very acidic, and can even veer into the alkaline range. Tests have shown that pH levels increase about 14 days after soil is treated with coffee compost, but then decline.
Are you one of the many people who use the one-cup coffee pods? Those little bit of grounds won't do much for your garden. If you can't part with your Keurig, ask your local coffee place to save their grounds for you.
Spent coffee grounds needn't be relegated to the trash. Don't spread pure coffee grounds around your garden, but do make them a balanced part of your compost or mulch.
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