Before taking those spent coffee grounds to your yard, learn the facts about giving your garden a caffeine fix. Home gardeners have heard they can count on used coffee grounds to do all kinds of things. Spread on planting beds like mulch, grounds are said to repel pests, fertilize soil, kill slugs and keep weeds at bay. A coffee mulch is also rumored to beckon earthworms and acidify soil. Other gardeners work coffee grounds into beds, swearing it aerates and acidifies soil.
Here are our top three tips when using coffee grounds for plants:
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Compost Coffee Grounds. The safest way to use coffee grounds is adding to compost. Take care to add grounds so that they comprise only 10 to 20 percent of your total compost volume. Any higher, and they might inhibit good microbes from breaking down organic matter. Another way to approach this volume is to add 4 parts shredded leaves to 1 part coffee grounds (by weight).
Some folks still suggest adding lime or wood ash to the compost to offset the initial acidity of the grounds. You can do that, but it's not really necessary. If you want to do it, aim for a ratio of 1 cup of lime or ash to 10 pounds of grounds.
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Spread Grounds Thinly and Cover. Using coffee grounds as a thick mulch isn’t a great idea because they tend to compact, forming a barrier that doesn’t let air or water pass. If you want to spread grounds on soil, use a thin layer (half an inch, tops) covered with a thicker layer (2-4 inches) of organic matter, such as shredded bark, wood chips or compost.
- Shift Soil pH With Grounds. If your goal is to acidify notoriously alkaline soils west of the Mississippi River, take a soil test first to know your soil’s pH. If you need to acidify it, dig grounds into soil to a depth of 7 to 8 inches.