Coffee Ground Science for Plants

If you start the day with a steaming hot cup of java, you’re not alone. Americans drink 700 million cups of coffee per day, and we create tons of coffee grounds in the process. Instead of throwing coffee grounds in the trash, why not put them to use in the garden? While there are plenty of uses for coffee grounds, they’re a secret weapon to help grow a beautiful garden. However, some of the advice you’ve read about using them may be wrong. Let’s dive into the research about the best (and worst) way to use coffee grounds to help plants grow.

I save all my coffee grounds
I save all my coffee grounds

Are coffee grounds good for plants?
Coffee grounds provide phosphorus, potassium, magnesium, and copper to growing plants. Moreover, as grounds biodegrade they release nitrogen, a vital element essential for plant growth. (A cubic yard of coffee grounds contains 10.31 pounds of nitrogen, according to an analysis done by Sunset Magazine.)

However, many people misunderstand how to recycle coffee grounds in the garden. Some blogs and articles advise gardeners to work coffee grounds into garden soil to feed plants, but research suggests this practice may inhibit the growth of many plants.

In one study, coffee grounds were mixed with urban agricultural soil in different concentrations. Every concentration level decreased the growth of five plants: broccoli, leek, radish, viola, and sunflower. Coffee grounds have also inhibited the growth of Chinese mustard, geranium, and other plants.

London botanist James Wong conducted his own experiment by planting two identical gardens of tomatoes, lettuce, herbs, and flowers. He used coffee as fertilizer in one bed and the results were disastrous. “The crop yield and growth of pretty much everything in the coffee bed became noticeably worse within about two weeks of application,” he writes. “Plant growth slowed, some developed leaf yellowing, others defoliated and died.” It may be the caffeine or phytotoxins in coffee grounds that stunt plant growth.

Hydrangeas benefit from coffee grounds
Hydrangeas benefit from coffee grounds

But to confuse the matter, a few plants may get a boost from coffee grounds. Coffee grounds may aid the germination of sugar beet seeds and the growth of soybeans and cabbage, according to Linda Chalker-Scott, a certified Master Gardener and professor at Washington State University.

What’s a gardener to do with this conflicting information? Many horticulture experts advise against working coffee grounds directly into garden soil. However, that doesn’t mean you can’t use them. There are two effective ways to use spent coffee grounds to grow beautiful plants.

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Perk Up a Compost Pile
Coffee grounds add nutrients to a compost pile. Moreover, they help a compost pile sustain the high temperatures needed to kill weed seeds and pathogens. Informal research conducted by the Oregon State University Extension Service suggests coffee grounds can keep compost temperatures between 135°F and 155°F for two weeks. That’s long enough to kill significant amounts of weed seeds and pathogens. Moreover, coffee grounds compost sustained more heat than animal manure in the trials.

Ready to transform coffee grounds into a nutritious soil amendment? It’s easy! (If you’ve never composted, choose a compost bin, and get started.) The basic concept of composting is to layer nitrogen-rich materials—including coffee grounds, grass clippings, and fruit and veggie scraps—with carbon-rich materials, such as dry leaves, straw, twigs, and sawdust.

Compost bins come in all sizes and shapes
Compost bins come in all sizes and shapes

Here’s how to:

Start a pile by laying a generous layer of carbon-rich materials on the bottom.
Add a thin layer of cooled coffee grounds on top. If desired, mix coffee grounds with other nitrogen-rich materials, such as fruit and veggie scraps or grass clippings.
Cover the nitrogen-rich layer with a generous layer of carbon-rich materials.
Repeat the process, aiming for a ratio of one part nitrogen-rich materials to three parts carbon-rich materials.
Repeat the process, aiming for a ratio of one part nitrogen-rich materials to three parts carbon-rich materials.
Coffee grounds should make up between 10 and 25 percent of the total volume of compost. Composting usually takes a few months, but it may take longer depending on the mix of materials, moisture content, and other factors. You’ll know it’s done when it’s dark brown and smells like earth. Work the finished compost into the top three to four inches of garden soil to add organic matter and nutrients, and improve the soil structure.

Benefits of composting
Benefits of composting

Mulch Acid-Loving Plants
Perhaps you’ve heard coffee grounds acidify soil for plants that prefer a low pH. It’s true; coffee grounds can be acidic, and many plants prefer acidic soil. Those include:

Azaleas
Blackberries
Blueberries
Camellias
Hydrangeas
Rhododendrons
Roses
Strawberries
Zinnias

Blueberries benefit from acid soil
Blueberries benefit from acid soil

There’s one problem though: It’s hard to know the pH of coffee grounds without testing them because acidity leeches out of coffee beans during the brewing process. In experiments, some spent coffee grounds are highly acidic, some are neutral, and some are even alkaline. Thus, coffee grounds may help acidify soil, but they won’t predictably do so.

Coffee grounds may offer other benefits though. As mentioned, they supply nitrogen and other nutrients to soil. Moreover, they may deter the growth of weeds. In one study, a mulch made of coffee grounds completely controlled weed growth around blueberry plants when used in combination with a weed mat (a barrier material that blocks weeds).

Bottom line? Experience is the best teacher in the garden. It won’t hurt to try using coffee grounds as a mulch around acid-loving plants, and it may benefit the plants. Follow these steps:

Apply mulch in the spring after the soil warms up, and then again in the fall.
Sprinkle a half inch or less of coffee grounds onto the top of the soil around the plants, keeping grounds away from the roots.
Cover the coffee grounds with a generous layer of dry leaves or bark mulch.
Drink Up!
The waste from your favorite morning beverage can help grow healthy plants in the garden. Be cautious about working coffee grounds directly into garden soil as a fertilizer. Instead, add them to a compost pile or use them as a mulch around acid-loving plants.

Shoveling out my rich compost and adding it to the garden
Shoveling out my rich compost and adding it to the garden

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