How To Use Coffee Grounds In The Garden, According To Experts

If a cup of coffee (or several!) are part of your morning or afternoon routine, it's likely that you've tossed a lot of grounds. Coffee grounds are actually full of organic matter and are wonderful to add to the compost pile to create a nutrient-rich amendment. There have been some misconceptions about used coffee grounds and whether it's a good idea to add them directly to your soil, and your houseplants. 

We spoke with gardening and soil pros about the best ways to use coffee grounds in the garden and why they shouldn't be added to your houseplants. 

Coffee Grounds As A Soil Amendment 

Coffee grounds are packed with nutrients and full of organic matter. They are often erroneously referred to as a fertilizer instead of what they are, a soil amendment. "Coffee grounds are certainly useful as a soil amendment—that is, something added to garden soil to improve its properties," says Elizabeth Murphy, author of Building Soil: A Down-To-Earth Approach: Natural Solutions for Better Gardens & Yards. "Although coffee grounds contain Nitrogen and minor amounts of other macro and micronutrients, their nutrient content is variable and, in many cases, they cannot be applied in sufficient quantities to provide all of a plant’s fertility needs; I wouldn’t consider them as a reliable fertilizer."

According to Matthew Evans, author of SOIL: The Incredible Story of What Keeps the Earth, and Us, Healthy, "Coffee grounds are made of organic material, lots of cellulose, hemicellulose and lignin, which break down at differing rates, making it a slow release soil amendment." Halleck adds coffee grounds need to be decomposed by microbes first to release nitrogen and other nutrients.

Can I Add Coffee Grounds Directly To My Soil?

When you're preparing the soil before planting, it's common to add a soil amendment, such as compost, humus, or manure, to boost the nutrients in the soil. A small amount of coffee grounds can be added directly to the soil. "Coffee grounds can be used in moderate amounts directly to the soil around plants," says Evans. Wondering how much to add to your soil? "Don’t have it more than about 1-2 cm (.4-.7 inch) thick and only have a band about 3 cm (1.1 inch) wide," he says.

You also don't want to put coffee grounds directly in contact with stems or foliage. Evans suggests leaving a small gap, around half an inch or so between your plants and the coffee grounds, to avoid any mold from growing.  

Will Coffee Grounds Change The Soil pH?

There are a lot of misconceptions that because coffee is acidic, if you add coffee grounds to the soil it will increase the acidity but experts explain this isn't correct.  "Contrary to popular belief, once brewed coffee grounds are close to neutral in pH, in the 6.5-6.8 pH range, so you're not getting an acidifying effect," explains Leslie F. Halleck, author of Tiny Plant and Certified Professional Horticulturist. "Also, certain types of soils that are buffered—both high in pH and alkalinity (carbonates) are difficult to 'acidify' even with truly acidic products." Knowing your soil's pH or whether it's alkaline, neutral, or acidic is important because plants require different nutrients and soil types to grow successfully.

How To Use Coffee Grounds In The Garden

If your plants are already established in the garden, coffee grounds can be applied. Make sure you only add a small amount of used coffee grounds. "With coffee grounds, moderation is the key, and watching your plants' response is a good rule of thumb," says Murphy. "A small amount can give plants an in-season Nitrogen boost, while too much may negatively affect soil and plant life."

Nitrogen is an important mineral for plants. Used coffee grounds that aren't decomposed won't be putting nitrogen into the soil, though and could be removing it. "Remember that nitrogen gets pulled out of the soil during the decomposition cycle," explains Halleck. If you're adding a lot of coffee grounds or other organic matter to the soil that hasn't decomposed, you're likely causing the opposite effect—providing less nutrients than you think. "Putting too much under-composed organic matter in your beds can actually cause nitrogen deficiencies in your plants," says Halleck.

Applying coffee grounds directly to seedlings or to soil where you're sowing seeds isn't advised. "Take caution with young plants and seedbeds, as grounds can inhibit germination and root growth," says Murphy. The best way to ensure you're reaping all the nutrients from coffee grounds is to have them break down first by placing them in the compost pile.

Why It's Best To Add Coffee Grounds To The Compost Pile

One of the best ways to use your leftover coffee grounds is to toss them in your compost pile. You reduce organic matter from ending up in the landfill where they'll contribute to methane production and you create a usable, nutrient-rich amendment to use throughout your garden. 

Compost needs a mixture of carbon or "browns" and nitrogen "greens" to break down properly. Browns are leaves, newspaper, shredded wood and greens include fruit and vegetable scraps, eggshells, grass clippings, and coffee grounds. "Coffee grounds benefit compost by adding Nitrogen, a “green layer,” to help heat up the pile," explains Murphy. Coffee grounds also stabilize nutrients for better distribution in the final product, she adds..

The goal is to create layers of greens and browns so everything in the compost pile can decompose. "To put it very simply, you just want to achieve a 1:3 ratio of greens to browns, by volume," says Halleck. "Too much nitrogen or too much carbon can stall your compost decomposition." Size also influences how quickly or slowly the compost takes. "The larger the pieces are, the longer the composting process takes, so a preponderance of coffee grounds does make the compost cook faster," says Deborah Barber, Director of Land Conservation, The Nature Conservancy Maryland/DC. "The finished compost provides nutrients, beneficial fungi, moisture retention and that formerly sterile soil is growing thriving plants now." 

How Often Can You Add Coffee Grounds To the Pile?

"If you're dumping a lot of coffee grounds into the compost every day, then you need to balance it out with browns, such as dried leaves, dried grass clippings, newspaper, wood ash, etc," says Halleck. Soil Science Advisor, Niamke Shropshire-Boykin for Rosy Soil adds, "There is no such thing as too many coffee grounds if you have the correct ratio of carbon in your compost pile."

Once the compost is ready, you can incorporate it into your raised beds, containers, or even houseplants. "If coffee grounds are just one ingredient of a well-rounded compost, use as you would any other compost," says Murphy. "Apply continuously as a mulch or work in a roughly ½ to 1- inch layer to the soil before spring planting." 

Can You Add Coffee Grounds To Your Houseplants?

When you have a surplus of coffee grounds, you may be tempted to mix them into your houseplants' soil, especially if you spend a lot of time watching "garden hacks" on social media. Think again. "Do not dump coffee grounds in your houseplant pots or outdoor patio pots (or egg shells or banana peels for that matter)," says Halleck. The nutrients in the coffee grounds aren't broken down and your indoor plants won't be able to reap any of the benefits. "You need microbes to break down that organic matter," says Halleck. "In mostly inert houseplant potting mix, all you're going to do is grow mold and fungus gnat farms!"