How to Use Coffee Grounds to Enhance Your Garden, According to Horticulturists

If you start each day with a cup of joe, then you know how magical coffee beans can be. What you might not know is that coffee grounds also offer a host of benefits for your garden, from providing much-needed minerals to helping with soil structure and deterring pests. 

But before you start serving up fresh java to your beloved plants, it’s important to understand all of the intricacies involved—including why it works, how to administer the grounds, and some common mistakes to avoid. Here, we spoke to experts about how your cherished morning cup can perk up your plants. 

Plants That Thrive With Added Coffee 

“Plants that thrive in more acidic soil can benefit from freshly brewed coffee grounds,” says Elizabeth Jones, a seed librarian at the Jefferson Public Library. “Some of these include blueberries, tomatoes, carrots, fennel, beans, beets, aster, and blue and pink hydrangeas.”

Maricel Gentile, the renowned chef and ingredient expert who owns Maricel's Kitchen, has seen success with leafy green and fruiting plants. “Tomatoes like nitrogen early in the season, and the coffee seems to give them a boost,” she says. “My peppers, lettuce, spinach, and carrots also seem to do well.”

However, avoid adding fresh coffee grounds to plants that prefer alkaline soil. “Some examples include lavender, geraniums, snapdragons, rosemary, and most of the Mediterranean herbs,” says Howe.

Using Coffee Grounds in Compost

Coffee grounds can be a great compost ingredient. Shane Brill, the campus garden director at Washington College, collects the grounds and mixes them with other organic materials for compost, balancing nitrogen-rich items like coffee with carbon-rich materials like dried leaves or straw.

“Layer the coffee grounds in your compost pile or bin, turning the mixture occasionally to keep it aerated,” he says. “It’s important to avoid overusing coffee grounds in the compost—they should not exceed one-fifth of the total volume. Be sure to turn the pile once a week and monitor moisture levels, keeping the pile damp but not soggy. The finished compost will be ready in about three to six months.”

Using Coffee Grounds as a Targeted Soil Amendment

Coffee grounds can be scattered lightly around plants to help suppress weeds and deter slugs, according to Brill. “It feeds soil microbes that can enrich the structure and improve drainage,” he says. “But rather than viewing coffee grounds as a standalone fertilizer, we can use it thoughtfully as a targeted soil amendment.”

Using Coffee Grounds for Fertilizer

Dried coffee grounds: Gentile dries used coffee grounds on a tray in the sun, then directly sprinkles them in her raised beds and containers as fertilizer. “Wet coffee grounds clump together and crust up, and don't let the nutrients seep out,” she says. 

Gentile lightly scatters the grounds around the base of her plants but avoids the stems. “I gently work the grounds into the soil, just using my fingers to lightly rake them into the top inch or so,” she says. “Garden gloves are very handy, and I always water to help the nutrients seep into the soil. I like to think I'm giving my plants a nice drink of coffee.”

Liquid fertilizer: You might also choose to toss your morning coffee grounds into a half-gallon bucket of water. “Let this sit overnight and drain, then this water is ready to fertilize your garden,” says Juliet Howe, horticulturist and the founder of Twigs Design.

Mixed in potting soil: You can also mix grounds directly with soil, says How. “Simply mix 10 parts potting soil with one part coffee grounds for a built-in boost to your potting soil.”

Why pH Matters

Soil is made up of various minerals, water, gases, organic matter, and microorganisms. All of these factors play a role in pH—a metric that determines how acidic or alkaline your soil is. pH can impact plant growth and can vary widely across different areas of the country—or even in different sections of your garden.

Therefore, it's important to test your soil’s pH every few years so you know exactly what you're working with. Before you start growing, you should also learn what type of pH level each plant prefers. 

So, what does pH have to do with coffee? Fresh coffee grounds are acidic, and conventional wisdom says that they can very slightly lower the soil's pH level. Brewed coffee grounds, meanwhile, are nearly neutral, notes Jones.

“For plants that are very sensitive to acidity, coffee grounds aren't your best choice,” she says. “However, repeated heavy application may slightly lower soil pH over time.”

Mistakes to Avoid

• Fertilizing too frequently. Too much fertilizer can't be absorbed by the soil. “It evaporates, leaving salts behind,” Howe says. “This high sodium content can damage roots, causing brown or yellow leaves.” 
• Not testing your soil. Experts recommend pH testing every three years or so to understand what nutrients you might need.
• Over-caffeinating your plants. “Used coffee grounds are preferred over fresh ones,” advises Jones. “They're less acidic, and they won't shock plants with excessive caffeine content.”
• Applying too closely. Adding coffee grounds too close to the root or stem may burn the plants, according to Jones. You also want to avoid sprinkling the coffee directly on the top of the plant.
• Harming young plants. “Always be mindful of applying coffee to very young plants or seedlings, as it may stress their delicate roots,” says Brill.