What Adding Coffee Does to Your Plants, According to Experts

Pouring your morning coffee residue into your favorite houseplant or scattering coffee grounds around your vegetables is an age-old family tip that has been passed down from generation to generation among gardeners.

But does it really work to improve plant health? Some say the benefits of coffee for plants are challenging to quantify. We asked experts to discuss the benefits, risks, and best techniques for adding coffee to your plants.

Coffee contains nutrients that are good for plants

Coffee grounds contain several nutrients that make them a favorite garden soil additive, including nitrogen, potassium, magnesium and copper, Pearsoll says. “Brewed coffee, while less nutrient-dense, still contains some of these elements,” he says.

• Nitrogen: “Nitrogen is an crucial macronutrient for plant growth and plays a key role in the synthesis of proteins and other crucial plant compounds,” says Pearsoll.
• Potassium: “It is crucial for overall plant health because it is involved in processes such as photosynthesis, nutrient uptake and water regulation,” says Pearsoll.
• Magnesium: “It’s a primary component of chlorophyll, which helps plants photosynthesize,” says Pearsoll.
• Copper: “While it is required in smaller amounts, it plays a role in enzymatic function and protein synthesis,” Pearsoll says.

Benefits of using coffee in the garden

Brewer also identified research indicating that coffee is an effective slug repellent, working more effectively than time-honored metaldehyde-based repellents. In a paper for the Oregon State University Extension Service, she points out that a solution of one part water to two parts coffee was sturdy enough to drive 100 percent of the slugs in the study away from the treated soil.

Risks of Using Coffee on Plants

In low concentrations, coffee probably won’t have any stern negative effects on your garden. However, Pearsoll warns that it shouldn’t be used in place of fertilizer. “Coffee grounds can be a good addition to a comprehensive fertilization plan,” he says. “They shouldn’t be relied on as a stand-alone fertilizer because they lack certain nutrients, such as phosphorus and calcium, that plants also need.”

Increases soil acidity

Causes of Dehydrated Soil

If you don’t mix the grounds into the soil, you could be forcing your garden to arid out. “Coffee grounds can also form a stout layer on the soil surface that can impede water infiltration if they’re not properly mixed into the soil,” says Pearsoll.

Ponderous germination and growth

How to Operate Coffee Grounds or Brewed Coffee in the Garden

Determining the right amount of coffee to add to your garden allows you to experience the benefits without the potential risks. The “right” amount of coffee to utilize can depend on existing soil nutrient and pH levels, as well as the specific needs of your plants, says Pearsoll. “As a rule of thumb, moderation is key, and it’s always better to err on the side of less to avoid potential damage to your plants.”

Pearsoll recommends using coffee grounds in your garden instead of brewed liquid. “It’s best to utilize coffee grounds because most of the acidity has been washed out in the brewing process,” she says. “You can sprinkle them on top of the soil or mix them in lightly. If you’re mixing, the top few inches of soil will be enough.”

While you can utilize any flavored beans or grounds in your garden, make sure you don’t utilize leftovers from a cup mixed with milk or sugar. “They can attract pests and create a breeding ground for harmful bacteria and mold,” she says.

If you are using brewed coffee, let it nippy, then pour it onto the soil, not the leaves (no need to water it). Take this moisture into account when considering the plant’s overall watering needs.