What Adding Coffee Does to Your Plants, According to Experts

Pouring your morning coffee leftovers into your favorite houseplant or sprinkling used coffee grounds around the soil of your vegetables is a nugget of homespun growing advice that gardeners have been passing down for generations.

But does it really work to boost the health of your plants? Some say that coffee's benefits to your plants are hard to quantify. Ahead, we asked experts to break down the benefits, risks, and best techniques for adding coffee to your plants.

Coffee Has Nutrients That Are Good for Plants

Coffee grounds contain several nutrients that make it a favorite soil amendment for gardens, including nitrogen, potassium, magnesium, and copper, says Pearsoll. "Brewed coffee, although less nutrient-dense, still contains some amounts of these elements," he says.

• Nitrogen: "A vital macronutrient for plant growth, nitrogen is crucial for the synthesis of proteins and other essential plant compounds," says Pearsoll.
• Potassium: "It's essential for plants' overall health, participating in processes like photosynthesis, nutrient uptake, and water regulation," Pearsoll says.
• Magnesium: "It's a core component of chlorophyll, helping plants to photosynthesize," says Pearsoll.
• Copper: "Although required in smaller amounts, it plays a role in enzymatic functions and protein synthesis," Pearsoll says.

The Benefits of Using Coffee in Your Garden

When you add ground or brewed coffee to your garden, you can boost the quantities of those essential nutrients within your soil—though some of Brewer's research and literature reviews have shown the percentage increase of those nutrients is relatively small. "What [the coffee] is providing is a carbon source for soil microbes," she says. You may not be able to tie that to brighter flowers or tastier tomatoes this season, but over time, the "microbial glues" left behind by these bacteria and fungi can improve soil structure, reduce erosion, and retain nutrients, she says.

Brewer has also identified research that points to coffee as a successful slug deterrent, working more effectively than traditional 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 strong enough to push 100 percent of the slugs in the study away from the treated soil.

The Risks of Using Coffee on Plants

In low concentrations, coffee likely won't have any major adverse effects on your garden. However, cautions Pearsoll, you shouldn't use it in place of fertilizer. "Coffee grounds can be a good supplement to a well-rounded fertilization plan," he says. "They should not be relied upon as a standalone fertilizer because they lack certain nutrients, like phosphorus and calcium, that plants also need."

Raises Soil Acidity

Conventional gardening wisdom points to coffee as a way to lower the pH of your garden; though Brewer says these effects are often temporary, Pearsoll notes that raising the acidity of the soil can cause problems for plants that thrive in neutral or alkaline soil.

Causes Dry Soil

If you don't properly incorporate the grounds into the soil, you can force your garden to dry out. "Coffee grounds can also create a dense layer on the soil surface that might impede water infiltration if not properly mixed into the soil," says Pearsoll.

Slow Germination and Growth

Brewer also notes that previous research has connected higher levels of caffeine with lower levels of seed germinations and slower plant growth.

How to Use Coffee Grounds or Brewed Coffee in Your Garden

Figuring out the proper amount of coffee to add to your garden allows you to experience the benefits without the potential risks. "The 'right' amount of coffee to use can depend on your soil's existing nutrient levels and pH, 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 any potential harm to your plants."

Pearsoll recommends using coffee grounds in your garden over brewed liquid. "Used grounds are best because most of the acidity has been leached out in the brewing process," he says. "They can be sprinkled on the soil surface or mixed in lightly. If you're mixing it in, the top few inches of soil would be enough."

Though you can use any type of bean or flavored grounds in your garden, make sure you aren't using leftovers from a cup mixed with milk or sugar. "They can attract pests and create a breeding ground for harmful bacteria and mold," he says.

If you are using brewed coffee, allow it to cool and then pour it onto the soil, not the leaves (no need to water it down). Take this moisture into account when considering the overall watering needs of your plant.