What an expert wants you to know before putting coffee grounds on your plants

Cut through the bad advice and learn how to use coffee grounds properly to help your plants thrive.

Key Points

• Do not add coffee grounds directly to soil but use them in compost, where they decompose and release nutrients.
• Coffee grounds do not significantly lower soil pH or act as an effective fertilizer; use specialized products to acidify soil.
• Overusing coffee grounds, especially in potted plants, can harm roots, but they can improve soil structure in clay soils.

You might have heard of gardeners adding coffee grounds to indoor and outdoor plants to make them thrive, or you may have seen coffee shops giving away bags of their used grounds for customers to use in the garden. Coffee grounds are touted as a natural fertilizer and organic soil amendment. But is that really true? We asked an expert to sort out fact from fiction.

Meet Our Expert

Chelsea McKinley is a plant health care specialist at the U.S. Botanic Garden in Washington, DC.

Using Coffee Grounds for Plants

Coffee grounds are not beneficial to plants, at least not when you put them directly on your plants, says Chelsea McKinley, plant health care specialist at the U.S. Botanic Garden in Washington, DC.

That’s not to say used coffee grounds have no value to a gardener. They do, just not as a fertilizer or a soil amendment. “The best way to use coffee grounds in gardening is to compost them,” McKinley says.

“You don’t get the full potential out of coffee grounds by mixing them into the soil,” she says. “You’ll get the full potential out of them if you properly compost them and then use that compost to fertilize your plants because then the nutrients are more readily available and in proportions more appropriate for plant growth.”

Coffee grounds are considered a “green” material when used in compost, based on their carbon-to-nitrogen ratio. Mix the grounds with two to three times as much “brown” material in your compost bin to achieve the right balance for an efficient decomposition process. In a few months, all your grounds (you can throw in paper coffee filters too), along with dried leaves, grass clippings, and kitchen scraps, will turn into nutrient-rich compost that will benefit your plants.

Do Coffee Grounds Acidify Soil?

Another claim floating around online is that coffee grounds are a good soil amendment for acid-loving plants because they lower the soil pH, which is desirable for plants like azaleas, blueberries, and hydrangeas.

That’s not true, McKinley says. “Coffee grounds have a pH between 5.5 and 6.5,” she says. “Most plants like a soil pH between 5.8 and 6.8, so coffee grounds won’t lower the pH significantly.” It works about as well as mulching with pine bark.

If you want to lower your soil pH, use a fertilizer designed to lower soil pH, McKinley says. “That’s a faster way to acidify soil.”

Are Coffee Grounds Good for Houseplants?

While used coffee grounds contain a little nitrogen, an element that plants need to grow, the nitrogen isn’t in a form that’s easy for plants to use. Plus, it’s easy to overdo it.

“If you add too many coffee grounds to your potted plant, it’s like overfertilizing them, and you burn the roots of the plant,” McKinley says. “Burned roots can lead to dead plants.” 

Potted plants are especially sensitive to dramatic changes in nutrient levels because they’re growing in an artificial environment with limited space, McKinley says. “I wouldn’t put coffee grounds on potted plants.”

Do Coffee Grounds Improve Soil?

It’s true that coffee grounds can improve soil structure when put directly into garden soil, McKinley says. Soil structure affects how much water and air move through the soil. Soil with good structure is well-drained and holds nutrients vital to plant health.

“If you have heavy clay soil, adding any organic amendment, even coffee grounds, can help,” she says. “Microbes in the soil will break down the coffee grounds eventually, but you’re not going to get the full potential from your coffee grounds unless you compost them.”

Frequently asked Questions

Why should I not add coffee grounds directly to soil?

Many plants, such as tomatoes and rhododendrons, tend to respond poorly to direct exposure to coffee grounds, but using compost that contains coffee grounds causes no problems.

Can I use coffee grounds in the soil when I plant seeds?

Coffee grounds inhibit the germination rates of some seeds while enhancing the germination rates of others. To stay safe, use compost that contains coffee grounds instead of direct coffee grounds.

Read the original article on Better Homes & Gardens