Should You Use Coffee Grounds for Plants? There Are Pros and Cons

Many creative ways are shared online for using spent coffee grounds to fertilize plants. However, DIY garden remedies and anecdotal recommendations are not always grounded in fact. When it comes to benefiting houseplants, we dug into the science to understand the pros and cons of using coffee grounds as fertilizer. Read on for expert tips on using coffee grounds for plants and how you should use them (if at all).

Are Coffee Grounds Good for Houseplants?

Studies have shown both positive and negative impacts of using coffee grounds on plants. While coffee grounds contain nutrients that can benefit plant growth, they also contain chemicals that inhibit growth in certain plants. The effects of using coffee grounds on plants also vary depending on their use. "Coffee grounds are best indirectly applied to houseplants so as not to cause mold to develop by [directly adding] the coffee grounds [to the plant soil]," says gardener LeAura Alderson.

“When they are used properly, coffee grounds supply nutrients and provide other benefits that increase plant growth," says Horticulturist Linda Chalker-Scott. "In general, only composted coffee grounds should be worked in as a soil amendment" for plants. Composted coffee grounds also contribute organic matter to soils, which helps improve soil structure and promotes the movement of air and water through the soil profile.

Houseplants That Like Coffee Grounds

Through personal research, Alderson found the following plants responded positively to coffee grounds. “The list…is based on those [plants] that prefer—and/or can tolerate—acidity at or below the pH neutral of 7.0,” explains Alderson. It includes:

• African violet (Saintpaulia spp.)
• Aloe vera (Aloe barbadensis)
• Azaleas (Rhododendron spp.)
• Bromeliads (Bromeliaceae)
• Christmas cactus (Schlumbergera spp.)
• Golden pothos (Epipremnum aureum)
• Jade (Crassula ovata)
• Peace Lily (Spathiphyllum spp.)
• Persian cyclamen (Cyclamen persicum)
• Philodendron (Philodendron bipinnatifidum)
• Roses, miniature (Rosa chinensis minima)
• Snake plant (Sansevieria trifasciata)
• Spider plant (Chlorophytum comosum)

Benefits of Using Coffee Grounds on Plants

First, it is important to point out that we are discussing the use of spent coffee grounds, which have already been used to make coffee. This is important because fresh and used coffee grounds, as well as brewed coffee, all have different properties. For example, brewed coffee is highly acidic, but spent coffee grounds are not. Let’s look at the nutrients found in spent coffee grounds and what benefits they can have on plants.

Contain Nutrients

After coffee has been brewed, the remaining grounds contain a mixture of proteins, oils, and carbohydrates that were not extracted by the water. Nitrogen-rich proteins make up about 10 percent of the spent coffee grounds—and plants use a lot of nitrogen. Coffee grounds also contain potassium, trace amounts of phosphorous, and micronutrients utilized by plants, including calcium, iron, magnesium, and zinc.

Contribute to Healthy Soil

While coffee grounds contain beneficial nutrients, plants can't utilize them straight from the grounds. The coffee grounds must first be broken down through composting or natural decomposition. Through these processes, soil microbes transform elemental nutrients found in coffee grounds and other organic materials into compounds usable by plants.

Suppress Disease

Some research has demonstrated that coffee grounds used as part of a compost mix suppressed some disease organisms in experiments conducted with vegetable crops. Diseases impacted included common fungal rots and wilts, such as Fusarium, Pythium, and Sclerotinia species. However, disease suppression from coffee grounds has not been studied on houseplants or ornamental plants in a garden setting.

Help the Planet

By reusing spent coffee grounds for plants, you're helping reduce food waste. You can reuse coffee grounds from brewing coffee at home or collect used coffee grounds from local coffee shops. This is a great way to gather large amounts of coffee grounds to use in your garden.

Drawbacks of Using Coffee Grounds on Plants

In addition to the potential benefits of using coffee grounds on plants, studies have revealed detrimental impacts associated with this practice. While most of these challenges have been observed only on crop or landscape plants, they may also apply to houseplants.

Reduce Plant Growth

Just as caffeine has positive and negative effects on humans, the same is true for plants. A study investigating the impact of the direct application of spent coffee grounds to several plants in field tests resulted in reduced weed growth—which we can all celebrate—but also reduced plant growth. Unfortunately, few houseplants have been directly tested.

Poisonous to Some Plants

One of the many suggested uses for spent coffee grounds is top-dressing the soil of houseplants. As Chalker-Scott explains, fresh grounds are demonstrably phytotoxic (poisonous) to various plants, so their use as an amendment or mulch is not recommended.

Create a Moisture Barrier

Another reason to avoid direct application of coffee grounds is that the finely cut grounds tend to compact, creating a moisture barrier across the soil surface. This can encourage fungal growth and reduce airflow to the plant roots.

Harm Good Bacteria

Though adding coffee grounds to plant soil might help prevent harmful bacteria, it could also harm beneficial bacteria. Without good bacteria, plants may be exposed to disease and pests.

How to Use Coffee Grounds on Plants

Research suggests that adding spent grounds to your compost tumbler or compost bin and then using the finished compost is the best approach for using coffee for plant fertilizer. According to Chalker-Scott, compost composed of 10 to 20 percent coffee grounds has been reported as the most effective.

Gardeners have long used composted coffee grounds to amend garden soils—a practice that has many benefits—but how do we use this compost on potted plants? A popular recommendation is to make fertilizer tea from composted coffee grounds and use this to feed houseplants.

However, Chalker-Scott says, “...there is no science supporting the use of compost tea for anything. Compost is great; [however], the water that leaches through it is a very weak fertilizer, and that’s about it.”

Adding Coffee Grounds to Potted Plants

When it comes to houseplants, just as in the garden, the greatest benefit comes from using finished compost directly as a soil amendment or mulch. A simple way to use finished compost is to add a thin layer on top of the potting soil in containers.

Adding coffee-infused compost to houseplants can be done one to two times a year and is a good way to add additional compost between repotting plants. This method doesn't result in the same challenges as when coffee grounds are applied directly to top-dress soils because the composted material is lighter, does not compact the same way, and allows ready movement of air and water.