Using Coffee Grounds for Plants—the Pros and Cons

See how reusing coffee grounds on plants can help fertilize your garden.

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 (forgive the pun). When it comes to benefiting houseplants, we need to dig into the science to understand the pros and cons of using the different methods to fertilize or enrich them. Read on for expert tips on using coffee grounds for plants and how (if at all) you should use them.

Meet the Experts

• LeAura Alderson is a plant enthusiast, author, and editor of GardensAll.com.

• Linda Chalker-Scott, Ph.D. is an urban horticulturist and professor at Washington State University.

Are Coffee Grounds Good for Houseplants?

Yes, and no. 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.

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. We’ll discuss that aspect in detail later, but first, 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.

Horticulturist Linda Chalker-Scott notes, “When they are used properly, coffee grounds supply nutrients and provide other benefits that increase plant growth." Chalker-Scott adds, "...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.

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.

Helps the Planet

By reusing spent coffee grounds for plants, you're helping reduce 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.

Challenges 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.

Do Coffee Grounds Lower Soil pH?

As we separate fact from fiction, let’s look at recommendations for using coffee grounds to lower soil pH. Although coffee is quite acidic, the grounds that remain after brewing are not considered acidic. This is because the acids in coffee are water soluble and end up in your mug, not the grounds.

“The pH of decomposing coffee grounds is not stable, and one shouldn’t assume that it will always, or ever, be acidic...[so] don’t assume coffee grounds will make an acidic compost,” writes Chalker-Scott.

Studies investigating the impact of spent coffee grounds on soil pH have shown a lower pH level. However, the resulting pH of composted coffee grounds has varied considerably from one study to the next. Though this research has not been conducted with potting soil, we can assume similar results.

How to Use Coffee Grounds on Houseplants

Pulling this information together, we can identify the best use of spent coffee grounds for 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 this organic material. 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. But, 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.

This 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.

Adding finished compost to potting mixes is another great way to reap the benefits of composted coffee grounds. Finished compost can be integrated with potting mix to stimulate soil health and plant growth. Compost loosens soils and minimizes compaction, improving drainage and aeration, and increasing the nutrient-holding capacity of your soil. Potting mixes can be amended with up to 30 percent finished compost.

Plants That Benefit from Coffee Grounds

You can find endless lists regarding which plants benefit from coffee grounds and which do not. However, many of these lists are built upon the assumption that coffee grounds are acidic, which, as discussed previously, used coffee grounds aren't, particularly once composted.

On the other hand, many of these recommendations come from gardeners applying fertilizer teas made directly from spent coffee grounds and not composted coffee grounds. Because teas are made by soaking uncomposted grounds in water, which we know extracts acids, “it’s logical that there would be some residual coffee acidity,” says Alderson.

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, and 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)

While gray areas certainly remain regarding the benefits of using coffee grounds for fertilizing, particularly on houseplants, we can learn from the documented research and observations available. Plus, our own experiences and at-home experiments can further inform the best methods of utilizing coffee grounds to enrich houseplants. If you're up for a challenge and want to see what coffee grounds can do for your plants, be sure to note all the caveats shared above, and, for best results, opt for composted coffee grounds.

Frequently Asked Questions

Which plants do not like coffee grounds?

Some plants do not benefit from adding coffee grounds to their soil. Plants like lilacs and lavender prefer alkaline soil, so the slight acidity will harm them. The caffeine in coffee grounds can also harm some herbs and geraniums. Be sure to research the optimal conditions for your plants before adding coffee grounds.

What bugs or pests will coffee grounds repel?

In addition to fertilizing your plants, coffee grounds can also be used as a pest repellent. Many bugs or pests will shy away from the caffeine and texture of the coffee grounds. Some common pests you can help deter by using coffee grounds on plants include bees, wasps, slugs, fleas, mosquitoes, ants, foxes, and cats.

Can you compost coffee filters?

Depending on the coffee filter material, you may be able to compost it along with the coffee grounds. You can compost coffee filters made from paper or other compostable materials. Do not compost those made of synthetic materials (like nylon or plastic).

Related: The 40 Most Popular House Plants for Your Home

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