Here's What You Should Know About Using Coffee Grounds on Houseplants

Thinking about sharing your Morning Joe with your houseplants? The internet abounds with creative ways to use 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 better understand the pros and cons of using the different methods to fertilize or enrich them. So, keep reading to find out why some people use coffee grounds on houseplants and how (if at all) you should use them on your own plants at home.

Are Coffee Grounds Good for Houseplants?

The answer is complicated. 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 can inhibit growth in certain plants. The effects of coffee grounds on plants also vary depending on how they're used. “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 LeAura Alderson, author and editor of GardensAll.com.

Let’s look at the pros and cons of using coffee grounds to supplement plants and identify the best way to make use of spent grounds.

Benefits of Coffee Grounds on Plants

First, it is important to point out that we are discussing the use of spent coffee grounds, those that 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 itself is highly acidic, but spent coffee grounds are not. We’ll discuss that aspect in detail below, but first let’s look at the nutrients found in spent coffee grounds, and what benefits they can have on plants. 

Coffee Grounds 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 grounds—and plants use a lot of nitrogen. Coffee grounds also contain potassium, and trace amounts of phosphorous, as well as micronutrients utilized by plants, including calcium, iron, magnesium, and zinc. 

Coffee Grounds Contribute to Healthy Soil

While coffee grounds contain beneficial nutrients, plants can't utilize those nutrients straight from the grounds themselves. 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.

In her article “Using Coffee Grounds in Gardens and Landscapes,” Dr. Linda Chalker-Scott, urban horticulturist at Washington State University notes, “When they are used properly, coffee grounds supply nutrients and provide other benefits that increase plant growth." She adds that, "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, promoting movement of air and water through the soil profile.

Disease Suppression

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. Disease suppression from coffee grounds has not been studied on houseplants or ornamental plants in a garden setting. 

Challenges of Using Coffee Grounds on Plants

In addition to the potential benefits of using coffee grounds on plants, studies have also 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.

Coffee Grounds Can Reduce Plant Growth

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

Challenges of Top Dressing with Coffee Grounds

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 a variety of plants, so their use as an amendment or mulch is not recommended. 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 fugal growth and reduce air flow to the roots. 

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 itself 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,” writes Chalker-Scott. So, she adds, “don’t assume coffee grounds will make an acidic compost.” Studies investigating the impact of spent coffee grounds on soil pH have shown that they do not consistently lower pH. In fact, the resulting pH of composted coffee grounds has varied considerably from one study to the next. This research has not been conducted with potting soil, though we can assume similar results.

How to Use Coffee Grounds on Houseplants

Pulling this information together, we can identify an approach to making the best use of spent coffee grounds for our houseplants. The research referenced above suggests that adding spent grounds to your compost tumbler or compost bin and then using the finished compost is the best approach to making use of this organic material. According to Chalker-Scott, utilizing compost with a composition of 10 to 20 percent coffee grounds has been reported as 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, Dr. Chalker-Scott said, “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.”

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 per 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 in 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 used as part of a potting mix to stimulate soil health and plant growth. Compost loosens soils and minimizes compaction, improving drainage and aeration, and increases the nutrient-holding capacity of your soil. Potting mixes can be amended with up to 30 percent finished compost.

Which Houseplants Benefit from Composted 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.

In her research, Alderson found the following plants listed as responding 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,” says Alderson. With limited research, our best approach is to utilize these suggestions passed on by others to guide our own experimentations with composted coffee grounds.

Houseplants that Like Coffee Grounds

• 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, particularly on houseplants, we can learn from the documented research and observations that we do have. Plus, our own experiences and at-home experimentation can further inform the best methods of utilizing coffee grounds to enrich houseplants. So, if you're up for a challenge, and want to see what coffee grounds can do for your own plants, be sure to make note of all the caveats shared above, and, for the best results, opt for composted coffee grounds.