Coffee Grounds for Plants

You’ve heard that adding coffee grounds to your compost speeds decomposition and adds much-needed nitrogen to the compost. Coffee grounds are good for plants in the garden. We explain why and identify specific vegetables, plants and flowers that can benefit.

In a related article, read about coffee grounds and other techniques to improve soil.

Coffee grounds as mulch

Coffee grounds by weight contain about 10% nitrogen-rich proteins, which are vital for seed germination and plant growth. In this way, coffee grounds are good for plants.

“The carbon-to-nitrogen ratio of coffee grounds can be as low as 11:1, an ideal ratio for plant and soil nutrition. Since coffee is extracted in water, most of the hydrophobic compounds, including oils, lipids, triglycerides, and fatty acids remain in the grounds, as do insoluble carbohydrates like cellulose and various indigestible sugars. Structural lignin, protective phenolics, and the wonderful aroma-producing essential oils are also left over from the brewing process. It’s this last group of chemicals that are reported to have antioxidant and antimicrobial properties.”

According to Linda Chalker-Scott, Extension Urban Horticulturist and Associate Professor, Puyallup Research and Extension Center, Washington State University:

Coffee grounds are highly acidic, they will lower the pH of your soil, making it more acidic. As a consequence, they can be beneficial to acid-loving plants.

Coffee grounds can contribute nitrogen to soil. Nitrogen is a key nutrient for plant growth and development. Coffee grounds help repel slugs and snails.

In scientific studies, adding coffee grounds has shown varying results, from creating a mildly acidic finished compost of 4.6 to an alkaline compost with a pH of 8.4(!). When composted coffee grounds were added directly to garden soil, researchers found that the pH of decomposing grounds was not stable. After showing an initial increase in acidity levels, the soil pH decreased shortly thereafter.

Plants That Like Coffee Grounds

Blueberries, strawberries, carrots, radishes, roses, hydrangeas, rhododendrons, azaleas, lilies and hostas are some plants which benefit from more application of coffee grounds. Here’s a round up:

• For blueberry and strawberry plants, sprinkle 4 or 5 cups of coffee grounds around the ground beneath each blueberry bush, then rake the coffee grounds into the top layer of soil. This can be done at any time of day and can be repeated every two to four weeks or as necessary.
• Tubers such as carrots and radishes flourish well in coffee grounds. Mixing coffee grounds with soil at the planting process helps in the production of strong tubers.
• Sprinkle coffee grounds in small quantities around your rose bushes.
• Mixing it directly into the soil can help balance alkaline soil or give a boost of acidity for plants that prefer a lower pH, like hydrangeas or rhododendrons.
• If you are planting azaleas, place coffee grounds in the hole before inserting the plant.
• Hostas will benefit from an application of coffee grounds used as mulch because of their relatively high nitrogen content, but you need to use the grounds judiciously. Too much coffee grounds spread around Hostas can form an impermeable layer that hinders water and air from reaching the roots.
• Because nitrogen supports green growth, using coffee grounds as compost around trees and shrubs encourages them to grow lush and leggy. … Maintaining a regular fertilizing schedule in addition to composting can keep your trees and shrubs healthy, blooming and, in some cases, fruiting.
• You can sprinkle coffee grounds evenly over your lawn. Rake them for good coverage and contact with the soil.

Plants That Do Not Like Coffee Grounds

Plants that do not like coffee grounds are plants that do not like acidic soil, including tomatoes, lavender, pothos, rosemary, and orchids.

It is not a good idea to spread coffee grounds across your gardens. If applied too thickly, coffee grinds might actually injure seedling roots by impeding growth. Use coffee grinds solely on certain plants.

Positive Results and Nutrients

Coffee grounds have shown substantially positive results. As with any good organic mulch, coffee grounds:

• moderate soil temperature and moisture in the soil.
• Bind pesticide residues and toxic heavy metals such as cadmium. As a consequence, they preventing their movement into the surrounding environment (very beneficial in keeping these pollutants out of local waterways).
• Can increase the availability of important plant nutrients like nitrogen, phosphorous, iron and zinc.

In one recent study, researchers found that Starbucks’ coffee grounds contain .6% potassium, .06% phosphorous and 2.28% nitrogen. There was also a bit of copper and a trace of magnesium in the grounds. When applied as a soil amendment, the coffee grinds provided some nutrients right away and released some gradually. This makes them a good choice to give your plants an immediate boost along with ongoing nutrition.

Note that acidic levels are affected by the variety of coffee bean combined with the brewing method.

Should you add fresh coffee grounds to garden soil?

Bacteria and fungi break down coffee grounds when added directly to garden soil. As a result, decomposition creates humic substances. Earthworms pull coffee grounds into the soil and use them as a food source which improves soil structure.

Nitrogen is the primary ingredient for plant growth and coffee grounds contain a high proportion of nitrogen. But not every plant responds well to them. In fact, fresh grounds can be poisonous to some plants (known as phytotoxicity), so never let the grounds come into direct contact with roots or stems.

Coffee grounds can suppress seed germination and inhibit growth in many plants. To explain this, some scientists speculate that decomposition of coffee grounds releases toxic substances. These might be responsible for weed suppression. Other scientists have proposed that caffeine is the nutrient that causes this allelopathic activity. To be safe, never add coffee grounds directly to soil or use as a mulch with germinating seeds.

Coffee grounds control fungi and bacteria.

Coffee grounds show some remarkable properties in suppressing fungal rots and wilts. In studies, the establishment and growth of certain pathogenic fungi and bacteria was prevented. This was the case with cucumbers, spinach, and tomatoes, for example . No studies as yet show conclusive proof of their efficacy in disease suppression on other food crops, trees or shrubs.

Keep Pests Out Of Your Garden & Attract Beneficial Creatures

Slugs and snails hate coffee grounds, and earthworms adore them! This makes coffee grounds an absolute must-have for any garden. Earthworms eat coffee grounds and then distribute them throughout the soil of your yard and garden in the form of highly nourishing casting of worms. This is one of the best soil amendments ever.

Consider other uses for your Coffee Grounds

Coffee is an incredibly versatile plant, and coffee grounds are full of nutrients and good stuff even after a good hoy brew. They have other uses for you to consider.

Your Yard & Garden Will Be A Delight For The Senses

When you use coffee grounds as fertilizer on your yard, you will never have to deal with a manure from organic matter or chemical smell from chemical fertilizers. Additionally, coffee grounds act as a natural cat repellent. If you have had problems with neighbors’ cats (or your own) scratching in your yard, your problem is solved.

When you use coffee grounds to feed your yard, you’ll have beautiful, healthy, lush green plants in a pleasant, inviting setting. You are sure to be proud of your beautiful garden, not to mention being proud of yourself for doing your part to reduce, reuse and recycle.

Composting coffee grounds helps prevent landfill overflow and supports you in creating some healthy, green space to benefit yourself, your family members and friends and local beneficial wildlife.

Video on using Coffee Grounds in Soil

Conclusion

Coffee grounds can safely makeup to 20% of total compost volume. More than that may be detrimental.  Twenty-percent total volume is sufficient for effective disease suppression and for building soil, and should not acidify your soil (pH levels vary according to type of soil, microorganisms present, rainfall, nature of compost and many other factors).

Using coffee grounds alone as a mulch is fine, but don’t lay them on too heavily, as they compact easily and may actually serve to block air and water exchange. It’s best to use coffee grounds in combination with an organic mulch, or layered underneath a mulch like wood chips to avoid compaction.

Coffee grounds will not permanently acidify garden soil as do peat moss or sulfur-based additives; they’ll only affect the acidity in the immediate area. My best recommendation is to add coffee grounds to your compost along with dried, shredded leaves and grass clippings.