How to Use Coffee Grounds in the Garden

Instead of sending your morning coffee leftovers to the landfill, you might consider adding them to your garden or compost pile. Used coffee grounds are high in nutrients and—contrary to popular belief—relatively neutral in pH. However, coffee grounds may suppress plant growth, and, in large quantities, can be toxic to dogs, so it’s important to use coffee grounds thoughtfully.

Building a good compost pile relies on maintaining a balance between “green” and “brown” materials. Although they are brown in color, coffee grounds are high in nitrogen, magnesium, calcium, and potassium, which makes them “green” as far as your compost is concerned. Most food scraps and grass clippings are considered green compost materials. By contrast, brown compost material, such as dried leaves and newspaper, is rich in carbon.

• Compost bin or pile: Simply add your leftover coffee grounds, and even your paper coffee filter, to your compost bin or pile. Add approximately four times as much brown material to your bin as green material. When your compost is ready—the organic material should be mostly broken down and have a sweet, earthy smell—use as a soil amendment or apply around established plants as fertilizer.
• Vermicomposting or worm bin: Worms like coffee grounds, but try not to add more than one cup of grounds to your worm bin each week. Once the worms have made their way through your food scraps and newspaper, use your vermicompost as a natural fertilizer.
• Municipal composting: If you bring your compost to a municipal or nonprofit organization for processing, follow that organization’s guidance on what materials are allowed in the compost. Most municipal compost facilities will accept all food scraps, including coffee grounds, while a small community garden may be more choosy about what they accept.

You don’t need a compost pile to incorporate coffee grounds into your garden. It is a good idea, however, to dilute your coffee grounds before throwing them into the garden.

• Dry method: In a large bucket, mix about a cup of used coffee grounds with about ten times as much potting soil and/or compost. You can then use this mix in containers or sprinkle it over a raised bed.
• Wet method: Fill a half-gallon bucket with water and add two cups of used coffee grounds. Let sit for a few hours or overnight, then drain. Use as you would liquid fertilizer, by applying to the roots of your plants, or foliar feed by spraying the mixture onto your plants’ leaves.

• Use coffee grounds to suppress weeds. Studies suggest that amending garden soil with coffee grounds can suppress the germination of seeds and hinder plant growth. In most cases, this is a negative thing, but if an area of your garden is being taken over by an invasive weed, coffee grounds may be a great natural solution.
• Use coffee grounds to feed worms. Worms eat coffee grounds, so they can be a great addition to your worm bin (in small quantities). Additionally, earthworms that live in the soil may also be attracted to coffee grounds. When earthworms digest coffee grounds, they deposit the grounds deeper in the soil and aerate the soil above, which can improve soil structure.
• Use coffee grounds to improve soil’s water-holding capacity. Coffee grounds are great for water retention. Therefore, you should add coffee grounds if your plants need well-draining, but moist soil.
• Use fresh coffee grounds to increase soil acidity. While brewed coffee grounds are fairly neutral in pH, fresh coffee grounds are much more acidic and can assist in dropping the soil pH if needed. Some acid-loving plants that may benefit from unused coffee grounds include hydrangeas, rhododendrons, azaleas, lily of the valley, radishes, and blueberries.
• Use coffee grounds to add nutrients to the soil. In addition to nitrogen, coffee grounds contain calcium, potassium, iron, phosphorus, magnesium, and chromium. These nutrients are excellent additions for slow-release fertilizers or used in composting.

Sometimes, coffee grounds can cause more harm than good in the garden.

• Do not use coffee grounds for pest management. You may have heard that coffee grounds can be used as a natural pesticide around the base of plants to keep slugs and ants away from your vegetable garden, but this has not been proven. Insects are just as likely to be a problem whether or not you use coffee grounds.
• Do not use coffee grounds as mulch. While a thin layer of spent coffee grounds may appear to be a solid option for mulch, the grounds’ structure actually creates a barrier as the particles lock together and get in the way of a plant’s growth.
• Do not use coffee grounds around dogs. While using coffee grounds as mulch won’t harm insects, it can harm your pets. Consuming large amounts of the coffee grounds can be poisonous for dogs. If you want to use coffee grounds in your garden, you can protect your dog by incorporating the coffee grounds deeper into the soil, or by adding coffee grounds to your compost pile, rather than applying them directly.

Grow your own garden with Ron Finley, the self-described "Gangster Gardener." Get the MasterClass Annual Membership and learn how to cultivate fresh herbs and vegetables, keep your house plants alive, and use compost to make your community—and the world—a better place.