While coffee grounds are a source of plant nutrients, they can cause problems. The best practice is to compost them first before using them as a soil amendment.
If you want to improve your soil, also try making leaf mold from fallen leaves.
Coffee Grounds in the Garden
Coffee grounds are commonly recommended to improve plant growth and repel pests.
And it sounds like a good idea. Feed your plants while reducing waste.
But, like many garden tips, the benefits tend to be exaggerated.
Coffee grounds have their value but it’s nothing extraordinary—and can be overdone.
Tossing handfuls onto the soil now and then won’t make a difference—good or bad.
For larger volumes, it’s best to compost them first.
Contents
Frequently Asked Questions
What are coffee grounds?
When coffee—the beverage—is made, hot water is poured over ground coffee beans, releasing their flavour into the water. The spent coffee grounds are the leftover by-product.
Are coffee grounds good for fertilizing plants?
While not the magic beans some promise, coffee grounds can provide some benefits to the garden if used properly.
Coffee grounds are a source of plant nutrients including nitrogen (2%), phosphorus (0.06%), and potassium (0.6%), along with micronutrients like boron, calcium, copper, iron, magnesium, and zinc.
One (of several) reasons to compost coffee grounds prior to use is to release this nitrogen which only becomes available as the grounds decompose.
How to compost coffee grounds
Despite their brown appearance, coffee grounds count as “greens” for composting. This has more information on greens and browns and how to make compost.
- Add used coffee grounds to your compost bin or pile along with your usual compostable materials like fruit and vegetable scraps (greens) and leaves (browns).
- The coffee grounds should be limited to around 20% or less of the total volume.
- Compost is ready to use—or “finished—when it has transformed it into an earthy substance, uniform in appearance with no unpleasant odors.
Can I use coffee grounds as mulch?
It is not recommended to use fresh coffee grounds as mulch but it is fine to use finished compost (that includes coffee grounds) as mulch. This explains how and why to mulch your garden beds.
On their own, coffee grounds used as mulch can become compacted, blocking air and water from the soil below and may also negatively affect plant growth.
Do coffee grounds repel garden pests or diseases?
Despite what you may have heard, there’s no evidence that coffee grounds will repel pests or attract beneficial insects or earthworms or prevent plant diseases.
Do coffee grounds change the pH of the soil?
Coffee grounds are sometimes recommended for acidifying soil. While coffee is acidic, spent coffee grounds are only mildly acidic and this acidity is temporary.
If you’re trying to lower soil pH for crops like blueberries, there are products made for this purpose including one with elemental sulfur. You can learn more about soil pH and why it matters here.
Do coffee grounds affect seedlings?
Coffee grounds have been shown to harm some seedlings and slow plant growth. This is another good reason to compost them first and avoid direct application.
Can I use coffee grounds to fertilize houseplants?
It’s best to compost coffee grounds first before adding them to soil or potting mix—indoors or outdoors. You will not get the benefits of the nutrients until the grounds are decomposing. Coffee grounds also compact over time which will restrict the air and water flow to plant roots.
A Worldwide Problem
It’s mind boggling to imagine the volume of coffee grounds generated around the world each day.
A report from the UK says that the average coffee shop produced 20 kg —over 40 pounds—of spent coffee grounds every day. That’s just one shop!
In North America, it’s estimated that nearly two million tons of coffee grounds are disposed of every year.
Much of that goes to landfill where it contributes to greenhouse gases and other problems.
Any diversion from landfill is a start.
If you do use them in garden, compost them first and be mindful of the recommended ratios.
Summary
- Small amounts of coffee grounds (1/2 inch thick or less) tossed onto your soil aren’t going to have any measurable effect (positively or negatively) on your garden.
- Larger amounts should be composted first, a process which makes the nitrogen available.
- Any acidity in coffee grounds is temporary. They will not lower the pH of your soil.
- The idea that coffee grounds can repel pests or stop plant diseases lacks scientific evidence.
Resources
Read More
- Using Coffee Grounds in Landscapes and Gardens WSU.edu (pdf format)
Learn more about other household items and how they affect the garden:
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Garden Definitions
Soil | The upper layer of earth in which plants grow, a black or dark brown material typically consisting of a mixture of organic remains, clay, and rock particles.
Mulch | Placed on soil, organic mulch can protect soil, retain moisture, and gradually fertilize the garden.
Leaves | Finely chopped fall leaves make excellent mulch.
Leaf Mold | Decomposed fall leaves beneficial to soil structure.
Compost | Decomposed organic matter providing nutrients for the garden.
Potting Mix | Contains no soil: designed to optimize plant growth in pots.
Seed Starting Mix | A lightweight potting mix for sowing seeds in containers.
Soil pH | Knowing your level (which may vary) is informational, not a call to action. Most soils fall in the range of 5 to 8 and accommodate a wide range of plants.
Free Soil Calculator Tool | Estimate how much you need and what it will cost
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