Can You Use Coffee Grounds In your Garden To Deter Pests and Encourage Plant Growth? What Works & What Is a Myth? This Article Provides Some Answers & Surprises.

It’s relatively easy to source coffee grounds from our kitchens, coffee shops or even factories, and to recycle them in our gardens. Even commercial growers use them to grow things such as mushrooms and fungi. But do all the things claimed for coffee grounds online actually work. What is fact, what is hearsay and what is total BS?

What Are Coffee Grounds?

It’s a basic question, but often misunderstood. They are the leftover product from when coffee is made. More importantly, from a gardening perspective, they are an organic product , even if not grown organically. So they can be composted and used in the garden to provide nutrients that plants can use.

In France they are also mixed with sawdust to form a briquette for burning. So there are some other commercial uses for coffee grounds.

Are Coffee Grounds Acid?

No, not very. Very rarely they can be acidic when fresh but generally speaking they are only slightly acid and adding a few kilo to a square meter of soil is going to make very little difference to soil pH.

Contrary to popular belief, it’s a myth that coffee grounds are acidic and will lower the pH of the soil. After brewing, the grounds are close to pH neutral, between 6.5 and 6.8. Research shows that whatever change coffee grounds bring to the soil is short-lived,

Linda Brewer Oregon State University

One research paper from Indonesia claimed their coffee grounds had a pH of 6.2 and the research was meant to ascertain how this impacted soil pH. But nowhere in the paper can I find any indication of what the soil pH was at the beginning or end of the research!

Are Coffee Grounds A Rich Source of Nutrients?

I often read how coffee grounds are rich in nitrogen, which is essential for plant growth. But the reality is that the normal value is between 1-2% which is not very rich in nitrogen. The levels of potash, phosphates and trace elements are also not significant.

Here are typical levels.

  1. Nitrogen (N): Coffee grounds typically contain around 1.45% to 2% nitrogen by weight.
  2. Phosphorus (P): The phosphorus content in coffee grounds is relatively low, usually ranging from 0.05% to 0.15%.
  3. Potassium (K): Coffee grounds contain approximately 0.3% to 0.6% potassium.
  4. Calcium (Ca): Calcium content in coffee grounds is generally around 0.05% to 0.15%.
  5. Magnesium (Mg): The magnesium content in coffee grounds is typically around 0.05% to 0.1%.

Coffee Grounds As An Organic Matter Source

Coffee grounds are an organic material, which means they decompose over time, adding valuable organic matter to the soil. This can improve soil structure, increase moisture retention, and encourage beneficial fungal and bacterial activity. However, don’t spread them too thick as they can cap on the soil surface which limits water and air penetration to the soil pore space.

Pest Control Using Coffee Grounds

I recently read that a layer of coffee grounds around plants will prevent slugs from eating plants. the clim is that the abrasive nature of the coffee grounds repels slugs. I doubt it! Slugs produce mucous and slide over the coffee grounds without actually touching them. So they are not going to prevent slug movement. Elsewhere on the website we show slugs climbing over a razor blade … if they can do that why would coffee stop them?

However, coffee can be toxic to slugs. When used as a soil drench.

Linda Brewer, from Oregon State University, highlights a potentially useful aspect of coffee grounds beyond their composting benefits: their efficacy as a slug deterrent. Studies reveal that employing a solution of 1% to 2% coffee mixed with water as a soil drench prompts a complete departure of slugs from treated soil, leading to their demise due to caffeine toxicity.

To prepare a 1% to 2% soil drench, simply combine 1 part water with 2 parts strong brewed coffee. For instance, mix 1 cup of water with 2 cups of coffee. To deter slug feeding on foliage, dilute the coffee solution with 9 parts water to 1 part brewed coffee and administer it as a spray. But test it for a few days on the foliage before spraying all your plants.

Are Coffee Grounds Cost Effective In the Garden?

If obtained without cost they are worth applying to the soil. But should transport costs be high I have my doubts. In most cases I suggest they are better composted with other organic matter rather than applied to the soil. Unless applied as a solid drench to kill slugs.

Research Sources

Tag: Coffee Grounds in your garden
Image attribution: Msmurugan1, CC BY-SA 4.0, via Wikimedia Commons, & Lamiot, CC BY-SA 4.0, via Wikimedia Commons

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