Gardening trends on the Internet might lead you to think you're getting a ton of free fertilizer from your daily cup of coffee. This well-known trick has become so popular that local coffee shops have started offering free coffee rides to any customer. The following content also has some reference value for raised garden beds.

But before you pour used coffee grounds into garden soil, you should understand the uses (and misuses) of this unique soil amendment.

Coffee grounds may be bad for some plants, but good for others. They are also best used after composting or microbial breakdown, rather than throwing them straight into the garden.

raised garden bed

Let’s dig into six surprising ways you can use coffee grounds in your garden.

Safe garden uses for coffee grounds

Like most kitchen waste, coffee grounds have been used as compost ingredients, mulch and even fertilizer for centuries. Long before landfills and waste services, our coffee-drinking ancestors had to keep their coffee grounds somewhere!

These grounds are still a valuable garden resource, but modern research has shed light on the most effective ways to use waste coffee in your garden without putting your crop's health at risk.

Mix into the compost pile

The most popular and beneficial way to use coffee grounds is to compost them. This is also the safest way to use coffee on your plants, as microbes can break down any caffeine that remains after brewing. Studies have shown that caffeine inhibits the growth of most plants, which is why we do not recommend applying caffeine directly to garden soil.

Instead, use the used ground as a "green" material in the compost bin. They contain about 2-3% nitrogen by volume and need to be balanced with carbon-rich materials to ensure proper decomposition.

For each cup of coffee grounds, add at least an equal amount of "brown" material, such as shredded paper or leaves. If you use a biodegradable coffee filter, these count as extra carbon material!

A good compost pile ratio should consist of one-third coffee grounds, one-third leaves, and one-third grass clippings. The amount of coffee grounds should not exceed one third of the total volume of the pile. Don't forget to turn your compost regularly to keep it ventilated.

raised garden bed

1. Feed Worms (vermicompost)

Does coffee harm worms? The jury is still out on this! While there is some evidence that espresso grounds harm earthworms, there have also been studies showing that used coffee grounds do not harm earthworms, or red jiggers or other worms used in worm compost.

The key here is to make sure your coffee is thoroughly cooked (the hot water is filtered) before pouring it over the plants.

The study showed that used coffee grounds increased the nutrient content of worm compost after being treated in the worms' stomachs. Interestingly, the study found that a mix of 25 percent waste soil and 75 percent straw particles resulted in the largest increase in earthworm and vermicompost biomass.

As a general rule, used ground should comprise up to 1/8 of the worm bedding material.

Once your worms have thoroughly broken down all compost components, it is safe to use worm compost in your garden as it is rich in nutrients and promotes microbiome modification.

2. Slightly acidified soil

One of the most frequently cited benefits is their ability to acidify soil. Coffee itself is acidic, so it seems reasonable that used grounds would lower the soil pH of acid-loving crops.

However, research from the University of Arizona suggests that abandoned ground may be less acidic than we thought. This is because the acid in coffee beans is water soluble.

After brewing coffee, the amount of grounds varied from 4.6(slightly acidic) to 8.4(alkaline). The pH ultimately depends on the type of coffee, the strength of the brew, and the type of compost or microbial decomposition.

raised garden bed

3. Plant disease control

Coffee grounds have significant antibacterial and antifungal properties and have even caught the attention of the pharmaceutical industry. In the garden, they inhibit some common plant pathogens, including:

  • Fungal rot caused by sclerotinia
  • Fungal wilt (fusarium and deromycosis, usually causing disease)
  • Some bacterial pathogens (E. coli and Staphylococcus)

If you have a major problem with soil-borne plant diseases, consider mixing ground coffee compost and letting the soil lie fallow for a season before planting crops in that area.

4. Environmental weed control

Because coffee grounds are high in caffeine and acidity, they can be used as mulch to keep weeds from growing. They act like "natural herbicides". Research has shown that coffee ground compost inhibits the germination of weed seeds and reduces the need for chemical herbicides.

However, this study found that they also have an allelopathy (inhibition of germination) on lettuce seeds. To be safe, we recommend keeping them away from freshly sprouted beds.

Instead, scatter the ground at least 4-6 inches away from the bottom of crop plants or non-crop (ornamental) beds and walkways. Scrape or mix on the surface of the soil to ensure they don't dry out and form a waterproof barrier on the top layer of the soil.

5. Deter slugs

Slugs like cool, wet weather, but they don't like sliding through gritty, acidic textures. Anecdotal evidence from gardeners across the country insists that used ground keeps slugs away from your crop.

You can try spraying and scraping the soil surface around slug-prone crops like strawberries or brassica to create "protective circles."

6. Soil remediation

If you're worried that an area of your yard or garden has been contaminated by a toxic pesticide or herbicide used previously, coffee grounds might be a saving grace!

The major study found that vermicomposting of abandoned land could help detoxify the soil of persistent pesticides. This form of remediation can have a significant impact on neutralizing contaminated or heavily sprayed soil.

In your garden, ground coffee vermicompost can help repair areas where plants are unable to grow due to previous contamination.

You can apply it to contaminated soil, like compost or mulch. To be safe, we recommend using the ground only when planting ornamental, inedible plants. Never grow vegetables in soil that could be contaminated with chemicals.

When to avoid coffee grounds

Coffee grounds are not a panacea for nutrient-poor garden soil. Mix the soil into the soil with care until the effects are fully understood!

Avoid direct application to crops

With so much conflicting information, it is safest to compost or vermicompost coffee grounds before applying them to your garden.

Avoid applying foundation directly to the soil in your vegetable bed, or you may harm your crop. This is especially important for vegetables that require a neutral or alkaline pH, such as tomatoes, brassica, and lettuce.

raised garden bed

Avoid decaffeinated coffee

Given that caffeine can cause many of the problems in the debate over the use of coffee grounds in gardens, you may be wondering if you should start drinking and using decaffeinated coffee. Unfortunately, this may not be such a good idea.

Chemical solvents such as methylene chloride or ethyl acetate are commonly used to remove caffeine from coffee beans. Using decaffeinated soil in a garden can leave a residue of chemicals in the soil.

Final thought

Needless to say, the use of coffee grounds in the garden is subtle and not as inconceivable as the hype on the Internet. Still, they are useful when your used land is composted with microbes.

When in doubt, compost first, mostly for decorative or acid-resistant plants. Better yet, use them as worm compost ingredients for worm farms!

April 04, 2023

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