Can you imagine the amount of free fertilizer you can get from a daily cup of coffee? The well-known trick has become so popular that local coffee shops have started offering free coffee grounds 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.

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Coffee grounds are bad for some plants and good for others. They are best used after composting or microbial breakdown, rather than throwing them straight into the garden or your garden bed.

Let's dig into seven surprising ways to use coffee grounds in your garden.

Safe garden uses for coffee grounds

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

These grounds are still a valuable garden resource, but modern research has given us some of the most effective ways to use waste coffee in our gardens without endangering the health of our crops.

Mix into your compost pile

The most popular and beneficial way to use coffee grounds is to compost them. It's also the safest way to use coffee on 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 don't recommend using caffeine directly in garden soil.

Instead, put waste soil in the compost bin as a "green" material. They are about 2-3% nitrogen by volume and need to be balanced by carbon-rich materials to ensure proper decomposition.

For every cup of coffee grounds, add at least an equal amount of "brown" substance, such as shredded paper or leaves. If you use biodegradable coffee filters, they are considered extra carbon material!

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The optimal ratio for a compost pile can include 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 the compost regularly to keep the pile ventilated.

Feed Worms (Vertical compost)

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

The key here is to make sure your coffee is fully brewed (the hot water is filtered) before spreading it on the plants.

The study showed that after the used coffee grounds were processed in the worms' stomachs, the used coffee grounds increased the nutrients of the worms' compost. Interestingly, the study found that a mixture of 25 percent waste land and 75 percent straw particles resulted in the largest increase in compost biomass of earthworms and worms.

In general, used soil should make up at most 1/8 of the earthworm bedding. Too much can "burn" the worm's skin, slowing the worm's composting process.

Once your worms have thoroughly broken down all compost components, it is safe to apply worm compost to your garden for rich nutrients that promote microbiome modification.

Slightly acidified soil

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

However, research from the University of Arizona suggests that waste soil may not be as sour as we thought. This is because the acid in coffee beans is water soluble.

After brewing, the concentration of the 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.

Plant disease control

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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 decay caused by sclerotinia
  • Fungal wilt (fusarium and dermophyte, which usually cause wilt 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 bed lie fallow for a season before planting crops in that area.

Green weeding

Because coffee grounds contain a high concentration of caffeine and acidity, they can be used as mulch to prevent the growth of weeds. 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 allelopathy (inhibition of germination) on lettuce seeds. To be safe, we recommend keeping them away from newly sprouted beds.

Instead, spread the ground at least 4-6 inches away from the base of the planted crop, or over non-crop (ornamental) beds and walkways. Scrape or mix abrasives over the surface of the soil to ensure that they do not dry out and create a waterproof barrier over the top layer of the soil.

Deter slugs

Slugs love cool, wet weather, but they hate sliding through the rough, acidic texture. Anecdotal evidence from gardeners across the country insists that used land can keep slugs away from your crops.

You can try sprinkling some powder over the surface of the soil to create a "protective ring" around slug-prone crops like strawberries or brassica.

Soil remediation

If you are concerned that your yard or garden has been contaminated by toxic pesticides or herbicides previously used, coffee grounds may be a desirable option!

The major study found that waste soil composted by worms can help detoxify persistent pesticides in the soil. 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 not growing due to previous contamination.

You can use it on contaminated soil, just like compost or mulch. To be on the safe side, we recommend using soil for this purpose only when planting ornamental, non-food plants. Do not grow vegetables in soil that may be contaminated with chemicals.

When to avoid coffee grounds

Coffee grounds are not a panacea for nutrient-poor garden soil. Be careful not to mix the ground into the soil 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 using them for your garden.

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

Avoid decaffeinated coffee

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Given that caffeine is causing many problems in the debate over the use of coffee grounds in the garden, you may wonder 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 often used to decaffeinate coffee beans. Using caffeine-free soil in the garden may leave a residue of chemicals in the garden soil.

Final thought

Needless to say, using coffee grounds in a garden is subtle and not as magical as the Internet hype makes it out to be. Still, they are useful when your used soil is composted with microbes.

When in doubt, compost first and use it primarily for ornamental or acid-loving plants. Better yet, use them as a worm compost ingredient for your worm farm!

April 23, 2023

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