How to Use Coffee Grounds in the Garden

Don’t Ditch the Coffee Filters!

Ever since I spotted a sign outside the Carlow Coffee Company offering gardeners free coffee grounds, the aromatic waste product has been on my mind. I’ve heard people mention they’re good for keeping slugs and snails at bay, but can they be used for anything else? Will coffee grounds help to prevent our lettuces becoming lacy or keep our potatoes perfectly pert?

After a bit of digging around, I’ve found that yes, coffee grounds can be pretty useful in the garden, but we should take a bit of care and not throw them over the soil willy nilly. Here’s why…

Can Coffee Grounds Change the Soil pH?

Adjusting the soil pH is something we’re going to have to consider at the School of Food in Kilkenny as the topsoil that was brought in is measuring just under a pH of 8 on my over the counter kit. This is great for alkaline loving plants such as asparagus which are happy growing in a high pH, but not so good for the potatoes which prefer a more acidic pH of 5.3 to 6.

There are slightly conflicting views on just how acidic coffee grounds are as research shows the pH can vary depending upon studies. Linda Chalker-Scott from the Washington State University reports that:

“While two studies on coffee ground composting reported mildly acidic pHs of 4.6 and 5.26, others have measured neutral (7.7) to somewhat alkaline (8.4) pH levels. One researcher found that the pH of soil treated with coffee compost increased after 14 to 21 days of incubation, gradually decreasing thereafter.

Obviously the pH of decomposing coffee grounds is not stable and one shouldn’t assume that it will always, or ever, be acidic.”

That pretty much answers that question.

Composting Coffee Grounds

With the above results in mind, it seems that composting our coffee grounds with piles of other waste material such as brown cardboard and vegetable peelings might be the best use for them if we’re not trying to adjust our soil pH. However, Professor Chalker-Scott recommends that we don’t add any more than a ratio of 20% coffee grounds to our compost heaps or they could have a detrimental effect on soil.

She also suggests that a 20% ratio will help with the suppression of certain diseases such as E. coli and Staphylococcus, thanks to their “biocontrol on bacterial pathogens”, but those experiments have only been carried out in a controlled environment and could differ once out in the open.

Coffee Grounds as a Soil Conditioner

Earthworms enjoy eating coffee grounds and find them a great food source. As a result they will help to condition and open up the soil, pulling the grounds deeply down through it. According to research undertaken for Sunset Magazine by the Soil and Plant Laboratory, coffee grounds can add small percentages of phosphorus, potassium, magnesium and copper to soil so it does seem that composting them is a good idea. Why compost and not just sprinkle them all over the soil surface? Keep reading…

Coffee Grounds as a Mulch

Coffee grounds tend to compact when they get wet, something that’s not great for our soil as the hardened layer will prevent moisture and air getting through, which in turn will dry out or suffocate plant roots if you use a thick layer. If you’re trying to use them as a soil conditioner and haven’t had a chance to compost them, mix some coffee grounds with some well-rotted compost or manure and then add them to your soil as a mulch, but don’t forget that 20% ratio mentioned above.

Using Coffee Grounds to Deter Slugs and Snails

No matter what type of gardener you are, slugs are likely to be a topic of conversation when you start chatting about your plants. Whether you garden on a balcony, an allotment, community or back garden, these plant munching pests get everywhere and we’re always looking for ways to get rid of them. I wrote an article a couple of years ago that mentions 15 ways people have found of getting rid of slugs and snails without using artificial chemicals but be warned, using anything as pest control that hasn’t been approved by Brussels leaves gardeners open to heavy fines from the EU!

According to an article I found on eartheasy.com in relation to caffeine and slugs and snails:

A study in June 2002 reported in the journal Nature found that slugs and snails are killed when sprayed with a caffeine solution, and that spraying plants with this solution prevents slugs from eating them. The percentage of caffeine required in a spray (1 – 2%) is greater than what is found in a cup of coffee (.05 – 07%), so homemade sprays are not as effective. Look for new commercial sprays which are caffeine-based.

So given that coffee grounds don’t appear to be great for our soil if added directly to it, and neat caffeine would have to be sprayed onto the slugs and snails directly at a high concentration, the grounds aren’t going to be much use to us in that respect. Even if we were to risk breaking the law, we wouldn’t be able to filter a high enough concentration of caffeine out of the grounds and into a spray bottle.

Coffee Grounds as an Ant Deterrent

In researching this article I’ve come across a few sites that recommend using coffee grounds as an ant deterrent. While some scientific research by an Entomologist in Texas has concluded that this method of pest control doesn’t work on fire ants, another study by gardenmyths.com suggests that their ants didn’t like coffee at all. They did, however note that the coffee grounds didn’t create a major problem for the ants, more that they were an inconvenient barrier. I’ll report back on this one following my own study as I have a red ant nest in the polytunnel at the moment and often find them nibbling the strawberries.

Conclusion

Given everything I’ve read, I’ll still collect coffee grounds when they’re offered and will continue to save Mr G’s grounds out of his Aeropress, but I’ll be adding them to my compost heap so that their slow-release nutrients will help to improve the soil in the heap and perhaps help to prevent E-coli and other bacteria building up from the well-rotted manure that’s sometimes added to it.

Have you used coffee grounds in your garden? Have you found it helps? Did you find that slugs or ants hated it? I’d love to hear your thoughts.

[print_link]