by Jane Malec, EMGV

A morning cup of coffee is a necessity at my house. It is just a part of getting the day started and I love everything about it — making it the night before, turning the pot on in the morning, and the aroma when it’s brewing. Over the years our coffee habits have changed due to the ever-fluctuating studies on the healthiness of, optimum consumption levels, or brew strength of a cup of coffee. It’s mostly decaf and fewer cups for us now.  Still, it remains a force of habit to pour the same level of water and ground coffee beans into the brewer regardless if we are running out the door or having a leisurely morning. Even if I reheat a cup later in the day, there is always a little left over brew sitting in the pot.

So I often walk around the house pouring the remaining cold coffee onto thirsty house plants. Our current home has a private well and I am ever more conscious of water usage. I make sure to pat myself on the back over my dedication to water conservation with every cup of coffee that soaks into the potting soil. I even wander outside to share the wealth with my container plants. A win-win, right? Or, maybe not. If there is a debate on human coffee consumption, I suddenly wondered if there might be one on plant consumption.

Coffee comes to us through a complicated path. To begin with, there are several species of coffee plants or trees, the two most commonly grown are Coffea arabica and Coffea canephora which is also called Coffea robusta. Arabica coffee is considered superior and commands a higher price than the robusta. Coffee production is a labor intensive process. The berries are picked by hand, sorted and allowed to ferment which removes a gelatinous tissue layer. Then they are washed with large amounts of fresh water. The seed of each berry, which is the bean, is roasted and then ground before becoming the beverage we enjoy. The degree of roasting the beans will determine the flavor, but not the caffeine level. Interestingly, the lighter roasts contain more caffeine so watch out you breakfast blend drinkers!

Which leads back to my curious question — is liquid coffee good for plants? There is a good amount of anecdotal information (on the Internet) concerning the use of brewed coffee for watering plants. However, there is little research-based information on this nor on liquid coffee’s benefits to soil.

However, there are some basic guidelines to help us understand if my “coffee watering practice” is helping or harming to plants. While coffee grounds tend to have a pH around 6.5 to 6.8 which is nearly neutral on the scale (neutral is 7), brewed liquid coffee is reported to have a pH level ranging from 5.2 to 6.9 depending on the type of coffee and how it is prepared. The lower the pH the higher the acidity; A drop in one full pH equals a factor of 10. For example, if the brewed coffee is 5.5 pH and the grounds are 6.5 pH, the brewed coffee is 10 times more acidic than the grounds.

Understanding that most plants like a slightly acidic to neutral pH and factoring in that most municipal water is slightly alkaline (pH greater than 7), it is possible that the acidity of the coffee could benefit plants. Also there are small measurable amounts of magnesium and potassium which are plant nutrients. Keep in mind that this information can only serve as a guideline as there can be other factors in play, for instance, if you have a private or community well, and whether or not you use a water softener.

The best recommendation is to follow some simple rules. Make sure the coffee is cold and then water it down. Never water with coffee if you add milk, cream or sugar. Know your plants pH happiness levels and then watch the impact this is having on your plants over time.  Adjust your morning coffee routine.   

Remember too much of a good thing is, well, too much of a good thing. Resign yourself to pouring some of the leftovers down the drain or on your compost pile. 

Maybe I could starting singing to my plants instead?  Hmm. 

Resources:

Coffee as Fertilizer?
https://web.extension.illinois.edu/dmp/palette/110109.html

Yard and Garden
https://extension.unl.edu/statewide/buffalo/Watering%20House%20Plants%20With%20Water%20%26%20Coffee%2C%20March%205%2C%202011.pdf

Clay soils
https://durham.ces.ncsu.edu/files/library/32/UNH%203.PDF