I love coffee grounds and have been using them for years in the garden. In fact, Starbucks became my best friend since they supply me with old coffee grounds. So, over the years, I have used coffee grounds for the garden in a variety of ways with some mishaps. My favorite uses are as follows with some warnings. Read on.
Using Coffee Grounds for the Garden
#1 Worms Love Coffee Grounds
If you vermicompost give your worms coffee grounds. Coffee grounds to worms are like chocolate to us.
Even if you don’t vermicompost, still put coffee grounds in your composter. Worms will find your plain old vanilla composter. I remember the first time I put coffee grounds in my composter, the next week it was teaming with worms. I couldn’t figure out who sent out the “party” invitations. In fact, I swear I heard those worms talking late into the night.
I am going to have to collect their keys next time.
#2 Coffee Grounds Heat up Your Composter
Informal trials by Compost Specialists in Lane County, Oregon found that coffee grounds help to maintain necessary high temperatures to kill weeds and vegetable seeds and dangerous pathogens. In their trials, their compost was comprised of 25% coffee grounds and temperatures were sustained at 135 to 155 degrees for at least two weeks. During the two week time period, a significant portion of the pathogens were killed according to Cindy Wise, the coordinator for Extension’s Compost Specialist program in Lane County.
Dr Linda Chalker-Scott, Associate Professor, Puyallup Research and Extension Center, Washington State University warns not to add more than 10-20% by volume of coffee grounds into your compost. 30% or more would be detrimental.
#3 Amendment to your Soil
Notice, I said the word “amendment” and not fertilizer. According to Soil and Plant Laboratory Inc., Bellevue, WA, coffee grounds can benefit your soil using up to 35 percent by volume coffee to soil. The grounds must be rototilled to a 6- to 8-inch depth into the soil.
Coffee grounds contain nitrogen,phosphorus, potassium, magnesium, calcium and copper. However, not all of these minerals are immediately available to the soil. In the short term, all the potassium and magnesium are available and half of the calcium and copper are immediately available.
On the other hand, nitrogen is not readability available since it bound up in organic matter, which must degrade in order to release the nitrogen. Note, the grounds contain a 24:1 ratio of carbon to nitrogen. So in essence, adding the coffee grounds to the soil acts as a slow releasing fertilizer.
Interestingly, the Compost Specialists at Lane County, Oregon conducted an informal study of the use of coffee grounds as an amendment. Beans were planted in various plots where no grounds, 2 inches and 4 inches of coffee grounds were added to the soil about six months prior to seeding. The harvest and vigor of the beans which were planted in the coffee grounds soil were less than the beans planted in the plots without coffee grounds. The group concluded that the coffee grounds needed to break down for about a year before the nitrogen was available to the plants. Further controlled studies were advised.
As I mentioned above, I sprinkle coffee grounds around my plants. Read here how much you should use.
#4 May Inhibit Fungal Rot and Wilt
Dr. Chalker-Scott stated in her article that researchers found that coffee grounds prevent common fungal rots and wilt (Fusarium,
Pythium, and Sclerotinia species.) During the study, coffee grounds were part of a compost mix. Remarkably, in one case, the mix contained only a .5% coffee grounds.
The study was only conducted on certain vegetables such as cucumbers, tomatoes, beans and spinach under controlled conditions.
Before you run out and start sprinkling, note, coffee grounds may inhibit germination of certain seeds. A Mississippi State University study noted that aqueous coffee extracts added to petrie dishes inhibited the growth of white clover, periennel rye and palmer amaranth. Chalker adds that alfalfa and both white and red clover seed germination are inhibited along with the growth of Chinese mustard, komatsuna, Italian rye grass, inch plant, geranium, and asparagus fern.
In my experience adding coffee grounds to any of my indoor pots has been a disaster. The plants were unhappy. I assume it was because the grounds make the soil too heavy in the pots.
#5 Use as Mulch
A study found spent coffee grounds inhibited 95% of weeds in blueberry patches when coffee grounds were added to the soil to the depth of 1.6 to 3.1 inches.
Before you dump a truck load of coffee grounds as mulch around in your garden beds, heed the advice of Dr. Chalker-Scott. She recommends using a layer of no more than a half of inch under a layer of coarse organic mulch like wood chips.
I didn’t list slug control since I can’t personally speak to using coffee grounds for this purpose. If you have, please let us know in the comments if it works, how much coffee grounds do you use, and how often do you apply the grounds.
Join the Conversation:
How do you use coffee grounds in the garden.