There must be hundreds of articles about using coffee grounds in the garden. I even have one! But the more I read and try these ideas I find that some of these ideas don’t work. At least, they don’t work for me.
Here are my top 5 coffee grounds in the garden myths.
Myth #1 Coffee Grounds Deter Flea Beetles.
For the last five years, I have tried to grow eggplant. However, every year, the flea beetles have destroyed the plants.
If your plant doesn’t have leaves, it will die.
Sure, I have tried covering my plants and rotating their location in the garden, but the flea beetles just show up as uninvited guests.
As a obsessed gardener who has grown alfalfa, amaranth, and buckwheat, you can’t imagine how hard it is for me to face my eggplant gardening failure. I was ready to just give up until I read a forum suggestion to sprinkle used coffee grounds around the plant and the flea beetles will leave (suitcase and all.)
Since I store my coffee grounds, I decided to give it a whirl. Well, guess what? The flea beetles just laughed at me. It started as a small chuckle and became a collectively belly roll type laugh.
I even tried sprinkling coffee grounds on the plants as well as sprinkling the leaves with coffee grounds diluted water. Nada. Nothing. The beetles thrived and sent me a thank you note.
If this method works for you, I am all ears.
Myth #2 Coffee grounds kill slugs.
Well, they don’t kill or deter my slugs. And no, I don’t have zombie slugs.
Years ago, we had the rain summer from h*ll. The slugs just came out of the woodwork. (Read my slug hunting story here if you need a good laugh.) So, I did my research looking into every possible internet solution from spray coffee grounds on them to setting up a coffee ground line of defense.
One study showed that large slugs were killed by spray applications that contained 1–2% caffeine. A cup of instant coffee has .05% caffeine.
The study further states that spraying a caffeine solution of .01% on cabbage plants reduced slug consumption.
Given this study, I applied coffee grounds around my seedlings. Perhaps if I would have continuously sprayed a coffee ground solution, it would have deterred them eating my brassica plants.
Honestly, if you want to try this method, it doesn’t hurt the plant since it is just organic matter. I just won’t rely on it to stop the slugs. It may deter some.
Myth #3 coffee grounds stop aphids.
Two years ago, my tropical hibiscus were covered in aphids and whiteflies. These two pests love this plant along with collard greens and kale. Again, I turned to the gardening forum, and found a comment that aphids hate coffee grounds and will leave.
I put a circle of coffee grounds around the plants.
Again, nada. Nothing. The aphids left me a present. More aphids.
Myth #4 coffee grounds are only good for acidic plants
According to Dr.Hepperly of Rodale Institute, when coffee grounds start to break down, their pH becomes neutral. However, simply throwing coffee grounds around your plants isn’t wise either. Read here how much coffee grounds you should use.
Note certain plants aren’t happy when you use coffee grounds. Linda Chalker-Scott, Ph.D., MasterGardener WSU editor Extension Urban Horticulture, states
“Seed germination of alfalfa (Medicago sativa) and white and red clovers (Trifolium repens and T. pratense)
was inhibited by water leached through coffee grounds. Growth of crops such as Chinese mustard (Brassica juncea), komatsuna (Brassica campestris) and Italian ryegrass (Lolium multiflorum) were all inhibited by coffee grounds, as was that of ornamentals including inch plant (Tradescantia albiflora), geranium, and asparagus fern. One investigator speculated that toxic substances released from decomposing coffee grounds were responsible for their inhibitory effect. This effect also reduces weeds, and perhaps in a landscape dominated by large shrubs and trees, only germinating seeds and seedlings would be injured. But as there has been no experimental research on coffee grounds and woody plants, this is only speculation on my part.”
From own observation, I find that potted plants do not do well with coffee grounds. I don’t know if the coffee grounds cause too much soil compaction so the water cannot percolate through or they just don’t like them. Others swear their potted plants love the grounds.
Just observe . If the plants don’t like it (dropping leaves or looking unhappy) either remove the grounds or just don’t give it to them again.
Myth #5 Add coffee grounds to your compost
Earthworms love coffee grounds. When I added coffee grounds to my compost, the next week you would have thought Mardi Gras came to my composter. There were so many worms crawling throughout my compost.
However, simply dumping endless amounts of coffee grounds into your compost is not a good thing.
Kit Smith, UCCE El Dorado County Master Gardener recommends the compost volume should not contain more than 15 to 20% of coffee grounds. I am guilty of pouring in two huge bags of coffee grounds from Starbucks.
To read more about coffee grounds:
- Coffee Grounds in the Garden: Foe or Friend
- Coffee Grounds and Roses
- How to Get your Coffee Grounds for your Compost just right
Join the Conversation:
- How do you use your coffee grounds?
- Do you have any coffee ground myths to dispel?
Marla says
Well Anna you coffee grounds certainly didn’t help save your plants but hopefully they did help fertilize the ground through the compost. It is distressing to have bugs eat your plants up and not know how to stop them. Better luck next year.
James Paulson says
I knew there was some kind of use for the pounds of coffee grounds that I toss each and every day!
rachel sarnoff says
Great post! What about coffee grounds directly on outdoor bushes, does that work? My aunt suggested that for flowering bushes. Hm…
Anna@Green Talk says
Rachel, coffee grounds work with all plants but many people like to put them on acidic plants although the benefits are temporary. Read my coffee ground post–friend or foe ( http://www.green-talk.com/2007.....nd-or-foe/) It will give you the guidelines of how much to put on around your plants. Anna
Karen Wood says
Heh, well, I’m glad to have run into you and this post just today. I drink a lot of coffee. Good home roasted from organic green beans, coffee. Unfortunately I only have a very small yard space and grow my plants in large pots and have been making compost that’s heavy on the coffee grounds. I know I don’t have a big enough yard for the coffee that I drink and I’m sure I have over loaded my plants with coffee compost ( none have been as happy as I thought they should have been). I will have to read the rest of your coffee ground related posts. I am wondering if coffee can be good for the lawn. Maybe I can sprinkle it out there for some benefits.
Anna@Green Talk says
I sprinkle coffee grounds everywhere so I can’t imagine why your lawn wouldn’t love them. Earthworms love coffee by the way. Anna
Misha says
My hubby likes spiced coffee–I always put pumpkin pie spice in with the grounds when making coffee for him. Would those “spiced” coffee grounds still be beneficial in the garden or compost? I’ve always just tossed it but would love to be able to use it to help (and not hurt!) our plants…
Anna@Green Talk says
Misha, I don’t see why not. They are just spices. Anna
Jonathan says
Hi Anna,
If you’re still looking for a (non-chemical) solution to your flea beetle problem, use diatomaceous earth (ideally food-grade). You can find the food-grade variety in any large pet store; its usage on pets is the external application on dogs for fleas. Wikipedia and other online sources have all the info you need regarding its application and why it works, but it is essentially the fossilized remains of trillions of diatoms, which are so microscopically abrasive that they kill most arthropods via dehydration (see “Fick’s law of diffusion” for the super-science-y explanation). The flea beetles that were eating my tomato leaves (and are fond of all nightshade leaves, including your eggplant) have been completely eradicated from my garden after one week of light dusting. If it’s windy and dusting is difficult, you can dissolve (well, create a suspension, since it doesn’t really dissolve) a few tablespoons in a standard spray bottle and spray your leaves. It’s completely safe to mammals and pets and harmless if ingested; it is essentially desiccated million-year-old algae-turned-sand. Some farmers even advocate internal use in livestock and pets for parasites (and some peculiar folk eat it themselves for health–go figure).
Look for diatomaceous earth labelled either mechanical insecticide or food-grade, and not the kind they use for pool filtration systems–the latter is not fine enough to be effective against pests. Finally, because food-grade diatomaceous earth is a rather fine dust, accidentally inhaling it can irritate your lungs. Some recommend wearing a mask during application, but I’m lazy and just stand upwind… Haven’t died yet. Hope this helps with your flea beetles 🙂 Thanks for your great posts re: coffee grounds.
JT
Anna@Green Talk says
I heard flour works as well, Jonathan!
claudia roulier says
well when I worked at a green house my boss said it was the actually coffee that killed the aphids, water with coffee instead of using the grounds you just can’t get enough coffee into the plant from just the used grounds although you could do both it’s the sort of thing that works from the inside out…….it works
Anna@Green Talk says
Claudia, that is interesting. So did you water with dark coffee or dilute and then water? Anna
Anu says
Hi,
I have used coffee grounds very effectively in whitefly. My hibiscus was badly infested. I cut off the infected parts, wrapped the discards IN newspaper to dispose off so the infestation wouldn’t spread. Then I made a spare of coffee grounds with salt, adding about a quarter spoon of salt to a cup of grounds. I applied this on the cut off ‘wounds’and sprinkled the rest around the base. Repeated after ten days and another ten days. Whitefly gone completely!
Anna says
Anu, that is such a great idea. Does it matter if the grounds are wet or dry? Anna
Justin says
I feel like there is a lot of misinformation regarding coffee grounds and caffeine being used synonymously. When you make coffee you are taking most of the caffeine out depending on the brew strength. Try using fresh grounds, a lot more used grounds soaked in water to multiply the amount of caffeine, or caffeine pills if you really need stop the bugs.
Adrienne says
Thank you for this information. If coffee grounds didn’t work to help with aphids, what method did you use?