In the two previous posts I looked at ways to use Coffee Grounds in the Garden and the effect of coffee grounds on ants. There was one outstanding question that did not get resolved. Do coffee grounds deter slugs from eating your plants? Since I could not find any scientific work on this topic I decided to run some tests myself.
Photo Source: All About Slugs
How to Get Rid of Slugs
The claim that coffee grounds will kill slugs was discussed in a past post–they don’t. What was not clear is whether coffee grounds prevent slugs from reaching your plants. Do slugs hate sliding on the grounds so much that they will not cross a barrier of coffee grounds on the ground? If slugs do not cross a barrier of grounds, then it might be a good way to keep them from your plants.
For other common ways to get rid of slugs see the reference list below.
Experiment Setup
I set up a plastic tub with moist soil on the bottom. I collected 3 slugs. For some reason I have very few slugs in the garden this year so the test was limited to these 3 lucky molluscs. I added some plant leaves that had been eaten by slugs, so I knew they liked the leaves. Everything was left for 2 days to let the slugs get accustomed to their container, and to make sure they did not mind sliding around on the soil. They seemed to be quite happy, moved all over the container and ate some of the leaves.
On day three, a 2 inch wide, 1/4 inch deep, circle of coffee grounds was placed on the soil. Some plant leaves and the 3 slugs were placed inside the circle. If the coffee grounds work, the circle should keep the slugs inside the circle. The wait begins…..
Test Results
It was not a long wait. Slug #1 slid up to the grounds, and without any hesitation, crossed the grounds and was out of the circle in about 10 secs. Who says slugs are slow! Slug #2 did the same and was free in 20 secs. Slug #3 decided to have a nap, and crawled under a leaf. It never went near the grounds.
The slugs were returned to the center of the circle to repeat the test. The same thing happened. Two slugs left in less than a minute, and the third stayed put.
The Latest Science
The latest testing found that bread dough and bread dough slurry were the best attractant for various species of slugs and snails. It was more effective than beer.
Getting Rid of Slugs and Snails
There are all kinds of methods for getting rid of snails and I have reviewed several of these including:
- Eggshells Control Slugs – Do They Really Work?
- Getting Rid of Slugs With Coffee Grounds
- How To Get Rid of Slugs With Beer
- How to Get Rid of Slugs With Copper
- How to Get Rid of Slugs with Diatomaceous Earth
Conclusions
It is clear that slugs do not mind walking on the coffee grounds. The myth that coffee grounds stop slugs is busted!
To be fair, I only tried this with 3 slugs, and they were of one type. Other species of slugs might behave differently. But given the speed at which my slugs crossed the barrier, I would be surprised if the physical characteristics of the grounds would stop any slug.
The above test was done with wet coffee grounds. Would dry ones work better? Maybe, but how long would they stay dry laying on soil?
References:
1) a good source for facts about slugs: http://www.slugoff.co.uk/slug-facts/facts
2) All About Slugs: http://www.allaboutslugs.com/how-to-identify-slug-or-snail-damage/
3) Getting Rid of Ants with Coffee Grounds: https://www.gardenmyths.com/getting-rid-ants-coffee-grounds/#more-1854
4) Do Beer Traps Kill slugs: https://www.gardenmyths.com/do-beer-traps-kill-slugs/#more-1784
5) Does Copper Repel Slugs: https://www.gardenmyths.com/copper-repel-slugs/#more-1249
Nice experiment. I did a similar thing. But with 20 small slugs and lots more coffee ground. They walked around as usual at first but after a while they all died. Bigger slugs died a bit later than the small ones.
In another experiment I used copper tape. Some slugs avoided to cross some don’t mind it at all. ☺️ None of them die out of it.
It is important to split the slugs into 2, treat both batches the same, except for the coffee. Without a control, we don’t know why they died.
Assuming they did die from constantly walking over grounds, this is not a normal situation in the garden, unless it is completely mulched with coffee grounds.
I am not here to start an argument, all I know is after years of using beer traps, copper tape, and salt, I’d resigned myself to ragged petunias left by slug feasting. This year I’ve been adding daily used coffee grounds to garden soil just for recycling purposes, and haven’t had any slug damage. Can’t prove correlation, but I’m going to keep doing it.
Yesterday I decided to put my coffee remains from my nespresso cups on the garden soil where i grow my vegetables. In the night I went out to check and I saw many slugs. (More than nights before) I saw them eating the coffee ground i put on the soil. That surprised me. Right now its March so there nothing growing. Maybe they were really hungry. Or the coffee might actually be something that attracted them.
I have a tomato plant in a pot. I regularly put brewed up coffee grounds from my daily coffee to the pot, and it had build up quite a layer. yet, i have literally hundreds of tiny snails (allopea) on the top of the ground, they are doing so well that i can already see dozens of tiny micro little snails ready to grow. so I am pretty sure coffee doesn’t kill/harm the snails – they literally thrive on top of a layer of coffee! also i started adding coffee to the pot around 6 months ago, so the argument that “coffee kills snails in the long run” don’t work, at least in my case.
If humans are forced to take drugs, will they like drugs? Yes, everyone will get addicted. But do drugs kill humans in the long run? Yes!
Your experiment was based on the fact slugs were afraid of coffee grounds or not. It really proves nothing on how coffee ground would help in the long run.
Nothing in this post talks about slugs being afraid of coffee grounds??
I haven’t done extended research, but I have spread a decent amount of coffee grounds yesterday in my raised bed and the next day none of the plants had been nibbled yet I had two dead slugs.
There’s almost no vegetation on the garden bed yet so I haven’t seen any slugs around it so for some reason the were attracted to it yet died. So I’d assume some correlation with the slugs and coffee.
Don’t assume correlation – that’s how myths get started.
Robert Pavlis,
I’m replying to your September 2 question to me about active ingredients, but I can’t get the reply link to work on my phone. Unfortunately the answer isn’t simple.
The Sluggo OMRI-certified “organic” snail and slug bait I mentioned with the unimpressive results is 1% iron phosphate.
The Corry’s Snail and Slug Death that gave me such superior results CURRENTLY uses ferric sodium EDTA. I just discovered this by looking it up now. However, the Cory’s I used in my head-to-head comparison had metaldehyde as its active ingredient. And I have to say, I haven’t been seeing many empty shells lately now that I think about it.
Now I want to do a test of Sluggo (iron phosphate) versus new Corry’s (ferric sodium EDTA) versus a bait that still uses metaldehyde. My guess is that only the last of those will give me the joy of seeing empty shells. My recent reading tells me that the iron-based baits are slower acting, so the lack of empty shells may not actually mean that the baits are ineffective, just that the snails are dying later where they hide during the day, not immediately after eating the bait.
To really judge effectivenes, I’d need to do something like setting up a camera trap with beer bait for before and after population counts, but that’s more work than in interested in.
https://www.nature.com/news/2002/020624/full/news020624-8.html
Your experiment started with a false premise: that coffee prevents hosta damage by repelling slugs. It’s not to repel, it’s to stop them from feeding and eventually kill them. Might be worth exploring again. Also, dark roast has less caffeine than light roast.
How do coffee grounds ‘prevent feeding’by slugs? If it does not repel them they it would not prevent them from feeding? You can’t coat the leaves with coffee grounds.
Most suggests by gardeners is to sprinkle them around a plant, to keep slugs away – that is repelling them.
The reference is about using caffeine. It is a well known pesticide. That does not mean caffeine in grounds affects slugs.
It prevents the feeding because they have to crawl through the coffee to get to the plants. They are exposed to the caffeine when they crawl through it, are poisoned, lose their appetites, and die. Most coffee grounds have the correct amount of caffeine necessary to kill them. I’m not trying to start an argument, I just think your experiment would have been more interesting if you had tried to feed them in addition to making them crawl through the coffee, and then noticed how long they lived compared to a control group.
The evidence says that is not true. It does not stop them from crawling over it, and they don’t die from it.
I have never used ground coffee as the cafeine is minimal. However, I do mix mix coffee with hot water, wit to it cools than spray it on and around the plants – it has never affected my plants as I also avoid spraying on flowers. The snails and slugs quickly absorb the coffee when they ‘slide’ over it and die very quickly. AS you know, even for humans too much coffee is harmful with many side affects as well. May I suggest you try this and see what happens. By the way, this method also kills off their reproductive ability and any eggs that these creatures quickly exude once they know they are in trouble do not escapse the coffe ‘poisening’. Try it and I look forward to your results. I have NEVER had a problem with slugs or snails in decades, despite I have many ashded areas growing many shade like plants and a large variety of ferns.Enjoy great Gardening Dr Len Smith