Slug Glorious Slug
The huge bright yellow banana slug is my favorite. I've loved them since I was a kid, & like best the black-spotted ones that look like slightly rotten bananas. I've kept them in vivariums as pets, & they are extremely entertaining.

The banana slug is native to North American forests & is common here in the Olympic Penninsula's temperate rainforest. They're not usually garden pests as they like densely wooded places with lots of mouldering conifer needles on the ground. The common garden slug, either solid black or brown, is an invader from Europe, introduced into North America a very long time ago & now so aggressively naturalized that many native slugs have been completely displaced, to the point of being endangered.

This banana slug appears to be nibbling on Deadly Nightshade. The photo is by Cagan Sekercioglu, & is used courtesy of a website you should visit for many stunning nature images.

Do Coffee Grounds Really Kill Slugs?

The Dream vs. the Reality

"All Nature seems at work. Slugs leave their lair -
The bees are stirring - birds are on the wing -
And Winter, slumbering in the open air,
Wears on his smiling face a dream of Spring!"

-Samuel Taylor Coleridge


One sees the "spent coffee grounds as slug deterrant" allegation about as often as one sees the plastic bottle full of water as mole repellant & the latter is definitively falacious folk lore. The former appears to have some small basis for the expectation, though the expectation is so exaggerated as to likewise be falacious.

Coffee grounds are a good soil ammendment for plants that like acidic soil & can be used directly in gardens for this purpose & can be regarded as a mild, slow-release fertilizer when applied thinly in particular around evergreens. It is even better when added to the compost pile, filter & all. If used uncomposted & a little too thickly directly in the garden, it molds & may invite fungal disease, but thinly dispersed, it has the same effect as leaf-fall in revitalizing soil through its decomposition. In many areas, Starbucks Coffee franchises save their spent espresso grounds, & make these available for free to gardeners. In all, coffee grounds certainly are useful for the garden & should not be wasted in the garbage. Even so, the idea that spent coffee grounds per se also harm or repel slugs is at best debatable.

Most of the "It works! It works!" testimonials come from garden forums & newsgroups where sundry individuals misremember something they overheard. The first of the two prevailing amateur theories is that coffee grounds are harmful to the slug's guts, cutting them up inside. This is completely false, & is borrowed from the effects of saltwater Diatomaceous Earth which is categorized as an Organic Pestitide because it is a completely "natural" product (ground up silica derived from petrified prehistoric planktons). This natural product is actually quite harmful, killing beneficial insects, & a little bit like sprinkling finely ground glass in the garden; & organic gardeners have to be on-guard knowing the exact effects of such "natural" products that are no better than chemical pesticides.

The second amateur theory is that caffeine is poisonous to slugs. This is in the main correct, though spent coffee grounds have not to date been shown to be a good delivery method for the toxic ingredient, caffeine. The mechanism is not fully understood by researchers, but it is believed that caffeine may be a slug neurotoxin. It will indeed kill slugs & snails — though only at concentrations greater than found in spent coffee grounds.

The following assessment (of coffee per se, & of spent grounds) was conducted for & reported by & was based on extremely limited trials. Their method did not meet any standard of a controlled study, but at least it goes a little beyond the usual idle testimonies for or against:
"From Hilo in Hawaii comes the story that coffee can be used to control slugs & snails. Well, Sainsbury's Medium Roast Instant Coffee strong, black (without sugar) was sprayed onto Little Gem lettuce leaves to no effect. So we made some Lavazza Gold Italian coffee with likewise no effect when sprayed onto lettuce leaves. As a further check the Lavazza coffee grounds were dried, spread out & a nasturtium leaf laid on the layer of coffee grounds with other leaves as an easier option to feed on. The next day all leaves were gone. Eventually all the coffee grounds were eaten by the slugs with no immediate effect. So, back to the drawing board."
The problems with the amateur trial from fantasy-gardening are sundry. First, the credible Hilo study was of caffeine, not coffee; but the fantasy-gardening trial used actual coffee, then spent grounds, neither of which reach the 1 to 2% caffeine level that would slowly kill young slugs & snails. Even if they'd achieved a sufficient concentration, caffeine does not kill adult slugs & snails but only baby ones. And since caffeine as neurotoxin takes two days to kill even a young slug, that wouldn't keep them from doing in the Little Gem lettuce or the nasturtium leaves in the meantime.

So the rational hypothesis for the trial as designed by fantasy-gardening should have been that the Little Gem Lettuce & other test plants would be eaten to the ground whether or not the slugs later died, with nothing definitively proven one way or the other about liquid sprays or a scattering of spent grounds. While the outcome of the fantasy-gardening trial is probably accidentally correct (that coffee grounds or brewed coffee have no noticeable deterrant value against slugs) the study was not really designed well enough to qualify as evidence.

Two legit preliminary studies exist, one with follow-up. The first & most important was completed by Richard Hollingsworth et al of the US Department of Agriculture Research Station, Hilo, Hawaii. Hollingsworth's widely misrepresented study proved that what retarded slug activity was to use a liquid spray to coat entire plants with caffeine. This says very little about coffee per se, let alone spent grounds scattered at the base of a plant.

A spray of 1 to 2 percent caffeine is sufficient to work as a neurotoxin killing very small slugs & snails within 48 hours of ingesting a treated plant (so for it to work, the plant must be eaten, hardly the effect gardeners are shooting for). Hollingsworth "expected" (but in an interview in the British magazine Nature wouldn't say this is definitively so) that this caffeine solution should at least partially retard the activity of adult slugs & snails, but not kill them or stop them from reproducing. The deadly effect was upon baby slugs & baby snails only. Over time the number of actively chomping snails & slugs should be lowered by right of fewer of the critters reaching adulthood.

The other study comes from Peter Usherwood of the University of Nottingham & upholds Hollingsworth's findings. Usherwood too showed that if an entire plant were sprayed top to bottom with liquid caffeine in 1-2% solution, then the smaller slugs & snails that ate the plant would die within two days of having eaten the plant. But Usherwood found that plants also had a negative response to exposure to the caffeine. Leaves became yellow from toxification. So Usherwood suggested adding polymer to the solution to negate the harmful effects caffeine upon plants.

So while in the popular & misguided misrepresentation these studies have proven coffee grounds kill & repell slugs, they have in fact proved nothing of the sort. Spraying the plant with 1-2% caffeine is what has the effect, not coffee grounds, not even coffee. The average cup of brewed coffee contains less than .1% caffeine which is a fraction of what it would take to kill baby slugs & snails. Instant coffee has about .05%. However, Hollingsworth did show that as little as .01% caffeine solution was sufficient to make some slugs stop eating & move on to something else, therefore brewed coffee would presumedly annoy, though not kill, a percentage of the slug population.

Once again, this would require the entire plant to be regularly doused in brewed coffee merely to be slightly unappealing to the smallest slugs; coffee will not ordinarily brew strong enough to function as a slug neurotoxin. Personally I'm not apt ever to be out in the garden dousing my plants with caffeine sprays, even though it might cut down on slug activity along their generations by keeping fewer babies from reaching reproduction age. It has the weak potential of a slight immediate effect of protecting plants, but since applications must be rigorous & recurring, & even then plants must be eaten to in the long run effect the slug population, in the shorter term it does little to nothing.

One thing both the Nottingham & Hilo trials agree on: Caffeine in 1-2% solution is more effective than the accepted commercial slug bait, metaldehyde. But among gardeners, slug bait is usually at the foot of a plant — so it is comparing apples & oranges since caffeine must douse the entire plant which is then eaten to kill slugs, rather than used as a bait-barrier to actually save plants.

Hollingsworth noted that a future sprayed-on slug repellant "might" qualify as organic & is definitely nontoxic to humans. By contrast, metaldehyde or methiocarb on harvested produce would legally ban it for human consumption. Again, personally, I would not regard chemical caffeine & polymer solution as actually organic, but I expect it will within the next couple years be marketed as such, since indications are it would be both safer, & work better, than the popular but not very effective metaldehyde or methiocarb slug-bait products.

Because spent grounds were not the focus of the two studies above mentioned, it's still an open issue to what degree, if any degree, coffee grounds might echo the effects of liquid solutions dousing whole plants. The factors, however, would be more complex than gardeners seem to realize. Spent grounds don't usually have much caffeine in them; & anyone who brews decaf beans, well, the active ingredient just won't be there at all. Fresh 100% Robusta beans do in fact contain 2% caffeine — Robusta is the most bitter tasting of beans as a result, but if you could get whole fresh Robusta beans, grind them up, & sprinkle them fresh (rather than spent) throughout the garden, then it'd contain the amount of caffeine known to act as a slug neurotoxin. But it would have to be control-tested to be sure, as slugs would have to ingest it to be harmed, & if they bypassed it entirely, in favor of a tasty plant, then it would be useless as a bait.

To date, no study supports the effectiveness of coffeegrounds per se as slug deterrants, though hypoethetically a minor effect could be inferred to the grounds on the basis of the predictable effects of the liquid solutions sprayed on entire plants. The scores of testimonials of gardeners for or against are in the main valueless as evidence. This valuelessness is exemplified by the trials where they overlooked a half-dozen elements that made their negative conclusion a baseless though accidentally correct finding. The bulletin board testimonials in its favor are just as baseless. When gardeners believe it works because they think there was less slug activity after spreading grounds under plants, they are just wrong, since the limited trials that have been done show that this wouldn't retard adult slug activity in any immediate manner. The random testimonials are murky & mistaken, based on wishfulness, hearsay, poor observation, & failure to take into consideration such other factors as humidity & temperature which better explain fluxuations in slug activity.

Well-designed controlled studies are still required to assess coffee grounds per se. My reading of the scant evidence to date suggests coffee grounds might have a barely detectible repellant value on baby slugs & snails, but would have no effect whatsoever keeping plants from being eaten.

For a realistic solution, see:
Iron phosphate: The first honestly effective snail & slug control


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