A new use for your cuppa

Doug Green pointed out quite rightly in his blog yesterday that used coffee grounds, while having a lot of wonderful uses in the garden, aren't really much use in fighting slugs. He's more or less right. They've certainly never slowed them down much in my garden. (Now isn't that a thought? Slugs that are even slower!) He then threw in as a joke: "while I didn't exactly tell her she was wasting her time spraying yesterday's cofee (sic) on them to "kill them", I didn't exactly tell her she wasn't."

As it happens, the newbie gardener was on the right track, if she was talking about liquid coffee. Seeing as Doug jumped from grounds to spraying coffee in a single sentence as if he were talking about the same thing, it's not too clear what she actually said. I fired off a comment to him after reading his post and link rather inattentively and he quite rightly ignored it.

Now, however, as public penance - ahem - I will clarify the issue. A group of scientists in Hawaii were conducting research on ways of controlling invasive frog species and discovered quite inadvertently (gotta love serendipity) that they were killing off slugs. Here's a link to their very dry abstract which basically tells you that a caffeine solution applied to leaves or soil kills slugs and is not harmful to food crops. Now Hollingsworth and crew were using caffeine concentrations somewhat higher than that found in your standard cup of Joe, but they found that reduced concentrations were also effective, although more slowly and at a lower rate. Not only that, but slugs would turn away from leaves sprayed with a caffeine solution. If you want to read more of the details, click here. Or for a longer article from Science News, here. It is worth noting that Hollingsworth is a gardener, which helps explain his enthusiasm in following up the unexpected lead.

So you might just try pouring that cold coffee on your hosta or better yet, on the soil around it. It's probably not a good idea to do it too often, as caffeine is slightly toxic to plants too and some tender-leaved plants could sustain some damage. Oh, for what it's worth, Hollingsworth says that the grounds do deter slugs somewhat, but not as effectively as a caffeine solution.

Now if somebody could just tell me what the caffeine concentration in a cup of espresso is... I certainly don't bother with that weak-kneed instant stuff.

Technorati tags: Coffee, Slugs