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Become a Smarter Gardener in 2019: Can coffee grounds slay slugs?

Drinking a lot of coffee over the holidays? Pondering what to do about all those slugs in your garden next summer? Here's potentially a good reason for gardeners to reach for a cuppa!

 Are slugs and snails repelled by caffeine?

In 2002, positive results were reported in Hilo, Hawaii.  Scientists with the U.S. Department of Agriculture tested 1-2 percent caffeine sprays to see if it would kill invasive frogs and learned instead that slugs and snails had died.

We should point out, the scientists used high doses of caffeine in their study, at a much higher percentage found in coffee we normally drink.

Promising results here in Minnesota

Fast forward to today. The restaurant at the Minnesota Landscape Arboretum has been donating its coffee grounds for staff members to use in the Arb’s Hosta Collection. They’re experimenting with it, hoping it will repel slugs.

This past summer, a trial was conducted by IPM Arboretum volunteers, who spread coffee grounds around hosta plants. Erin Buchholz, Integrated Pest Management (IPM) Specialist at the Minnesota Landscape Arboretum, observed, “the slugs would not cross the ring of coffee where it was applied.  The heavy rains this year favored slug damage, but areas where we applied it had less damage.”

The project at the Arboretum is in its first year, and while the results are visible, this is not a true experiment with controls and specifically measured quantities of caffeine.

Why trial caffeine to keep slugs at bay?

Caffeine is a drug. Coffee varies in the amount of caffeine from 50-235 mg, which amounts to about .07% caffeine in the strongest cup of coffee…far less than the 1-2% caffeine solution used in the Hawaii study. They also found concentrations as low as 0.01% repelled the pests. 

So for now, the word is, it can’t hurt. Go ahead and pour your leftover coffee around the hosta in your garden or use coffee grounds as mulch.  The organic matter in coffee grounds is good for the soil and you may find slugs are avoiding your hosta as well!


Results of the research in Hawaii by USDA Agricultural Research Scientists was published in the journal NATURE:

If you’d like to add coffee grounds as a mulch, here’s some “how-to” information: 

Author: Mary H. Meyer, Extension Horticulturist and Professor

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