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!
Resources
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
