Use Coffee, Not Coffee Grounds

While caffeine will kill slugs, sprinkling coffee grounds around your plants won’t keep the pests from devouring your garden, says Linda Brewer, a certified professional soil scientist with Oregon State University. “There isn’t enough caffeine left in coffee grounds to repel or kill slugs,” Brewer explains. “A lot of the caffeine has been washed out in the process of making coffee.”

However, there is a coffee-related way to keep slugs away from your plants. Dilute brewed coffee with water and pour it on the soil around the base of the plants. A study by the USDA Agricultural Research Service1 found that using a weak solution of coffee as a soil drench caused 100 percent of the slugs to leave the treated area and 92 percent to die of caffeine poisoning. Caffeine acts as a neurotoxin, disrupting slugs’ nervous systems.

Treat Slugs with Weak Coffee

To make a batch of super watery coffee that will slay slugs, mix 1 cup of cold coffee with 4 cups of water, Brewer says. Pour it on the soil in a ring around the plants you want to protect. Slugs will come out of the soil, crawl away, and die. One downside: Earthworms and other beneficial soil residents could be killed by the coffee solution, too.

It's best not to overdo the coffee near your plants so you don't damage their roots, so don't keep reapplying it regularly as a preventative. Instead, only reapply if you notice slugs returning to the area after the first treatment has been washed away by rain or watering.

Try a Coffee Spray

You can also make a foliar spray that will protect your plants from slugs. Mix 1 part cold coffee with 9 parts water and spray it on plant leaves and stems, Brewer recommends. Rain will wash the diluted coffee spray off the leaves, so you may need to re-spray after a heavy rain.

Homemade coffee spray won’t harm beneficial insects like bees because their water-repelling exoskeletons protect them from the caffeine. Slugs have no such protection, so the diluted cold coffee solution is deadly for them.

So brew a pot of coffee and offer it to some slugs you don't want around. As for those coffee grounds, they can help your plants in another way: Making compost with them