Why Are People Leaving Coffee Grounds on Their Porch? An Entomologist Explains

Some gardeners and homeowners sprinkle coffee grounds on porch, patio, and other outdoor spaces to deter flies, mosquitoes, and assorted insect pests. But does this tactic actually work or are there better ways to use old coffee grounds in the garden?

A bug expert weighs in on whether or not coffee grounds make an effective pest deterrent, and offers their all-natural tips for repelling unwanted insects without harming pollinators.

Why People Leave Coffee Grounds on Their Porch

Instead of throwing spent coffee grounds in the trash, some homeowners leave piles or lines of coffee grounds around their gardens and homes to keep flies, mosquitoes, and other pests away.

Coffee grounds are also often used to deter slugs and snails, reduce weeds without chemicals, and even repel neighborhood strays.

However, there isn’t a lot of actual evidence that coffee grounds deter pests effectively.

Want more gardening tips? Sign up for our free gardening newsletter for our best growing tips, troubleshooting hacks, and more!

How It Works

It’s often said that pests are repelled by the strong scent of coffee, or that mosquitoes won’t bite and ants won’t be able to navigate by their scent trails if coffee grounds are around. Some also argue that coffee’s caffeine content or abrasive texture deters pests from entering homes and gardens. But these claims may be more myth than fact.

“While some studies have investigated the effects of coffee extracts and caffeine on insects, the results are mixed and often involve concentrations much higher than what you would find in used coffee grounds or brewed coffee,” says entomologist Madeline Potter. “Plus, coffee or caffeine can potentially harm beneficial animals, like pollinators and aquatic wildlife,” Potter adds.

4 Other Uses for Coffee Grounds

While coffee grounds may not be the most effective method for keeping pests out of your space, that doesn’t mean you need to throw old coffee away once you’ve finished brewing. Spent coffee grounds still have a lot of use left in them and can be repurposed in a few different ways in the home or garden.

• Composting. Old coffee grounds contain a lot of nitrogen and can function as a “green material” in compost piles to help carbon-rich ingredients break down faster.
• Natural fertilizer. Spent coffee grounds can also be used as a natural fertilizer for garden plants of all sorts. Just sprinkle them around the base of your plants and scratch them into the soil with a hand rake or your fingers.
• Cleaning scrub. Thanks to their abrasive texture, coffee grounds also make an effective scrub for cleaning kitchen or garden items, or tidying up your barbecue after an evening of grilling. Just rinse the grounds off with water when you’re done cleaning, and don’t use them on surfaces that could stain.
• Deodorizer. Old, dried coffee grounds can be used in place of baking soda to soak up unpleasant odors in your potting shed, pantry, or other space. Once they’ve done their duty, simply toss the spent grounds in your compost pile.

5 Ways to Repel Pests Naturally

The jury may still be out on whether or not coffee grounds work for deterring pests, but there are a number of science-backed ways to repel mosquitoes, flies, and other insects without chemical pesticides than can potentially harm pollinators.

1. Eliminate standing water. “For mosquitoes, the most effective thing you can do is eliminate standing water,” Potter says. “Look out for clogged gutters, plant saucers, tarps, and even small crevices in outdoor furniture or toys where water can collect.”
2. Reduce food sources. Open trash or compost bins can attract flies and other pests, so it’s wise to keep these covered with a lid.
3. Install a fan. Mosquitoes and many other pests are not strong fliers and can be kept away from porches with the addition of a strong fan.
4. Add a barrier. Screening in porches or installing a screened-in tent in your yard is also an effective way to keep pest insects out.
5. Use natural pest repellants. Limiting the amount of time you spend outside in the early morning and evening, and wearing natural insect repellant sprays can help you avoid mosquito bites, too.