Used coffee grounds can be put to work in your garden in a few really useful ways. They can be sprinkled on your garden to deter pests like snails, slugs and ants. They won't stop a major invasion, but will definitely deter the usual traffic. (Believe me, I'm one of those people who have put piles of grounds across ant trains and they just walk right on over it.)Coffee grounds are excellent an soil structure improver. Mix your grounds with your soil to a depth of up to 10cm and to a maximum of 35 percent volume. You can do this in one go when you turn your garden over or simply mix the grounds in as you go. If you don’t drink coffee but would still love to try using used coffee grounds in your garden, simply ask your local coffee shop if you can have their grounds. Most coffee shops use at least 30kg of coffee every week, so there should be plenty on offer.
If you have a worm farm, don't forget to add some grounds to your worm farm too - up to 25 percent of volume, mixed with scraps. See also DIY Natural Morning Coffee Scrub.
Used coffee grounds contain essential plant nutrients such as copper, phosphorous, and are high in potassium and magnesium. The high carbon content helps feed the soil. Coffee grounds also release nitrogen as they break down so are an excellent slow release fertilizer.
Used coffee grounds are slightly acidic (around 6.2) and while this isn't high enough to be problematic, but do mix them with other organic matter rather than putting them directly onto a plant. Coffee grounds are particularly good for acid loving plants like tomatoes, roses, blueberries and camellias. All acid loving plants love used coffee grounds, including berries, potatoes, and camellias.