Used coffee grounds, those already brewed, can benefit your plants. Brewed coffee grounds sprinkled around your plants add organic material, aerate the soil and help retain water.
Fresh Coffee Grounds
Coffee grounds added fresh from the can add acid to the soil, but is only good for acid loving plants like hydrangea, lilies, blueberries, and azaleas. It can damage plants that don’t like acidic toil. Root crops such as carrots and radishes also like the acid boost. The acid in fresh coffee grounds used a mulch around acid loving plants, seems to suppress some pathogens and weeds. They also seem to deter cats from digging in your garden and stop slugs who don’t like to crawl over the gritty surface. Because of the high caffeine content of fresh grounds, apply small amounts until you test how it will affect your plants and use only on acid loving plants.
Used Coffee Grounds
If you compost, used coffee grounds are considered green material and will need some brown material added to balance it. You can throw the filters in also. Because the coffee grounds don’t break down until composted, either in the compost pile or in the ground, it’s good to scratch it into the soil or use it in the planting hole when putting in new plants. Using brewed coffee in the garden adds some nitrogen to the soil as it breaks down. After coffee grounds are washed with water through the brewing process, their pH levels are near neutral so they can be used on vegetables as well as flowers.
As they break down, brewed coffee grounds release minerals such as potassium which attracts earthworms and you can’t have any healthier soil than to have it teaming with earthworms. The castings they leave in the soil are like a five-course meal to plants. Other minerals such as copper, magnesium and phosphorus are released and taken up by plants.
You can add 2 cups of brewed coffee grounds to a gallon of water and let it steep overnight. Filter out the grounds and use the liquid as a foliar spray. Brewed coffee grounds are not high in nitrogen, only about 2 percent, but the other ingredients in them benefit plants well. If you need nitrogen in your soil, use a higher nitrogen granular fertilizer along with the foliar spray.
It’s always good to use caution with any new plant treatment until you see how it will affect your plants.