Many of us have heard that coffee grounds are good for plants. People recommend adding used coffee grounds to the garden, either directly or mixed with compost or other organic materials, as a means of adding nitrogen-rich organic matter. Some coffee shops even offer their used coffee grounds free to interested gardeners. Researching which plants might benefit from used coffee grounds and was surprised to learn that coffee grounds can be harmful if not carefully applied in the garden because they can dangerously increase the pH of the soil as well as suppress plant growth.
A consistent theme among various articles is that the coffee grounds directly applied to plants suppress plant growth. Coffee is rich in caffeine. Caffeine reduces competition from other plants by suppressing their growth. Thus, there is a possibility that coffee grounds can suppress the growth of plants we want to grow and not just weeds that we don’t want to grow.
In addition, some articles described the acidity of coffee grounds as being neutral in pH and other articles described the coffee grounds as highly acidic and best used on acid-loving plants like blueberries and azaleas. Regardless, too much acidity can harm some plants, even acid-loving plants.
There are some potential benefits to adding used coffee grounds besides (hopefully, safely) increasing the soil pH. Coffee grounds can increase the water holding capacity of the soil, which could be a good thing or a bad thing depending on the soil type. Coffee grounds are about 2 percent nitrogen by volume and composted coffee grounds can be a substitute for nitrogen-rich manure. (Uncomposted coffee grounds, however, are not a nitrogen fertilizer.) Due to their nitrogen content, coffee grounds are considered green compost material and should be balanced with the addition of brown, carbon-rich compost material. Be aware, however, that if your soil is already high in nitrogen then excess nitrogen can result in rapid foliage growth at the cost of flower and fruit production.
In sum, if you would like to use used coffee grounds in your garden, it is highly recommended to combine the grounds with compost rather than applying the grounds directly. You could even conduct an experiment to see how your plants grow with and without composted coffee grounds.
For more gardening information go to cecolusa.ucanr.edu ■