There are mixed opinions about coffee grounds and gardening. Are coffee grounds actually beneficial to your garden? Will they harm your plants? Are they too acidic?
Well...as with most things, it depends. Many studies and lab tests have been done about the decomposition of coffee and the use of grounds in a garden, and they don't always agree. There are methods that can be beneficial and methods that can be harmful. But when used properly, most people find that grounds can be a positive addition to a garden.
There are a few important points to note when using coffee grounds in a garden:
Coffee grounds are commonly considered to be acidic.
Of course, there are contradicting studies (see this summary from WSU). And used grounds have much less acidity than fresh grounds. But, high pH levels or low, most sources do agree that they can be used in compost. Some people like to take precautions like rinsing grounds with water or adding lime to offset acidity.
Coffee grounds can clump together.
When adding to compost, spread the grounds out over the soil and mix them in. This allows for a well-rounded compost mix, and keeps the compost particles relatively small.
Coffee grounds should be used in moderation.
Seemingly everybody agrees on this point: whether adding grounds to compost or using as a mulch, mix them with other materials and don't overdue the amount of coffee. The previously mentioned WSU study recommends keeping the coffee ground composition of your compost below 20%.
So, with all the difference of opinion, why not leave coffee grounds out altogether? Well, the otherwise wasted coffee grounds will be recycled. Also, legend attributes a few interesting benefits to coffee grounds: most notably, they deter slugs and insects, worms love them, and they are resistant to disease and fungal rots. These are no small thing to any gardener!