Why coffee grounds are good for plants
Coffee grounds starts at a fresh plant with leaves and seeds, so it makes sense that gardens would thrive from plant matter. In a sense, that's what coffee is—plant matter. It is a drink that comes from plants, so plants help plants in a way!
Aside from that, coffee helps the soil because it has a lot of nitrogen in it, and gardens love nitrogen. The soil benefits from nitrogen, and helps the plants make chlorophyll, process food, and grow taller. Some other benefits that coffee gives to plants are calcium and magnesium. Basically, like humans, plants need a ton of nutrients to grow. Coffee provides that.
What kind of plants need coffee grounds?
So coffee is pretty acidic. You should probably avoid putting coffee grounds in your house plants because 1) It will get smelly and moldy eventually. 2) It will kill the houseplants, which don't need the same kind of nutrients as plants that produce fruit and vegetables do. They are very low maintenance in comparison to plants that you grow outdoors and in your garden.
However, plants that do need a lot of acidity are ones that produce certain fruits and flowers. Think about it - Strawberries and blueberries are pretty acidic tasting compared to a house plant right? They need acidic soil to produce fruits, which have a high amount of acidity in them. To be more specific, the fruits and vegetables and flowers that love acid are carrots, rhododendrons, lillies, hollies, roses, blueberries, azaleas, carrots, radishes, rhododendrons, hydrangeas, and cabbage. So, if you plan on growing any of these you should definitely add coffee grounds!
How to Use Coffee in the Soil
Coffee grounds should be mixed into the soil for plants to get the full blast of "health" benefits. You can also use them as a mulch and sprinkle some on the top. To avoid adding too much acid though, you should mix the coffee with other mulch items like wood chips and nutshells.
When mixing the coffee with the soil, again. Don't use too much of it at a time. There should only be a 1:4 ratio of soil. For example, for every four parts of soil, only one part should have coffee grounds. The same applies to mulch; for four parts of shredded leaves, only add one part coffee. You can also include the coffee filters in your mixture so you don't have to spend time separating the two.
Finally, you can use coffee grounds to defend your plants from pests. Because of the acidity in coffee grounds, slugs and snails and worms do not like to touch them. So, if you have these or other pests in your garden, you can build a "moat" around each plant and you will get much less annoyance with your harvest!
Another word of caution: Do not mix coffee grounds with your other compost and put all your fresh compost in the soil! Food has a totally different way of breaking down, and it takes way longer for solid foods to break down for compost than coffee.
How I use coffee for plants
When I started using coffee grounds in my garden, I would usually wait for my own separate compost bin to fill up next to my coffee station. Then, I head into the backyard with my garden gloves and my full coffee bin (it's a small waste bin) and get to work! I try to break up the filter into torn pieces and sprinkle that with the coffee grounds across the bed. For context, I’m growing some flowers and herbs. Then, I work it into the soil surrounding the plants. Don't sprinkle the coffee grounds directly on the plants! It benefits more from the roots that contact the coffee.