Add a bit of unused coffee grounds to the soil of citrus plants, moss, and other plants that thrive in acidic environments. Be sure to rinse the grounds, first.
Alternatively, used coffee grounds are neutral in PH, and can help a lot of different plants grow!
Just sprinkle the grounds onto the soil and gently mix them in before you would usually water the plant.
Even if you use a Keurig, you can carefully open up used k-cups after they’ve cooled- they’re really hot before they cool, be careful! Puncture the aluminum and scoop out the used grounds for your plants, and voila!