Using coffee grounds in your garden as a fertilizer is a great way add nitrogen while you are using something that would otherwise take up space in a landfill.
The majority of my grounds get dumped out, directly on the soil, as sheet mulch around plants. My rule of thumb is to use a good half-inch thick layer atop the normal organic mulch. The grounds break down relatively quickly as worms and soil microbes go to work, and as this happens I add additional amounts over time.
Side-dressing for Heavy Feeders
Coffee grounds are high in nitrogen, at about 10%. Depending on the exact beans and extraction process, the carbon to nitrogen ratio of coffee grounds can be as low as 11:1, which is an ideal ratio for plant and soil nutrition. With nitrogen levels like that, pure coffee grounds make an excellent side-dressing for my leafy greens and hungry fruiting vegetables, like tomatoes, spinach, and squash, early in their growth cycle.
Natural Slug Deterrent
I use coffee grounds formed into uniform circles around the plants most susceptible to slugs as seedlings…I still use Sluggo here in my Pacific Northwest garden, but I believe this helps.
If you are composting using various worms you’ll find that adding coffee grounds has some major advantages for this process.
Suppression of Fungal Diseases
The natural mold and fungus colonies on coffee appear to suppress some common fungal rots and wilts, including Fusarium, Pythium, and Sclerotinia according to research. Incorporating coffee grounds into the compost does help to prevent build-up of nasty verticulum and fusarium wilt.
Where can I Get Coffee Grounds for My Garden
Local independent coffee shop will save back grounds if you ask, and chains like Starbucks and McDonalds will make them available for patrons in special displays.