Useful and free spent coffee grounds

Spent coffee grounds

Spent coffee grounds are one of the most useful, abundant, and widely available waste streams in America today.  For those who do not know, spent grounds are simply used coffee grounds.  Unfortunately, home coffee makers, offices, cafeterias, and coffee shops dispose of almost all of them every day.  There are several great biological uses for spent coffee grounds.

Biologically speaking, grounds are excellent life supporting material.  They are over 90% carbon, 5% nitrogen, and 0.5% potassium, and 0.5%  phosphorous.  All of which is biologically available.  PH is around 5.5.  If spent coffee grounds are left in the open air for several days, numerous molds of different colors begin to colonize the surface.  I have seen blue, green, white, gray, and yellow molds growing on spent grounds.

Here are some of the ways I have used spent grounds:

Garden soil amendment: Simply mix a small amount of grounds in with the soil.  From my experience there is no reason to mix in more then about 10% grounds into the soil.  Higher percentages of grounds can actually be harmful to the soil and plants by lowing the pH too low or by introducing too much nitrogen.

Compost: Adding coffee grounds to a compost pile lowers the pH and raises nitrogen.  Both speed up the composting process and increases the quality of the final compost.  However, I wouldn't make a compost pile more than about 20% coffee grounds.  Too much nitrogen from the grounds can again harm plants and make the compost process difficult.

Mulch: Spreading coffee grounds on the surface of a garden, or around other plants works well as a fertilizer and is a slight bug deterrent. 

Compost tea: Add into compost tea to lower pH and raise nitrogen.  Some people have found that by making a tea with only spent grounds, the tea can be dumped on plants to prevent and to "cure" insect infestations.

Growing mushrooms: Spent coffee grounds grow oyster mushrooms very quickly and successfully.  In-fact some people have started businesses doing this.  Check out Back to the Roots for a very cool example of a spent coffee grounds mushroom farm.

Fungi spawn substrate:  Not all fungi will form mushrooms on coffee grounds though.  But, even if mushrooms will not form for a particular fungal species, the mycelium will still grow so grounds can be used as a spawn substrate.  

Worm food: Add spent grounds into a worm bin to make worm castings.   I have never done this but would guess to add no more then about 30%.  From what I hear worms absolutely love coffee grounds.

A few years ago you could stop at any Star Bucks and they would have coffee grounds set aside for people to take for free.  Well apparently not enough people picked up these grounds so now you have to ask them to set them aside for you.  I have asked many different coffee shops and cafeterias to set aside grounds for me and they are all very willing to do it.  You may have to explain yourself to them as this may seem very weird and confusing to them. So, find a coffee shop and ask if they will set grounds aside for you.  I am sure, that soon enough you will have more grounds then you know what to do with, and all for free.