Used Coffee Grounds
Used or spent coffee grounds still contain a significant amount of caffeine.
A study conducted by The Department of Nutrition, Food Science and Physiology, School of Pharmacy, University of Navarra found that spent coffee grounds contained 3.59 to 8.09 milligrams of caffeine per gram of used coffee grounds.
For the amount listed above, we took the average of this and multiplied it by the amount of ground coffee typically used to make an espresso.
Caffeine in Used Coffee Grounds Varies by Brew
Brewing methods with longer brew times will result in less caffeine in the spent coffee grounds since caffeine is highly water soluble.
Take Care With Disposing
Care should be used when disposing of spent coffee grounds as caffeine has been detected by some municipalities in our waterways and reservoirs.
Want to add them directly onto your garden? Don't. Despite anecdotal advice - research shows that direct application of coffee grounds will inhibit growth of plants. So at the least, try to compost them first.