The first step is understanding what coffee can do for your plants. The main ingredient in the used grounds is nitrogen, but there are traces of the other important nutrients like phosphorus and potassium. Fresh coffee is highly acidic, so may be useful for acid loving plants like magnolias and blueberries or for gardeners wanting to turn their hydrangeas blue.
However, by using your coffee grounds fresh you will miss out on your morning cup, so we’re going to focus on used grounds which are much more gentle, coming in at an average pH of 6.9 – 6.2. When in doubt, test your soil using a home pH kit available at most nurseries.
If you have trouble with slugs and snails, coffee can provide a useful deterrent. Not only do these gastropods find the coarse grinds themselves difficult terrain to travel over, but the residual caffeine will turn them away as they are sensitive to even trace amounts.
For best results dry the used coffee on newspaper in the sun before spreading around worm affected areas. Be careful not to put the grounds too close to tomato plants as they can react poorly to uncomposted grounds.
Don’t let their appearance fool you. Coffee grounds are brown to the eye, but in composting jargon used coffee grounds are very much in the “green” category, with a carbon to nitrogen ratio of 20:1 – on par with grass clippings.
Nitrogen creates the heat in compost which allows all the organic matter to break down into plant food. Studies at Oregon State University found that coffee grounds were able to sustain high temperatures in compost heaps for up to two weeks – long enough to kill ‘a significant portion of the pathogens and seeds’ in the mix.
In order to keep your compost healthy, it’s important to balance your “green” elements with “brown” or dry elements such as dry leaves or newspapers to avoid the mix becoming too slimy and smelly. So if you’re introducing coffee grounds, be sure to add some dry material at the same time.
Worms love coffee just as much as we do! Break down the used coffee in your hands and work the grounds into the top of soil. The worms will go to work devouring the coffee grounds and aerating the soil.
If you’re working with larger amounts of used grounds, make a light fluffy mulch by mixing chopped dried leaves and grounds. You can experiment with wood ash as a mix to create a nutrient dense soil amendment which the worms in your garden beds will thank you for.
The last piece of advice comes from a neighbour who swears by coffee and banana peels applied directly to the base of her rose bushes. While I can’t find any evidence, I cannot argue with her results. As Peter Cundall would say, “that’s your bloomin lot!”