Recently I have been reading some conflicting information over the use of coffee in the garden, or more precisely, the grounds. I have always used it in with my blueberries (mixed with pine shavings that we have for the chicken coop). This also acts as a mulch over summer. Our blueberries thrive. There is no arguing that.
One train of thought is that they are a good source of nitrogen and that acidic loving plants (like berries) respond well to them. Some also add it to their compost as a nitrogen source – I know I have certainly done that in the past. And if you follow hugelkultur principles, it is also added, which I did last year with my hugelkultur approach to my grow bags.
Others also swear by using it as a pest deterrent. I will admit, I am on the fence there after a doing a science fair experiment with egg shells, coffee, and a combination of the two, and watching the snails climb all over both many years ago.
More recently I read that coffee grounds actually have a neutral PH, and that it is the actual coffee itself that has the high acidity levels. Interestingly, following along that train of thought is the two strawberry planters at work – one that is flourishing has been receiving those half cups of cold coffee I often leave on my desk.
There is also some that suggest that as coffee grounds break down it releases chemicals that can be harmful to worms and other beneficial insects in the soil, and that it has antibacterial properties that kill the microbes in your compost. Likewise, there is some research that suggests that due to coffee plants being allelopathic it stands to reason that coffee beans are too, therefore plant growth will be inhibited. But then again, so are pine trees so surely I shouldn’t be using that as a mulch either.
That being said, it hasn’t stopped me from sitting there a couple times a year, slicing the tops of my coffee pods, saving the grounds, stacking the pods and sending them off in my postage bag to be melted down again. My blueberries are now nicely mulched with a combination of pine shavings, coffee grounds, and a few pellets of Dynamic Lifter. I don’t over feed my blueberries, and only do this a couple times a year.