Coffee grounds and gardening might not seem like a match made in heaven as there’s some evidence to show that we should be using this very abundant waste product in our gardening efforts.
And most of us have lots of this product so here’s how to work both ends against the middle and use this product.
The first thing to be aware of is there’s a lot of misinformation and unproven data out there about coffee grounds in the garden.
The Research on Coffee Grounds & Slugs
The research on slugs and caffeine shows that concentrations of caffeine as low as .01 % reduces feeding by slugs (they avoid caffeine treated leaves) but that it doesn’t kill them at that rate.
A 1% solution can be expected to kill 60% of slugs while a 2% caffeine solution will knock out 95% of all slugs. This 2% solution is more effective than the chemical normally used in slug control. (metaldehyde)
The 2% solution also damaged some foliage on the tropical plants being used to feed the slugs. This calls into caution the use of caffeine on more tender leaved plants.
So where does this leave you with your morning coffee grounds – and your garden uses?
Fresh coffee contains approximately .05% caffeine. Which is a heck of a long way from the 1% solution you need.
This means that coffee grounds and fresh coffee will not kill slugs.
Coffee Grounds In The Garden Mulch?
Used coffee grounds make an excellent mulch. Note that they are acidic with a pH of between 3.0 and 5.0.
BUT when they finish decomposing, they will be neutral (finished compost tends to be neutral) So they do not turn the soil acidic.
They can be used thinly all over the regular garden as organic matter so you can simply toss your used coffee grounds onto the garden if you like. Unless you’re adding inches of this stuff (in truckload quantities) to the garden, you’re not going to see a difference in your soil pH.
If you do add a massive quantity in one spot, you may want to dig them into the garden as there are reports that they will develop a fungal layer if left exposed to the air.
Coffee Grounds and Worms
Coffee grounds are beloved by worms. I used to have a worm bin and you could almost hear the cheer when I toss in the morning’s makings of used coffee grounds.
So if you have a vermiculture setup, use the grounds as a food source. If not, simply toss them onto the garden and the worms will find them.
Toss them out daily into the garden or into the compost bin to avoid fly buildups.
Composting Coffee Grounds
And they should go into your regular compost bin because they compost very well in the compost bin. They have a carbon-to-nitrogen ratio of 20:1, roughly the same as grass clippings. After making the morning wakeup, coffee grounds contain up to 2% nitrogen. So for composting purposes, consider coffee grounds “green” material similar to grass clippings.
And one of the interesting things about composting coffee grounds is that the microbes that do the composting will turn the coffee from acidic to a neutral pH.
So coffee does not make compost acidic.
So that’s all the real news about coffee grounds and gardening.
Doug’s Summary Notes
I hope this clears up some of the worst of the coffee ground garden rumours.
The one I hear the most is that the grounds will kill slugs or deter insects. If you get lucky, they “may” deter (and I wouldn’t even count on that) but they surely don’t kill.
But they are excellent organic matter to add to the garden.