For what you are using them for, you wouldn't have a problem. Fresh coffee that hasn't been brewed has a lower pH because a lot of the acid is cooked out. Prolonged use of fresh coffee can make the soil too acidic over time. The plants you are putting it on love some acid and if either your soil, of your water is alkaline, that would be even more helpful. On an obscure note not relevant to this discussion, I have read there is a chemical in coffee that gets brewed out, that would be a problem with fresh coffee. It causes smaller tomato plants to degrade and sometimes die.

Coffee has well over 140 different chemicals in it including 3 carcinogens, and 2 neurotoxins, so coffee is a very complicated liquid.

Just look at Camelia sinensis ( what people make tea out of ) not only can you kill the same insects with tea, but in nature, the old leaves fall off the plant and that act as a pre-emergent herbicide, so other native plants don't take over the camelias.

So, is spreading used coffee grounds on azaleas (and working it in a little) almost pointless as a soil acidifier?

It would seem so. One of the guys I follow on YouTube has been using coffee grounds directly and in his compost for a few years. In a recent update, he had tests done on his soil to see how much he had improved it with all his organic methods and while everything had improved, the one thing that had not was the pH of his soil. Despite years of adding coffee grounds, and I mean huge bags from Starbucks and such, he had fairly alkaline soil.

Link to his video update.

So....doesn't indicate that the strategy was "useless", just that it did not lead to pH change. And this is extremely common across a wide range of methods used to either increase or decrease soil pH, due to buffering capacity-the tendency of soils to return to whatever pH they were, as soon as the person doing the work to change that pH leaves for the day! OK-I kid, but soil pH change can be frustrating. But even in the used coffee grounds example as given, soil organic matter-and possibly other less-well-identified factors-would have been changed for the better.

+oM

But I wonder if the coffee grounds lowered (acidified) watering? Whether from rain or irrigation, if water has to pass through the grounds, would it acidify the water and thus provide some benefit to acid-loving plants? Hmmm.

Carol in Jacksonville

If we're still talking about used grounds, I think what we're finding is that any result would be very, very minor in that regard. Couple that with many soil types' buffering capacity, and I think it would be a complete wash.

+oM

So we should all switch to using fresh coffee on our plants?

I put the grounds in our compost thing and eventually they get to the plants.

"So we should all switch to using fresh coffee on our plants?"

Sure if you want an expensive fertilizer, I drink the coffee and throw the used coffee grounds in my garden. Fresh coffee has a pH of 5 and used coffee grounds have a pH of ~6.2. You would have to add a lot of coffee grounds to make a dent in soil pH, it's cheaper to use sulfur.

What's In Coffee Grounds?

Fresh Coffee pH

Used grounds do add things to the soil. If anything, they help to attract worms. But they do not affect the acidity of the soil or deter slugs/snails. For that you would need unbrewed grounds and personally, if I am buying coffee, I am going to brew it.

As far as affecting acidity though, I imagine it is more that the plants simply detect and are happy with the acidity of the grounds than it is that the grounds affect the soil itself, but that is merely my own uneducated theory on the matter

From what I have heard using coffee isnt any better than not using coffee. I sometimes put used coffee grounds in soil just to add volume to the soil ( so i have more of it ) :P If you want to use it i'd just brew it up at least your house will smell good. Also if its less than a year past expiration date id just drink it. its not like expired milk or something that could make you sick from using it past the date. Another great use for it (and ONLY do this if you have a detachable shower head) is use it as a full body exfoliant in the shower. It will look like a national geographic mud party but the detachable shower head will make rinsing the shower out after easy. Your skin will feel amazing after. Use fresh or used coffee grounds but if you use them used, make sure its from the same day or kept in the fridge up to 4 days after before you use ( dont let it sit in shower or it can get moldy) Enjoy.