This is the first time I have ever heard that coffee grounds -when used in compost or soil - could hurt plants .
I have used coffee grounds for years and have seen no ill effects on plants .
I will do some research and see what turns up .
Mary

Wow ! I read about coffe grounds on this site and have had great luck with them!It made a huge difference in a lot of my plants . I really interested in what you find out Mary .

Well, that was what I thought, too. But this article listed chemical compounds that supposedly retard plant growth. I was really surprised and wondered if it could be true. I wish I could remember where it was...I think it was on another forum here...I'll do a search and see if I can find it. If so, I'll post the link, so you can see if you think there's anything to it.

Still Confused,
Marcia

My wife brings home 10 lbs at a time from her work and I save ours too, worms love um and so does the garden

Uh Oh!... I just mixed a huge batch from Starbucks with 2 large bags of potting soil and about 5 gallons of sifted compost. That was spread around the garden today.

Much was also used to fill several pots that await the birthday roses.

And, I did mention on another thread that several of my roses have sent up beautiful basal breaks and the only thing I am doing different is that I have been schlepping out the grounds every day and doling them out to the roses.

My oldest Duchesse actually has !3! basal breaks.

Was this study funding by a fertilizer company?

Marcia, that is confusing. I also have been using coffee
grounds on my plants directly and in compost for years.
It seems to be good for my plants.
Felix

here are my only complaints regarding grounds and both are due to human stupidity,mine!
If you do not use them right away they get moldy in the bags,Have also gotten soldier worm larvae but the ducks found them to be a REAL treat.
I did not mix them well enough into my soil and they formed a crust which DID inhibit the seeds from sprouting.However,after learning my lesson and turning it in(AND reseeding) the plants are quite vigorous.
I even heard they were suppose to help the dreaded Sago Scale when sprayed directly on plants.
I think it is like any food we eat they will always find something wrong with it.

I know they are good for the sago scale and other problems of that nature, and I always thought they were a good addition to compost, which is why we have always added them to ours. This article just blew my mind, and I wish I had saved the link to it, because I simply cannot find it any more. I KNEW when I read it I wanted to ask this group about it, so why I didn't at least make note of where it was, I don't know. (Where's all that wisdom that's supposed to come with age? I'm just as big a DODO as ever!)

Anyway, I'm glad for confirmation that none of you have read this and that you are using them in the garden with no noticeable problems. If I find the article/post again, I'll add it here.

Thanks for all the responses.

Marcia (Wondering if I hallucinated the whole thing??? I've gotta quit sniffing the Ben-Gay fumes!!) ;o)

from greendaily.com :

Tips for using old coffee grounds

Soften and add shine to hair. When washing your hair, rub coffee grounds through wet hair and rinse. For brown hair, coffee grounds add highlights.

Use coffee grounds as an exfoliant for skin. Pat on skin, massage over skin, rinse.

Add coffee grounds to your skin mask beauty routine.

Make homemade tattoos (temporary) with henna and coffee grounds.

Fertilize plants. Old coffee grounds are nutrient-rich for plants .

Add used coffee grounds to the pots of indoor plants.

Work used coffee grounds into your garden soil before seed planting. After your plants start to emerge, work in coffee grounds near the plants. Used coffee grounds are said to repel snails and slugs as well as adding nutrients to the soil.

Increase your carrot and radish harvest by mixing seeds with dry coffee grounds before planting the seeds.

Use coffee grounds to repel ants.

Keep cats from using your garden as a kitty box by spreading used coffee grounds and orange peels throughout flower beds.

Deodorize a freezer. Place a bowl with used coffee grounds in the freezer to remove unwanted odors.

Add a few drops of vanilla to coffee grounds.
Rub coffee grounds on hands to get rid of smells from chopping or cutting up pungent foods.

Make a used coffee grounds sachet. Fill old nylons or cheescloth with dry used coffee grounds. Hang in closets to absorb odors.

When you need an abrasive cleaner, coffee grounds can be used. Be careful of any surfaces that might stain.

Remove furniture scratches with wet coffee grounds.

Got a fireplace? Sprinkle wet coffee grounds over the ashes to keep from becoming engulfed in the plume of dust ashes create when you need to remove them.

Dye fabric, paper or Easter eggs. Simply add used coffee grounds to warm water and let sit a bit to create a dye.

After you give your dog a bath, rub coffee grounds through the coat of your pet. Coffee grounds are said to repel fleas.

Keep bait worms alive by mixing coffee grounds into the soil before you add worms.

Grow mushrooms on old coffee grounds.

I've found that top dressing with coffee grounds will crust easily
over the soil, and will inhibit plant growth, probably by not letting moisture enter. Or too acidic?
But added to my compost, they do no harm. I love them in the compost.

Found this info about coffee grounds - thought it might interest OP.

From container gardening forums - someone was asking about tea bags. First reply is the one that talks about not using grounds. (This is first time posting - so I hope that I include link correctly.)

Here is a link that might be useful: From container gardening forum - Tea Bags

Gatormom, that was great info. I have to get my DH to switch from instant to regular.

Julie

Hammockgal, thank you! That is the person whose post I read, but it was another post, much longer and going into more detail on the growth inhibiting qualities of coffee and tea. Here is a quote from the post you linked to:

"We also know that the tannins in both coffee and tea are known allelopaths (growth inhibitors). There are ongoing experiments to develop herbicides using extracts from both coffee and tea that cause me to want to say they might serve better as a nonselective herbicide than as a tonic. I would not use either on my plants; nor would I add tea bags/coffee grounds to my container soils."

This poster (tapla) is the one who "created" Al's Mix for container plants and is very knowledgeable about the chemistry of things, apparently. And it was definitely his longer post that concerned me about the coffee. I will keep hunting for it.

Thanks so much for posting the link to this shorter post. I knew I wasn't COMPLETELY crazy...teetering on the brink, maybe, but not completely gone.

Marcia (I will keep looking for the longer one)

Well, they also say that certain things like corn gluten meal will retard some seed germination so you could sort of use it as a weed stopper in an established lawn but then again, any sort of seed meal is gonna act as fertilizer. Coffee grounds are just seed meal. Used coffee grounds are leached seed meal.

I don't know that coffee grounds are a wonderful and perfect as some lists might tell you but they are also usually free and definitely not terrible for your garden.

I've started seeds in mixtures using coffee grounds and they came up.

Too much of any one thing can be bad. Too much coffee grounds on the surface can cause a crust so its probably better to rake it in a bit.

I do know that used coffee grounds if left in the bag or bucket will get moldy, for this reason, I would not use them for growing mushrooms without sterilization.

If there are chemicals in coffee grounds that could be bad for plants, I don't think they are so super bad as to kill your plants unless you were to completely bury your rose bush in a 10 food mound of used moldy coffee grounds.

I've not seen ill effects to compost or soil from used coffee grounds. And only heard once of the cold pressed coffee used coffee grounds perhaps being more acidic than normally processed coffee grounds and therefore being a bit too acidic for the worm bin when used in large quantity.

I suppose the moral to the story is, moderation. Most any medicine becomes bad when too much is used.

A lot of people have been using coffee and tea both grounds and brewed liquid to water/fertilize their plants for a long time now. If there was a problem with this it would have become apparent by now.

You can use too many coffee grounds around your plants. If you pour them out an inch deep or more they tend to want to become crusty, moldy, and impervious to water penetration. The solution is the apply them more thinly or mix them with some other material first. My roses and camellias get all of our coffee grounds and any liquid that might be left over and they like them. If I were picking up Starbucks coffee grounds by the ten pound bag I'd just have to be careful not to mound it up around the plants.

.....Alan.

Marcia, you're not having a old age moment.

Jeff Gillman is the horticulture professor at the University of Minnesota. In his new book, he says coffee grounds can inhibit growth for tomatoes.

"The Truth about Garden Remedies: What Works, What Doesn't and Why" (Timber Press, 2008)

Corrie

My grandfather always dumped his old leftover coffee and the coffee grounds on the soil around my grandmother's roses every day since they first got married.
Her roses were huge and beautiful with no pests or diseases. I think it must be okay for certain acid tolerant plants that are planted in the ground.
~Lenette

I'm not arguing for or against coffee grounds. Just reporting that a man who is apparently very knowledgeable about the chemistry of things (and one whose soil mixture is widely used by many on GW) says coffee contains high levels of growth retarding chemicals. He seems to have the science to back this up, and if I can find his longer post, I will put a link here, so those who are interested can read it and decide for themselves.

If studies are being done to use coffee to make an herbicide, it does make you think.

TC, it is my understanding that corn gluten meal only acts as a PRE-emergent weed control, and that it does act as a fertilizer on anything that has already sprouted. I don't think there is any doubt about it working if used before weeds sprout, though I haven't tried it yet myself.

As for the rest, I have no firm opinion either way. We have been mixing coffee grounds into our compost for a long time, just because all our kitchen refuse goes into the pile, except for meats and cooked things that might have fats in them. Whether or not it retards growth, I can't say from personal experience, because I have nothing to compare it to. In order for me to have an opinion, I would have to try one batch of compost WITH grounds and one without, and compare the results.

Please note that tapla doesn't say it will kill plants. Just that it retards (slows) the growth of them. I'm still curious about this, and still hunting for the original detailed post I read.

Marcia

Okay, I posted on the Container Garden Forum, and Tapla kindly clarified his comments on coffee for me. Here is what he says:

"Here is the comment, so you can link to your own thread, but it is directed primarily toward the use of coffee and tea as a "tonic" for containerized plants.

The use of coffee grounds in compost, as a mulch, or even incorporated in mineral (garden) soils would be less of a problem than the actual coffee or tea, which would contain far more of the potentially harmful compounds.
Forum discussions frequently center on the question of adding dilute coffee/tea to plants as a 'tonic', but Arabica (coffee) and Camellia (tea) are known for their toxic alkaloid (caffeine) content and their allelopathic affect on plants as well as autotoxic (to their own seedlings) effects on future generations.

Caffeine interferes with root development by impairing protein metabolism. This affects activity of an important bio-compound (PPO) and lignification (the process of becoming woody), crucial steps for root formation.
We also know that the tannins in both coffee and tea are known allelopaths (growth inhibitors).

There are ongoing experiments to develop herbicides using extracts from both coffee and tea that cause me to want to say they might serve better as a nonselective herbicide than as a tonic. I would not use either on my plants; nor would I add tea bags/coffee grounds to my container soils."

As you can see, he is mostly referring to containerized plants and potting soils, though the growth inhibitor factor is apparently true. But he suggests it would be much more of a problem if using diluted coffee itself on your plants.

As I say, I have no firsthand knowledge of any of this, but I thought his comments were very interesting, and they definitely make me think about how I would want to use coffee grounds. Our compost is often used in containers, and even though we don't use brewed coffee or tea, I don't think I want to have coffee grounds in my potted plants any more. Some of you may want to rethink the use of grounds, and some of you may not. If you are having good luck with using them, why change? (If it ain't broke, don't fix it.)

Basically, I just wanted to get more info on the subject and put it out there for general consumption.

And Corrie, thanks for the vote of confidence on my mental state. There are days I'm really not too sure.....*grin*

Marcia

Quick question - are we allowed to post links to a thread on a different forum - if it pertains to the topic at hand?

Thanks.

As far as I know. We often have. I just chose to copy the post so it would be quicker and easier for everyone to read.

Marcia

Well, I have to run, so will include what was said - by Jeff Gillman

"Here's the quick story on Coffee grounds -- a number of researchers have studied them -- they can decrease the pH of your soil or media, but not as effectively as something like sulfur which I prefer. Coffee grounds contain allelopathic chemicals which seem to harm some plants, however, for most plants coffee grounds just break down the same as any organic matter and provide food for your plants. Naturally the book goes into much greater depth -- but if you've got specific questions I'd be happy to give them my best shot.

Jeff"

"As a mulch coffee grounds work great -- the allelopathic chemicals may even help control some weeds. These chemicals shouldn't inhibit the growth of your crop unless you're incorporating the coffee grounds into the media -- which isn't generally a good idea. If you are incorporating coffee grounds into compost and are concerned about that -- don't be -- the composting process should take care of the problem as long as you take care to compost for a nice long period of time (I usually recommend 6 months).

Jeff"

He doesn't say it in the thread, but as corrie22 wrote (and others in the quoted thread), in the book he talks about it inhibiting growth of tomatoes.

My guess would be that it would include other plants in same family, I honestly do not know.

You can Google either quote to find forum/thread where I got that info.

Good luck - for what it is worth, I use coffee grounds in the compost, and we have had no problems. But, I don't use it for tomatoes either, that was something we were taught growing up.

;) You replied while I was copying and typing.

I was meaning a different forum on a different site. No biggie - quote(s) can be used to find original, if wanted.

Oh - you are welcome for original finding. I had just finished reading the tea thread and searched for coffee and found your thread, was like - hey, I just read that. :) Love when things actually fall into place.

Okay, all of this is starting to make more sense now. I can see that used properly for certain things, coffee grounds can be beneficial. Brewed coffee or tea probably should not be used as a tonic for plants, especially in containers, and in aged mulch the grounds should be fine.

So interesting. Like everything...moderation and specific circumstances play a big part.

Thanks for all the input, everyone. I have learned a lot of new stuff today.

Marcia

I get the impression that there is a gardening difference between used coffee grounds ( UCGs ) and brewed coffee or unused coffee grounds .
UCGS are usually added for their addition as an organic amendment to soil .
UCGs normally have a low amount of acid due to the brewing process .
I just know that having used Starbucks UCGs has improved the quality of my compost , soil and plants .
I have not tried to germinate seeds in this mixture .
After reading this post , I think I will be more cautious about the soil I use when attempting to grow things from seed .
Thank you for this post manature .

I cannot say about germinating seeds in a media with a large amount of coffee or tea grounds. Nor have I used brewed coffee or tea to water planted seeds. It may be that it does have a negative effect on them.

I can say that I and many others have used coffee/tea grounds around plants such as roses and camellias for many years with only positive effects. As I said earlier you don't want to mound them around the plants, but used properly they can beneficial. This applies as well to any container roses or camellias I may have at the time as they usually get first chance at whatever coffee grounds and leftover liquid there may be. I just planted a Pink Perfection camellia that waited an entire year for me to get around to it. It was horribly rootbound from the wait but was full of blossoms when I set it out and is still blossoming. I've been pouring coffee into it at least one day a week, sometimes several, for the whole year.

Tomatoes I haven't used grounds around so much as I don't usually have enough to go around, but I wouldn't hesitate to use them if I had them. In moderation of course. They don't need a lot of nitrogen.

Too much of anything is simply too much. Observe your plants and the way they are growing whenever you try a new nutrient or amendment and don't use large amounts of anything before you are sure it's the right thing to do. I think any effect on soil pH is transitory at best. Mostly the grounds and liquid are just a mild nitrogen boost and the grounds contribute oragnic matter for improved moisture and nutrient retention.

.....Alan.

As I said, I just wanted to get the info out so people can make an informed decision. For me, the grounds will continue to go into my compost heap, but will never go into a container plant. Nor will I ever pour diluted or leftover coffee or tea on any of my plants. No sense taking that chance when there are plenty of proven "tonics" out there. I need all the help I can get and don't want to do anything deliberately that might inhibit growth.

But that's just me and how I feel about what I've read. Everyone will make their own decisions, I'm sure, and for those who have beautiful plants with no problems, they should probably keep doing whatever they are doing.

Marcia

I have used coffee grounds for years with no bad effects.

Honestly, folks, if you read this thread in its entirety, there's nothing that says you can't continue to use coffee grounds. It's just recommended that if they go into compost, it be aged before being used. And it is apparently a known fact that concentrated caffeine (such as using grounds in container plants or pouring leftover coffee on any plants can retard plant growth.

By all means, I would feel free to continue to use the USED grounds in a sensible manner. But not in containers. And pouring leftover brewed coffee directly on plants apparently isn't such a good idea.

So say those who know a lot more than I do.

Marcia

Just thought I'd add my two cents on this.

Regarding the caffeine issue (where it retards root growth), I have read that most of the caffeine in tea is released in the first few minutes of steeping, so if you're using USED tea, you shouldn't have a problem. This, obviously, does not apply to herbal teas with no caffeine.

I assume that it's the same way with coffee.

Fox Kitty, it IS released, but it is released into the TEA. The tea leaves will be low in caffeine, but the tea solution will not be. That was what was meant by not using the brewed tea or coffee (the LIQUID itself) on plants, especially not on container plants. The used leaves and grounds are another matter and discussed pretty thoroughly above.

Marcia

By all means, I would feel free to continue to use the USED grounds in a sensible manner. But not in containers. And pouring leftover brewed coffee directly on plants apparently isn't such a good idea.

But some of us have been using liquid coffee and/or tea with our plants for years and have not seen this at all. I never throw away left over coffee. Any that we haven't drunk goes to my roses or camellias, the ones in containers get first chance at it.

It seems to me that there has been a bit more extrapolation of study results than is warranted here. Normally brewed coffee is not concentrated caffeine. The studies that I have seen stipulate the use of extracts of coffee. I have no doubt that if you sufficiently concentrate selected parts of brewed coffee it could do some harm, but that's not ordinary brewed coffee then.

Excessive use of coffee grounds as a top dressing can crust over and become water repellant. Most anyone who has used them much will know that. If you have a lot of them then it's better to mix them with some other organic materials first before applying them. I can't say about using in seed starting media as I've never tried it.

As a mild fertilizer around perennials I can say brewed coffee and tea works pretty well if used within reason. Coffee and tea grounds will do the same and add organic matter to boot. Just don't put it down thickly as a top dressing or try to use it as a major part of a potting media any more than one would use composted manure that way.

.....Alan.

Thanks for the extra info, Alan. I actually am not extrapolating anything, myself. I just quoted someone who seems knowledgeable on the subject, and asked what others thought. I have no opinion on this issue one way or the other, but don't see any reason why I should use brewed coffee if there's any chance it retards growth. BTW, I used the phrase "concentrated" referring only to using the brewed coffee on container plants, where by the nature of the container itself, everything is concentrated. I didn't mean to imply that the liquid was necessarily concentrated.

I appreciate what you are saying, but for me, I'll just continue using the used grounds in my compost, and not worry about saving the liquid. Actually, there is never any liquid to save. Only Mark drinks coffee, and he finishes his whole pot every morning. And I am the only one drinking tea, 1 cup at a time, and I finish those, too. So for us, the used grounds and leaves are the only thing we have to deal with.

I don't know if Tapla has additional information on controlled studies on this or not, but it seems to me that would be the only way to get sound information. It would be hard to prove whether pouring coffee on a plant slows the growth of that plant or not, unless you have two plants in identical conditions, one getting coffee and one not. Then you could see which one grew faster. But I don't need to know that badly, and I suspect you don't either. As I say, if you are doing something that seems to work for you, why change?

Me, if I want a tonic for my plants, I think I'll stick to proven formulas on the market rather than chance any retardation of growth. But I certainly don't expect everyone to feel the same way.

I found an interesting thread on another forum, and shared it here. I have no idea personally of the truth behind either side of the issue. But my general rule of thumb in life is, when in doubt, use caution. So that's going to be my own approach.

Marcia

Marcia, I had a similiar experience lately. I read in a rose book to never plant a rose where a rose had been due to something called soil sickness that the previous rose gives off. I was very upset. I have a tiny lot and I must plant where roses had been. I posted on the rose forum my concern. They said 'fogetaboutit'. It was basically nonsense. Sometimes I think people write from opinion rather than fact in articles and books.
For me, I try to trust common sense. I have put coffee grounds on my roses for years and I have to chop off 2' at a time to keep them in bounds. They apparenly haven't heard that the grounds are supposed to retard plant growth. Interesting subject, thanks.
Nancy

I know what you mean, Nancy. However, if you read Tapla's post quoted above, he isn't sharing so much an opinion as some results of studies done. And he does not say not to apply used grounds on plants in the ground. He just says he wouldn't use them on container plants, where they would be more concentrated. And he doesn't think using brewed coffee or tea (liquid) is a good idea.

The study results are there for anyone to read, and his actions are his own personal opinions based on the studies.

Again, the used grounds themselves are not suspected to retard growth on plants in the yard. There is a big distinction there that many seem to be missing, and I'm actually sorry now that I shared this information. I knew some people would rethink how they use coffee grounds, and some wouldn't. But I wasn't prepared for it to turn into quite such a big deal.

Sorry, folks.

Marcia

I see what you saying Marcia. I just have this common sense thing that keeps getting in my way. There are people who sound very profound, who quote all kinds of studies, etc. who basically don't know from beans. If you were to meet my brother in law. On basically any subject he sounds very bright and profound. But if you do know him, the only thing he really knows is how full of himself he is.
Back to the coffee grounds and my problem with common sense:
This is a subject that we've been reading about for 10 years, maybe more. In all of that time I've only heard good (except the crusting thing.) It's not like some other subjects like Milk, it's good for you, it's bad for you. Grounds have been consistant. They bring worms, worms generate wonderful manure, manure enriches the soil, etc.
The example I used was from a woman who wrote a rose book. I read it in her book. I'd think she's an authority. But it turns out... I'd never heard that before in all of my years (the rose transplant thing) so I feel it should have been a red flag that maybe I should just trust what I've been doing. I had a professor in college who said 'always remember, if something is working for you, don't change it.' I just can't see how coffee grounds on plants in the ground is OK but not on containers. If anything the stuff we add to containers gets washed out even quicker due to more watering. Have you had problems with the plants in containers while you've been using grounds in your compost?
OK. I'm done. Sorry for the rant.
Nancy

No need to apologize. The reason Tapla says he doesn't use them in containers is because by virtue of them being concentrated in a smaller area, the chemicals that retard growth are stronger. They might leach out later, but at the time of application, they would be concentrated right on the plant roots.

Our compost bins are huge. The amount of coffee grounds in them would be such a minute percentage they couldn't make any difference at all, I'm sure. But I don't use our compost in my potted plants, ever. I prefer a commercial mix with time released fertilizer in it, so I have no experience on the issue.

The comments were not about using compost with grounds mixed in, anyway. (Tapla thought that was fine.) It was about applying the grounds and/or the liquid directly to the potted plants. Not mixed in compost first, but dumped on the containerized plants.

As I said in just about every reponse on this thread, if what you are doing is working, why change it? I was just sharing what I read because I found it interesting, but I'm really wishing I hadn't. It seems to have really stirred up something, though why, I'm not sure. Who knew people were so passionate about the pros and cons of coffee grounds? I sure didn't.

Marcia

Ha ha, it's amazing what can get folks stirred up. I guess we all have pretty rich lives if we have nothing bigger than coffee grounds to worry about.
I've seen pictures of your containers and I am always envious, so definitely keep using your commercial mix, minus the grounds, it's working for you girl.
Nancy

That's it exactly...if it works for ya, stick with it. But I just like to keep informed on any possible problems. If someone is using the grounds on container plants, and the plants aren't doing well, it's a possible factor to consider, ya know?

I love my Miracle Gro potting soil. I'm too lazy to mix my own, though I'm always looking at Michael's formula (which I thing looks a lot like Tapla's) and thinking I'll try that one day. But so far, I just grab a few bags of MG and away I go.

One other thing learned from the Tapla's thread on the Container Gardening Forum is that you don't have to toss out the used coffee filters. They are biodegradable (and we use the unbleached ones, anyway), so that saves a step. Just toss the grounds, filter and all, on the compost heap.

Marcia

I stumbled on this thread while looking for another of my old postings. Here's what I actually said about using uncomposted coffee/tea grounds on plants ..... and you'll see my focus was on containerized plants. This offering is the result of my research on a broader topic, but I encourage anyone to do their own. I only posted it because I thought having the info more conspicuous would be beneficial.

Coffee/Tea Grounds

Forum discussions frequently center on the question of adding coffee/tea or grounds to plants as a 'tonic', but Arabica (coffee) and Camellia (tea) are known for their toxic alkaloid (caffeine) content and their allelopathic affect on plants as well as autotoxic (poison to their own seedlings) effects on future generations. Caffeine interferes with root development by impairing protein metabolism. This affects activity of an important bio-compound (PPO) and lignification (the process of becoming woody), crucial steps for root formation.

We also know the tannins in both coffee and tea are known allelopaths (growth inhibitors), and there are (perhaps WERE by now as I wrote this 12 years ago) ongoing experiments to develop herbicides using extracts from both coffee and tea that cause me to want to say they might serve better as a nonselective herbicide than as a tonic. I would not use either (stale coffee or tea) by applying directly to my plants - especially containerized plants; nor would I add tea bags/coffee grounds to my container soils.

Al

In this thread there is mention of the influence of uncomposted coffee on the pH of soil/medium, and there is mention of coffee in compost; unassociated mentions. I wish to know the extent to which the pH of ANY composting material (as major portion) directly affects the finished compost's pH.

four, used coffee grounds do not add acidity to the soil. By the time you make coffee with them, the acid is all leached out, and they are good for nothing but adding organic matter to the soil. Unused coffee grounds are very acidic, but I don't know how they would affect pH. The problem I've had with adding coffee grounds anywhere is that they breed fungus very easily, which I guess would mean they are slightly acidic, because fungus doesn't breed in alkaline conditions.