I have heard that adding coffee grounds to the soil - as a top layer/mulch - help potted plumeria. But what about this helps the plant thrive?

Bonus Q: I have heard the same of banana peels.

4 Answers 4


Coffee grounds benefit soil the same way compost and mulch do, they add organic matter. You might as well just compost the grounds and add the compost to the soil.

It is a myth that coffee grounds acidify soils. By the time they break down they have a pH close to rain water. Here is a related question about pine needles that has a very thorough answer debunking this myth. Here is a link specifically about coffee grounds.

  • Original source for this puyallup.wsu.edu/wp-content/uploads/sites/403/2015/03/… and references puyallup.wsu.edu/lcs/reference-coffee-grounds
    – kevinskio
    Sep 21, 2015 at 19:08
  • Thank you all to those who answered and commented. I have come to the following conclusion (summarized in this answer) after I followed some of the links through to read up more - it appears that the biggest benefit comes from the ability of the grounds to 1)provide a loose soil that drains quickly and 2) ability to retain moisture. These properties are likely also the reason to add it to compost.
    – user51113
    Sep 21, 2015 at 22:47

Coffee grounds are said to be acidifying, and Plumeria prefer a soil on the more acidic side, so that might be why you've been told that. Whether they really are acidifying depends on which resource you look up, and depends on which coffee they're from and at what stage of decomposition they're at, but they also may have a deterring effect on various fungal infections commonly present in open ground. Organic, fresh, coffee grounds do have a small nitrogen content too. As for the banana peels, they contribute nutrients such as potassium, magnesium, calcium and phosphorus, but should be cut up and buried in the soil, which I feel would be rather difficult to do with a potted plant. Of course, you could always just drape a banana skin in an artistic arrangement on top of the soil in the pot, and maybe pile up spent coffee grounds on top, but frankly, from an aesthetic point of view, its probably best to stick to plant fertiliser during the growing season and relegate the coffee grounds and banana skin to the compost heap. It's all down to what you think is acceptable though - I have an acquaintance whose front garden is always littered with banana skins, teabags and coffee grounds, which she feels benefit her plants. They might or might not, but they don't benefit the appearance of the local neighbourhood much...

  • This is most useful in areas where the tap water is neutral or alkaline. After a few years most soil less mixes become more alkaline. The coffee grounds counter act this and are also useful if you are growing a coffee plant indoors.
    – kevinskio
    Sep 21, 2015 at 17:45
  • Coffee grounds do not acifidy soil. See the links in my answer.
    – Philip
    Sep 21, 2015 at 18:11
  • @Philip - answer extended to cover that, see above
    – Bamboo
    Sep 21, 2015 at 18:18
  • "Whether they really are acidifying depends on which resource you look up"... Sorry didn't follow your logic there...
    – J. Musser
    Sep 21, 2015 at 20:44
  • @J.Musser - there are sources on the web which suggest coffee grounds are acidifying, and other sources which say the opposite... that's what I mean. My personal deduction is that they're best restricted to the compost heap and only occasionally, maybe once a week at most. Any value as a mulch appears to be minimal and it looks messy...
    – Bamboo
    Sep 22, 2015 at 11:49

Coffee grounds are great JUST FOR adding organic matter to the soil. The percentages of NPK are miniscule. Banana peels are OK. Again...pluses and minuses make banana peels OK for including organic matter to the soil. Banana peels take forever to decompose. After years I've found whole peels in my compost and have quit including them. Messed up my blender...it was an old whimpy one but still. I would never consider these things 'fertilizer'.

What concerns me is that we have such a huge responsibility in our gardens; have to know deficiencies, EXCESSES of chemicals necessary for plants to make their own food...when to add water, when not to, how to manage different types of soils properly...sandy, silt or clay. How to increase the biological and microbiological part of our soil. A simple, straight-forward all-purpose fertilizer for your plants is so very necessary. Getting at least 2 soil tests so that you know what your soil contains. Knowing what is necessary to add to your soil and no more. Just adding decomposed organic matter to the top of your soil is all one needs to do to improve tilth, drainage, add weed control, chemicals for plants(to make their own food in conjuction with sun energy) AND decomposed organic matter to feed micro-macro soil organisms.

How many banana peels is too much, too few? If they aren't decomposed, the nutrients will not be available for organisms or plants. Will they inhibit water penetration? Way too unscientific for me. The coffee has some nitrogen available that would be enough for the decomposers to use to decompose the grounds...but NOT a source of nitrogen for plants! And yup, coffee grounds are slightly acidic but not enough to worry about. The pH of your water, the rain, the normal environment's plant debris make much more of a difference. You have got to get a soil test to make proper decisions on managing your soil/plants/garden. Using coffee grounds/bananas/egg shells is pretty much equal to just using ANY decomposed organic matter in your soil. I like knowing what I am adding to the soil. By watching the growth of the plants, the leaf colors, spots, I understand how much to add, what might be lacking. This decomposed organic matter does add necessary chemicals to the soil for plants and animals but to depend on it for fertilizer is in my opinion crazy.

Gotta know why, how much, what you 'add' to your garden. How else will you be able to learn to recognise problems? In my opinion, coffee grounds/banana peels/egg shells...should be lumped into 'old wives tales' until better research is done. Grin, sorry. They are great in compost but should never replace soil tests and a good, basic fertilizer (organics actually make the best extended release) where you KNOW the percentage of chemicals you add.

Then one can get into stuff like...dried horsetail (equisetum) is incredible for plant hardiness by adding silicon. Fish emulsion used once or twice per year for all plants also makes plants...more hardy. This is similar to using...banana peels, egg shells. Even I have my 'little secrets' that work well and that science hasn't really explained. But I know my soils weaknesses, strengths and what I HAVE to add.


Much also depends on region, climate, dryness of the weather, soil of the region, etc. We grow plumerias over much of Southern California, but we often have long dry spells, and we tend to have alkaline soils. I grow mine in pots, so watering and feeding follows a different schedule than those in other areas.

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