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Hardiness Zone: 5a
Dawn from Elmira, NY
I'm glad you asked this question. Most gardeners assume that because it's slightly acidic, fresh coffee grounds or leftover brewed coffee can be sprinkled over the soil of any acid-loving plant. Not true. It's actually a bit more complicated than that.
First of all, not all acid-loving plants are created equal. Plants like Azaleas, Gardenias,Hydrangeas, Roses, Rhododendrons, and Blueberries all seem to respond well when grounds are mixed in with their soil.
Acid-loving African Violets, on the other hand, do not. Most edible garden crops also prefer slightly acidic soil, but adding coffee grounds also seems to affect them in different ways.
For example, lettuce seems to benefit greatly from the addition of coffee grounds, but tomatoes are actually affected somewhat negatively.
Plant scientists are not exactly sure why some acid-loving plants respond differently to fresh grounds than others, but it could be because coffee contains allelopathic chemicals that actually inhibit growth in some plants.
This might be helpful when you're trying to control weeds, but unfortunately, coffee grounds can't tell the difference between the plants you like and the plants you don't.
Secondly, mixing fresh grounds in with the soil has a tendency to tie up some of the nitrogen while the grounds decompose. Once the fresh grounds are fully broken down, however, a modest amount of nitrogen is released back into the soil. Mixing small amounts into the soil probably won't affect your plant's nitrogen uptake too much, but it's still better to add the grounds after they have fully decomposed or just sprinkle a small amount on top of the soil.
Finally, a word of caution on using coffee grounds for houseplants: Moist grounds encourage fungus growth that can be detrimental to your plants. If you use them on houseplants, do so sparingly and avoid watering your plants unless the soil is dry to the touch.
Unfortunately, there's no real recipe for how much is too much. Your safest best is to add leftover coffee and used grounds to the compost pile (it's unlikely you could add too much).
If you don't compost, you'll have to rely on trial and error. A cup or two around shrubs and a tablespoon or two around potted plants now and then should do no harm.
Because coffee is somewhat acidic, adding grass or leaves (browns) as mulch can help neutralize its acidic affects around plants that prefer more alkaline soil.
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Which plants like coffee grounds?
Hardiness Zone: 8b
By Janet Skinner from Jax, FL
Just to let everyone know, I have 75 tomato plants in pots. They are doing great, 3 ft. tall and growing. I used coffee grounds and organic fish and bat guano. Everything in my garden is organic, including the dirt.
I water my plants every two days in the evening and always at the same time. This is very important if you want your crop to produce. Timing is everything
Can I use coffee grounds on my house plants? I have a Christmas catus and a climbing plant that I am not sure what it is, other than it has large waxy leaves. I hate to throw them out and I can't compost them because I live in a apartment.
You can put a sprinkling of the grounds around your plants.
There are different opinions about using coffee grounds on house plants but most agree that using them outside (not heavy quantity in one place) is a good thing. Since you live in an apartment I see no reason why coffee grounds would be okay to use but do as Judy says and just sprinkle them on top of soil and not on the plants or the stems. Be sure to "sprinkle" as too much may become moldy.
Remember, coffee grounds added does not take the place of a regular schedule for fertilize.
Here is a quote from University of Florida Ag site:
Also, very weak black coffee/tea can be added (small quantities) to many plants as this seems to benefit my house plants more than coffee grounds.
This question has been asked on ThriftyFun before so here is a link that you might like to check out.
My best advice is not to add too much to indoor plants because they will definitely get moldy. I found this out the hard way :) If you use it, dig it in deep. It will be great.
So many terrific answers and good advice! My two cents? If you're looking for (additional) natural fertilizer, visit a local greenhouse and ask about worm castings.