While adding used coffee grounds to your plants works as a mild fertilizer ( 4, 1, 3 ) I recently read some articles where gardeners had used coffee grounds in place of soil for houseplants.
No one had success germinating seeds in pure coffee grounds but several plants did very well planted in coffee grounds instead of soil. If you are lucky you can just take out the coffee in the filter and drop the whole thing into a pot. Usually the pots and coffee filters won’t be so conveniently sized.
Occasionally plants in coffee grounds will develop a phosphorus deficiency or manganese toxicity as seen in leaves turning purple. Adding a bit of phosphate and lime to the soil will correct this.
The coffee also acts as a neurotoxin against slugs and other unwelcome critters.
The grounds are acidic so are a great way to add some acid to your acid loving houseplants like gardenias and azaleas.
The best successes came with tree and shrub houseplants. They seemed to do much better indoors planted in coffee grounds than dirt. Also plants that tend toward preferring drier soil to wetter soil seem to do better in coffee grounds.
I’ve read that expresso grounds do not work as well as regular coffee grounds.
See forum thread Coffee Grounds as Planting Medium