Hello,

I couldn’t find any research that stated houseplants do benefit from adding coffee grounds. One stated that moist grounds attracted fruit flies to a compost pile. Another stated there is a slight acidification of the soil. Applied to a soil surface they can form a barrier that prevents water and air from getting to the root zone. Some research showed some plants were stunted by using coffee ground  in the soil. 

Grounds can be added to compost bins, at 10-25% by volume. In Worm compost, the worms seem to like grounds. All references recommended working them into the soil or composting them.

If you decide to try them, use small amounts, and don’t use them first on a highly valued plant. Instead start with plants you wouldn’t mind losing. Here are some references-

COFFEE GROUNDS 

House plants- https://www.agriculture.purdue.edu/agcomm/newscolumns/archives/YGnews/2006/February/060216YG.htm

Compost etc- https://s3.wp.wsu.edu/uploads/sites/403/2015/03/coffee-grounds.pdf

Other- https://web.extension.illinois.edu/dmp/palette/110109.html

https://extension.oregonstate.edu/gardening/techniques/coffee-grounds-composting

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