Does decaf grounds work on plants, or does it have to be real coffee?

Decaf will work too. Any organic material is good for plants.

Decaf is real coffee that has gone through a process using ethyl acetate to take out the caffeine. The ethyl acetate is usually from a synthetic chemical form. A few companies may use a natural form of ethyl acetate obtained from the fermentation of sugar cane. As to whether or not this is bad for the soil is beyond me. Just thought I would let you in on the process of decaffeination. From that, you can make a more informed decision.