For this experiment, I took four different spider plants that were roughly the same health/size at the beginning of the experiment, and waited approximately ten days to note any differences. They each had roughly the same amount of light/fluid in their tubes.
I attempted three trials, with one Control:
- Control Water (original state of all four)
- Sample 1: Water with unbrewed ground coffee
- Sample 2: Water with brewed ground coffee
- Sample 3: Brewed coffee
The control grew predictably, with no pronounced damage.
The grounds were added dry, and floated briefly before becoming water-logged and sinking. The water remained the color of brewed coffee even after the grounds settled out.
The sample did not grow perceptibly, though it also showed no signs of damage or sickness. The root system seems to have expanded at roughly the same rate as that of the control, though clearly this is all estimated.
The water here too became 'dark-tea' colored after adding grounds, though obviously not as dark as that of Sample 1. The root system and leaves grew at the same rate as that of the control; presumably, this was the healthiest of the non-control specimens, and was possibly even healthier than the Control.
The coffee was allowed to cool to room temperature before being added to the sample. This sample exhibited identical behavior to the first sample: Increased root system, diminished growth of the exposed greenery.
It seems as though the caffeine did stunt the growth of the plants, just as
S. Albano proposed. However, it is possible these effects are temporary. I haven't followed it long enough to conclusively determine any long-lasting benefits/risks.
Add some used coffee-grounds to whatever growth medium (soil, water, etc) you're already using; but don't replace it entirely.
Perhaps if watered with coffee in normal potting soil, this would be a healthier setup? Additionally, I'd like to see what happens over a longer period of time, or with diluted coffee — or, indeed, with decaf grounds. I'll keep you posted if I find anything else out!