Long story short, I have stale ground coffee (whoops vacation whoops) and two spider plants growing in pure water. Can I combine the two, or is that too acidic? I know that spider plants like acid, but... perhaps that's too much. Or perhaps there's not enough in the ground coffee to feed them?
For context: I am a student living in university housing, so I can't easily access potting soil.
[EDIT] To be clear, I mean to use the coffee in lieu of soil, not as a liquid growing medium.
I would say it depends on entirely upon 1) the coffee and 2) whether you plan on using the brewed coffee grounds or the coffee itself.
According to a few books I have here at home, spider plants really only tolerate slightly acidic (range 5.5-7.5 on pH scale) environments.
If these sites(1)(2) are accurate and the pH of coffee grounds are neutral to ever-so-slightly acidic, I wouldn't be afraid of adding brewed coffee grounds to the plants. The coffee itself may be another story.
As someone who appreciates coffee, I can attest that the pH for coffee varies greatly depending upon brewing method, origin, type, roasting, and other factors. For example, cold-brewing or French pressing a decent quality coffee would likely result in the appropriate pH for adding significant amounts to plants. The coffee in your university dining hall, or a quickly-brewed cheap coffee, will probably have to be diluted to be used. I've seen some articles that compare an acidity of 4.5 for "traditional freeze-dried crystal coffee" to 6.5 for "gourmet French press coffee."
I regularly throw an old pot of coffee into the planter or mix grounds with compost without problem. But I personally think transplanting the plants from water to a soil of coffee grounds or a bowl of coffee would be a mistake; I've always only known coffee to be used as an additive and not a replacement.
All of this is to say, "try it and see." I would say spider plants are fairly hardy and will show signs of over-use of coffee. Add a portion at a time and wait a week to see how the plants adjust.
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:
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!
I'd caution against planting in straight grounds. Like jboneca, I've added used grounds to my potted plants, but if I do not mix them in well with the soil that is already there, I notice that they grow mold. Regardless of whether the plants did well growing in the grounds, I think you'd be at risk for a real mold issue if you were using straight grounds. Are you certain you could not find a bit of potting soil somewhere? I just purchased a small bag at my local grocery store for under $2.
I came across this discussion after I experimented growing spider plants in a mix of coffee grounds and some good potting soil. The coffee and soil is in layers. Pretty slow growing so far compared to a few control plants. I'll keep at it - it's getting a little warmer so I'll keep in touch.
Really, there is no landscaping or trees around the dorm where you can snitch a few scoops of dirt under the blanket of dark??? That's what I would do. Wait until dark, run out with a coffee cup or a ziplock bag and a spoon, scoop up some dirt. It will work better than keeping a plant in water. Eventually the plant will not survive. Annd the dirt GRADUALLY (A little everyday) to the water.
