Pasta water is in fact an excellent thing to save... As a cooking ingredient.
Specifically, what you can do is freeze it in an ice cube tray, and then store the cubes in a Ziploc. A handful of cubes added to a soup, stew, stock or sauce will give it a smoother, more silky texture thanks to all the starch.
Not sure if this is actually a good idea, either for inside or outside plants. The stuff that is dissolved in the water (mostly starch) does not help the plant at all, but it help microorganisms and they do not need to be good for the plant. Like mold.