Do houseplants like coffee?

Very weak, of course. I used to keep cold leftover tea (from the teapot, not the milked variety), dilute it a lot further, and water my houseplants with it. Everything thrived.

Also used it on garden plants in Oxfordshire during dry spells. We don't really have what you can really call dry spells in Argyll.

Anyway, there was some cold coffee in a pot and my houseplants needed watering so I watered it down a lot and watered the plants with it this evening.