You could probably buy a few dozen UV LEDs, those won't use much power at all.
Beyond that, you could look into a blacklight (I'm pretty sure those are UV) or a UV cold cathode the inverter for which often runs straight off 12v, as from a car battery. Check computer modding shops for the latter.
Is the no way of wiring this area to mains with an extension cord or something, or at least have say a 12v power supply plugged into mains outside the cubpoard this plant is in, because that would certainly simplify your project.