The absorbed energy is converted to heat energy. The scenario you mention with the LED is very close to the blackbody problem known as the ultraviolet catastrophe.. There is a Wikipedia article about it here. What happens in the situation you described is that the light proof box gets hotter.
Diagnosing rust on geraniums can be a challenge for growers. Rust begins with yellow spots on the underside of the leaves and can rapidly progress with brown fungal spores called pustules in the center of each spot. The pustules are raised up off the leaf surface and feel rough to the touch.
When saving geraniums for the winter in pots, dig up your geraniums and place them in a pot that can comfortable fit their rootball. Prune the geranium back by one-third. Water the pot thoroughly and place in a cool but well lit part of your house.dvertisement. If the cool area you have in mind does not have enough light, place a lamp or light with a fluorescent bulb very close to the plant. Keep this light on 24 hours. This will provide enough light for getting geraniums to last over winter indoors, though the plant may get a little leggy.