A slightly overcast day- not ideal for infrared photography, but when you're at a garden it still gives an interesting effect. I take IR photographs by placing an IR filter (only passes infrared light) in front of my camera lens. The exposures are much longer than without the filter, but the results are often worth it.
During the film camera days infrared photography was often used for architectural photography (blue skies are rendered as nearly black) but IR is also strongly reflected off the chlorophyll in leaves, grass, etc. Click here for much more information from Wikipedia.