If you could order ideal weather for photographing flowers, plan on a day of soft overcast, with its diffused light and low contrast. Fog or misty rain can add great water droplet effects. Since we can't always create perfect weather, manufacturers have created "diffusion tents," simply made of thin white fabric on a frame. You can make your own with a white sheet, although you'll lose one or more stops of light. You can tack dark, opaque fabric on one side of this "tent" to create directional lighting (in this case, also known as subtractive lighting). And you can bring along a spray water bottle, such as those found in gardening departments, for those dew-drop effects.
Autumn Leaves
As with flowers, soft light is best for close-up work. Overcast days also bring out the warm colors found in autumn leaves. Sunlit days are okay, too, great for dramatic backlighting and spotlighting. Good timing helps. If you can, go out just after a storm has brought down fresh leaves. See how the greens and other cool colors balance with the warm colors. Use a warming filter for photographing leaves in the shade. Color enhancing filters also help. A saturated film such as Fuji's Velvia produces eye-popping colors.