With long exposures, wind becomes a problem. Any movement will result in a blurred image. Some nature photographers carry sticks and plastic ties to hold the stem steady. Others use blinds to block the wind. And the really dedicated just wait for the wind to stop, or they come back another day.
Early morning is a good time to shoot flowers.
The wind often dies down and the light will be at its best. Avoid shooting flowers in direct, harsh sunlight. You will be disappointed in the results. Although flowers look good to our eyes in bright light, on film, there's just too much contrast and the colors burn out. An overcast day will will produce flowers with lovely, saturated colors.