As I found while photographing the Tulip Festival here in Ottawa, outdoor flowers make a tough subject for macro photography.
The reason for the difficulty is mostly due to wind.
So not only do you have to ensure that your camera does not move when taking a close-up shot (by securing it to a tripod or balancing it on a stable surface), but you also must ensure that your subject does not move - otherwise you'll end up with a blury image.
In order to prevent tall flowers from blowing around too much, consider bringing a sheet and two poles so as to set up a shield.
Depending on your shot (and how public the flowers are) you can stabilize the base of the stem by tying it to a short, sturdy pole, provide the pole is out of view of the lens.
The other alternative is to bring a friend along who can physically hold the stem still, or act as a mobile wind shield.
The best time of day to capture flowers is at sunrise or sunset when the natural light is at its warmest shade - I recommend you aim for sunrise with the chance that dew will still be on the petals.
Of course, the ground will also be wet, so If you plan to get the ant's view of your flowers - i.
e.
shooting from grass level up - then bring a tarp to lie on.
Use of fill flash can really help minimize the impact of shadows on the undersides of flowers, as can proper use of reflectors.
If the sun is really bright, you can even use a diffuser to soften its effects, although I prefer to shoot with the strong sunshine behind the petals, bringing them alive with an almost-fluorescent glow.