I am going to keep this simple- there is only one truly important thing to remember when you are taking a wildflower portrait- background. There are a few other things to be aware of certainly- how pretty the flower is, how nice the light is, is it windy, are you rushed- but all of them are inferior to the background. Nothing kills a flower portrait quicker than a bad background.
So how do you assure a good background? The first thing you do is to preview it in your viewfinder or on your LCD. Take a test shot and see if there is anything distracting in the background. If you use the depth of field preview button be sure to give your eye 30 seconds or so to accommodate to the sudden darkness. Things will appear more clearly after 30 seconds.
The second thing you do to assure a good background is to try to pick an angle that puts the closest part of the background at least a couple of feet away. If the background is closer than 2 feet from your subject than it will not be out of focus (that beautiful soft, poster board background) no matter what f-stop you use. I am forever trying to find a good subject with a background that is far away. In fact, I almost always look at the background first and then find the flower and angle that lets me use that background.
Third thing you can do is use your longest lens you can to get the shot you want. The longer the lens the narrower the viewing perspective and with a narrow viewing perspective the easier it is to pick a pleasing background. My flower portrait lens of choice is my Nikon 200mm macro lens- it allows me to get as close as I want and it has such a narrow angle of view that I can practically pick any background I want.
And background is all you will ever want- great background allows a great photo.