The following is a tip from Lynn Byczynski  from her book The Flower Farmer, Revised and Expanded: An Organic Grower’s Guide to Raising and Selling Cut Flowers .
One of the tricks to growing flowers organically is to cut them before insects have a chance to destroy their beauty. I learned this the hard way during the first summer I grew sunflowers commercially. The guidelines I had received from various sources said that sunflowers should be cut when about one-fourth of the disc flowers—the tiny flowers in the brown center—were open. But by the time this happened, cucumber beetles had chewed holes in all the petals.
So I started to cut sooner. I harvested the sunflowers when the petals had just opened, and they held fine. But a few beetles were still getting their bites in, so I started cutting earlier and earlier, until I was cutting the flowers before the petals had even unfurled. Those flowers eventually opened in buckets, were just as vibrant as those that had bloomed outside, and were cosmetically perfect.
This trick works well with nearly all the composite flowers, which have large, flat outer petals subject to insect damage. Rudbeckia, cosmos, gaillardia, and tithonia all can be cut early and bloomed indoors. Zinnias, although they are in the same family, aren’t as attractive to insect pests and don’t suffer the same kind of chewing damage, so it’s best to cut them once their blooms fully open. I also have found that some sunflower cultivars are less receptive than others to cutting early, so I recommend that you experiment with a dozen or so of each type that you grow in order to find out just how early you can cut. On the other hand, most of the spike-type flowers (delphinium, larkspur, etc.) can be cut when just one or two flowers on the stem are open. The alphabetical listing of recommended cut flowers in appendix 1 gives specific instructions about the best time to harvest each type of flower, and there you will find many others that can be cut in the bud.
It’s better to cut unopened flowers in the evening, when their stems are full of starches and sugars that will help them continue to open. You also should use floral preservative, which contains about 1 percent sugar. Some preservatives can be used at double strength to prompt buds to open; check the label. You can also make a bud-opening solution that contains 2 percent sugar by adding 5 ounces of sugar to 2 gallons of water. Leave the flowers in this solution in a cool place out of the sun (but not in a cooler) until the flowers open.