First, make a positive ID
Make sure to make a positive identification of each variety before using. Don’t assume that all flowers are edible – some are highly poisonous.
Obviously, one should avoid flowers that may have been sprayed with pesticides or other chemicals. It's useful to either grow organic flowers, or harvest them from a location where no chemicals are used. Organic or not, all flowers should be shaken and washed in cold water prior to use, as they may to be homes for insects.
Best time for picking
Pick edible flowers in the morning, when they have the highest water content. Keep them on some dampened paper towel inside a sealed container in the refrigerator for as long as a week. Prepare them for eating just before serving in order to prevent further wilting.
Remove the stamens and pistils from flowers before eating.
Stamen produce pollen can cause allergic reactions when eaten by some people so it’s good to remove it in most cases.
Eat petals only.
Stems and leaves can be tough, and may or may not be edible.
Are vegetable flowers edible?
Not all. Tomato flowers are not edible, but broccoli flowers are. They have a bit of a peppery taste. Cauliflower flowers taste a little sweet to my palate. Radish flowers are edible and trendy now. Summer and zucchini squash flowers are yummy and lovely stuffed with cream cheese flavored with herbs. The flowers of garden peas are edible, but those of the sweet pea ornamental are NOT edible.
Fruiting trees
Cherry blossoms are sometimes pickled, and apple blossoms have a lovely aroma. Plums, peach and citrus flowers are all edible.
Culinary herbs that flower are edible, but how is the flavor?
I think the flavor is more mild than the leaves. Nasturtiums are a good example.
Ornamentals
Fuschias are gorgeous and dry with a pretty color. Chysanthemums (not a great flavor), calendula, roses, dianthus, violas, pansies, petunias, dahlias and hibiscus are some commonly grown edible flowers. Hollyhocks and rose of Sharon are edible. Day lilies are edible but Easter lilies are not.
Calibrachoa, which looks like mini petunias, are not edible.
Eat the weeds.
Dandelion flowers, clover flowers, and violets are all edible. Dandelion petals make a nice wine, and red clover flowers are very flavorful.
Ways to use edible flowers.
Stir into some salt or sugar, process in food processor until finely ground, then dry in low oven to remove moisture. Mix & match:
For salts, do a combo of nasturtium petals and sage flowers. Great in stuffings and grains.
Toss in some mint leaves with rose and lavender petals for sugars to use in baked goods and drinks.
Crystallize them - a bit of egg white and fine sugar allows them to dry and stay nice for a year in a covered container. Brush all surfaces lightly with egg white, then lay in sugar and sprinkle sugar all over. Let dry on racks.
Dry flower petals separately, then crush together for trendy flower sprinkles to use on sweet & savory dishes, baked goods, and bath salts.