Many celebration cakes seem to be decorated using fresh flowers these days, and sometimes brides ask me about placing a particular kind of flower on their cake. It's worth knowing that a great many flowers and plants are poisonous, and even if the flowers aren't poisonous, chances are they've been treated with insecticides and/or preservatives. So, I try to make any customer who asks for fresh flowers on a celebration cake aware of that. Having said that, there are ways of 'styling' a cake so that any fresh flowers that form part of the display have no direct contact with the cake itself. I most often use posy picks, plastic 'petri' dishes, and acetate sheets as part of my toolkit in this regard. Also, it is becoming easier to get hold of fresh, organic, edible flowers that have not been treated with pesticides. There are many popular edible flowers (refer to the list below), but of course, edible does not necessarily mean tasty. It just means that you can consume those flowers without causing digestive upset. The most palatable edible flowers are probably roses, pansies, Johnny-jump-ups, and violets.
Of course, one way of circumventing this issue entirely is to choose handmade gum paste flowers for your cake, which are made from a sugar dough that is completely edible. My preferred method of dealing with any floral wires or other structural components of these sugar flowers that come into contact with the cake is to coat them first in melted white chocolate. I leave the chocolate to dry, and then the floral wire is considered food safe and can be inserted into the cake.
As mentioned, here's a list of popular edible flowers, from cookbook author Dede Wilson:
Angelica, Apple Blossom, Bee Balm, Borage, Calendula, Carnation, Chive Blossom, Cornflower, Cymbidium, Dandelion, Daylily, Dianthus, Elderberry flower, English Daisy, Gardenia, Geranium, Gladiolas, Herb flowers (Rosemary, Basil, Mint, Sage, etc), Hibiscus, Hollyhock, Honeysuckle, Hyssop, Iceland Poppy, Impatiens, Lavender, Lemon Verbena, Lilac, Marigold, Nasturtium, Pansy, Passionflower, Petunia, Rose, Safflower, Sunflower, Tuberous Begonia, Tulip, Viola, Yucca, Zucchini Blossom.
Sugar flowers can be just as beautiful (or almost) as the real thing. To give you an idea: a sugar peony (made by me) and below that, some amazing jade sugar roses by Ron Ben-Israel: