do i just cut off the stem before putting on cake and what kind of flowers can I use and what kind can i not use?

Safe Flowers for Cake Decorating
Make sure flowers are pesticide free before using.

Apple Blossom, Malus spp.
Begonia, (most tuberous), Begonia X tuberhybrida
Borage, Borago officinalis (must remove hairy sepals from center)
Cornflower, Centaurea cyanus
Daisy (English), Bellis perennis
Freesia, Fuchsia X hybrida
Geranium, Pelargonuim spp.
Hibiscus
Honeysuckle, Lonicera japonica
Impatiens, Impatiens wallerana
Johnny-Jump-Up, Viola tricolor
Lavender, Lavandula angustifola
Pansy, Viola X Wittrockiana
Rose, Rosa spp.
Tulips, Tulipa spp.
Violet, Viola odorata

Toxic Flowers and Plants:

Amaryllis, Hippeastrum puniceum
Anenome, Anemone spp.
Autumn or Meadow Crocus, Colchicum autumnale
Azalea, Rhododendrun spp.
Belladonna lily, Amaryllis spp.
Bird of Paradise, Strelitzia reginae
Bleeding Heart, Dicentra spectabilis
Buttercup, Ranunculus spp.
Caladium, Caladuim bicolor
Calla Lily, Caladuim bicolor
Cardinal Flower, Lobelia spp.
Clematis, Clematis spp.
Crocus, Dutch, Crocus vernus.
Daffodil, Narcissus spp.
Datura, Datura spp.
Delphinium, Delphinium spp.
Foxglove, Digitalis purpurea
Glorious lily, Glorious spp.
Hydrangea, Hydrangea macrophylla
Iris, Iris spp.
Jessamine, Gelsemium spervirens
Lantana, Lantana spp.
Larkspur, Delphinium ajacis
Lily-of-the-Valley, Convallaria majalis
Lupine, Lupinus spp.
Monkshood, Aconitum spp.
Narcissus, Narcissus spp.
Oleander, Nerium oleander
Periwinkle, Vinca spp.
Poinsettia, Euphorbia (Poinsettia) pulcherrima
Rhododendron, Rhododendron spp.
Star of Bethlehem, Orinthogalum spp.
Sweet Pea, Lathyrus odoratus
Wisteria, Wisteria spp.

Interesting topic, I was wondering the same thing. I am an avid gardner, people can see my gardens from one of the city parks, and numerous times we look out and there are people walking around in the gardens. At first it bothered me, and now I am honored, but I have had people ask why I don't use my own flowers on my cakes. I really never thought about it. I very rarely even gather flowers to bring in for Vases. We don't use any pestisides on our gardens or yard, however we do fertilize every Friday.

I have a cake coming up with Blue Hydrangea's on it, and wondered if I could use my own. I just figured that Florist's probably did something to the flowers before the put them on. It never even crossed my mind to use my own flowers.