There are plenty of reasons to have a cake which will include flowers. There are also several options for the type of flowers that can be used. Real flowers are the example from which all others are modeled, of course. But there are silk flowers, as well as flowers of icing or sugar.
The most common cake that will include flowers is a wedding cake, however, flowers are certainly suitable for anniversaries, weddings, showers, and other special events. For many simple celebrations, icing flowers will be the first choice. They are simple, edible, and no special arrangements are required. There is a multitude of varieties that can be made simply with buttercream or royal icing, and these will be sufficient in most cases.
For anniversaries, since there may be a renewal of vows where a bouquet is used, or for a wedding, the choice of flowers on the cake is more complicated. Many feel that the flowers on the cake should match the flowers used elsewhere in the décor. In this case, it must first be taken into account whether fresh or silk flowers are being used. If silks are used, then it is an easy choice to put silk flowers on the cake. But when fresh flowers are used, there are a few things which complicate the decision. In many cases, silk flowers will be available in similar colors, though these may have to be searched out. Silk flowers can be delivered to the Baker in advance of the event, and are easy to pre-arrange for setting on the cake. For these reasons, they may be a convenient option.
Fresh flowers pose a bit more difficulty. It will have to be estimated how many flowers are required. A decision will have to be made whether the cake artist or florist should arrange the flowers for the cake. Vials of water will need to be provided for any flowers that may wilt. Fresh flowers will have to be delivered to the Baker on the day of the event or dropped at the location to be arranged when the cake arrives. Most importantly, only edible or non-poisonous flowers should be used on a cake. Poisonous plants can cause a reaction merely by contact with the cake and do not have to be ingested to make a guest ill.
Another option is to have the flowers created out of sugar. This can be done in various ways, most commonly with gum paste or fondant. Typically, though the sugar portion of the flower is edible, the supporting structures are not. Flowers are formed on picks, and the wire is often used to hold pieces together or as stamen inside the blooms. While there is an advantage in that the flowers can be made well in advance, the drawback is that generally, it is cost-prohibitive. In the case of a special event where no expense is spared, sugar flowers that are entirely hand crafted are extremely impressive and will turn any cake into a stunning show piece.
When deciding what kind of flowers to use on a cake, it is important to take into consideration the advice of the cake designer. Ultimately, this is where the best information comes from. The primary goal of any good cake artist is to do right by the customer, and of course, provide a stunning cake.