Fresh flowers are part of food since ancient times and they have been used as cake decoration since forever.
Commonly used fresh flowers are roses, peony, daisy, lisiantus, gerbera, carnation, lavender, baby's breathe etc. Not all flowers are safe. Some has poisonous sap (frangipani), and all types of lilies (Calla, Tiger, Lily of the Valley) are toxic from the pollen to the leaves.
Being light weight, fresh flowers are used on buttercream cakes, semi-naked and naked style cakes, to achieve rustic design. Production is quick, suitable for current consumer's lifestyle of wanting everything yesterday. However, their limitations lie in not being able accomodate certain colour shades, limited variety of flower type, and limited arrangement styles due to softness of cake. Some would also argue on the use of pesticides and chemicals during the growing stage and therefore not use fresh flowers at all. Extra care need to be taken when clients want to use fresh flowers.
The above are examples of cakes with fresh flowers. Some are imported flowers such as Indian Roses with are bigger in size (bottom right), Ranunculus and Red Berries (top right) and some structures are in collaboration with florist (bottom left). Additional ideas are paring of flowers with berries (strawberries, blueberries, figs), herbs (rosemary, thyme) and spices (cinnamon bark, anise star).
An experiment on dying roses in blue in accordance to the wedding theme colour. Two shades on blue food colouring used, and the stem of the roses are slit half so that water reaches the petals easily. After two hours, do a check. The edges should have blue tinge, add more colouring if needed and leave overnight.
Though I would encourage the use of gumpaste flowers over fresh flowers, as gumpaste flowers gives a better overall look to a wedding cake and are limitless in arrangement and design, I am just as well versed in fresh flowers for rustic design, and to compare, rustic design is more fragile, everything needs to be fresh, a la minute and therefore is stressful indeed.