Edible flowers are grown specifically for eating, without pesticides or treatments that are not allowed on food. Violas, nasturtiums, calendula, borage, some rose petals: they are used on cakes, desserts, salads and cocktails.
The key point is the supply chain: a flower that would be edible in itself, but bought from a florist, may have been treated and should not go in food. That is why the flowers on the cake and on plates are chosen by whoever makes the food, and the florist decorates only around them or with protective barriers. Some ornamental flowers are also toxic. The guide to flowers near food and edible flowers explains the rules; the one on flowers on the wedding cake covers the most common case.
What to ask the baker or caterer: where the flowers come from, whether they are certified for food use, who will put them on the cake.
