Start with the right blooms

There is something quietly magical about a cake crowned with fresh flowers. It looks effortless, but a little knowledge goes a long way towards making sure your beautiful centrepiece is also safe to eat. The first thing to understand is the difference between edible flowers and non-toxic flowers. Edible blooms can be eaten and taste pleasant, while non-toxic ones are simply harmless if touched or accidentally nibbled — they are there for looks rather than flavour.

Reliable edible choices include pansies, violas, calendula, nasturtiums, borage, cornflowers, pinks (dianthus), rose petals and herb flowers such as rosemary, thyme, chive, mint and sage. Lavender works beautifully but is intensely perfumed, so use it sparingly. Apple and cherry blossom are lovely in spring. If you want a decorative bloom you would not actually eat, gerberas, sunflowers, freesias and many orchids are generally considered non-toxic — but always confirm the identity of a flower before it goes anywhere near a cake.

Flowers to keep off the cake

Some of the prettiest garden plants are best admired from a distance. If you cannot identify a flower with absolute certainty, leave it out. The following are well known to be harmful and should never decorate a celebration cake:

• Lily of the valley, foxglove, monkshood and delphinium
• Oleander, rhododendron, azalea and hydrangea
• Daffodils, hyacinths and tulips, including their bulbs
• Sweet pea, wisteria and laburnum
• Lilies, ivy, yew, hellebore and buttercup

It is also worth remembering that some flowers are safe to handle but not to eat, and that pollen can be an irritant on its own. If you are decorating for a party, a quick note on the table explaining which blooms are edible saves a lot of guesswork.

Source them like an ingredient

Treat flowers the way you would treat cream or eggs. Home-grown blooms are ideal, provided you have not sprayed them with pesticides, weedkillers or slug pellets. If you buy them, look for flowers labelled for culinary use — usually sold near salad leaves in supermarkets or by specialist growers. Flowers from a florist's bucket are grown for display, not for eating, and are routinely treated with products that are not licensed for food.

Avoid picking from roadside verges, parks or other people's gardens. Beyond the obvious question of permission, these spots see exhaust fumes, herbicides and plenty of dogs. Once you have your flowers, rinse them gently in cool water, shake off the excess and lay them on kitchen paper to dry completely. Any lingering moisture will bleed into buttercream or soften a fondant finish.

Wrap, sleeve and protect the cake

A flower stem pushed straight into a cake is a small reservoir of moisture and bacteria, and it will not do your icing any favours. The neat solution is to never let a cut stem touch the cake directly. Trim stems to a short length and wrap each one tightly in florist's tape or a small strip of cling film. For a more robust approach, cut a drinking straw or cake-pop stick to size and slide the stem inside it as a sleeve before pressing it into the cake.

You can also rest blooms on a disc of greaseproof paper or a small collar of sugar paste, so the flowers sit on top rather than sink in. Petals can be placed directly onto the surface once they are dry. Two rules are absolute: never use floral foam anywhere near a cake, and never allow a pick or water holder to leak. If you want a hidden water source for a large arrangement, keep it in a separate holder behind the cake, not inside it.

Assemble late and keep it clean

Flowers are at their best for a short window, so add them within an hour or two of serving. Up to that point, keep stems in a glass of water in a cool place, away from direct sun. Once the cake is dressed, keep it out of a warm kitchen and away from strong-smelling items in the fridge. Blot petals once more before they go on, and handle them by the stem rather than the petal.

Remove stamens from any large blooms, both for neatness and to reduce pollen. If children will be at the table, sit the tallest arrangements at the back and mention which flowers are for eating. Finally, check with your guests about allergies before you commit to a design.

Small touches that look professional

Restraint is what separates a pretty cake from a chaotic one. Group flowers in odd numbers, vary the heights and keep to a colour palette of two or three shades. A single perfect rose with a few herb sprigs often looks far more elegant than a crowded bouquet.

• Anchor flowers with a small blob of buttercream hidden beneath the bloom
• Sugar-glaze petals by brushing with lightly beaten egg white and dusting with caster sugar, then drying on greaseproof paper for 24 hours
• Scatter a few loose petals around the base for a relaxed, garden-picked feel
• Match the flowers to the season — spring blossom, summer herbs, autumn dahlias

Get these details right and your flowers will look as though they grew there, while you can relax knowing every guest can enjoy the cake safely.