Dried flowers can be preserved using various drying methods.
1. Commercially air dried flowers
This picture shows lavender bunches drying in a barn. The bunches are tied upside down to a drying rack until dry. Drying under cover in a barn is one method employed in the UK, where spoilage by damp air and rain is a significant factor. In sunnier climes, however, drying outside is the norm.
Dried rose petals from Pakistan, India and Iran may be laid out in the open air on giant sheets or tables to dry naturally.
In the UK, industrial drying methods may also employ the use of a drying floor, for example for dried lavender, where the circulation of warm air aids the drying process, and prevents the build up of moulds and other damp-loving organisms.
Recently in the UK, a few farms have started growing delphiniums in order to sell the dried petals as wedding confetti. These too are air dried in bulk.
The air drying of flowers is a bulk process, and so usually produces dried flowers which are less expensive than other drying methods such as freeze drying or glycerin preservation (see below).
2. Air dried flowers produced at home
Hang small bunches of flowers upside down in an airing cupboard to dry for at least a week.
This method is discussed in more detail in its own post: How to make dried flowers at home
The photo shows how some roses I dried at home have darkened in the drying process (they were crimson to start with). They are also more wrinkled than the fresh flower, and suffer from brittleness.
Other commercial methods have been developed to improve on air drying methods – in freeze drying, the colour is true to life and there are no wrinkles, whereas liquid preservation retains the suppleness.
3. Freeze dried flowers
Freeze drying is a method applied to delicate flowers to prevent the wrinkling or colour loss which may be associated with air drying.
It requires expensive specialist machinery, but results in a bloom that looks fresh as the moment it was picked. Because it is an expensive process, and there is almost individual attention paid to each bloom, only premium flowers and petals are used. Freeze dried flowers are expensive to buy but worth it if they are going to be viewed closely, for example as a wedding table decoration.
The freeze drying process works by reducing the petals slowly and carefully to temperatures well below freezing under vacuum. As the temperature is slowly brought up again (over a period of days), the water evaporates from the petals and freezes onto a specially cooled plate and is removed.
Clearly this can’t be done at home!
4. Liquid preservation of flowers
Technically, this is not a method of producing dried flowers, as the flowers remain “wet”. The process entails replacing any water in the flowers with a combination of glycerine preservative and dye. This results in a supple flower, not brittle. It means flower colour can be accurately selected and enhanced. But the additives mean that the petals cannot be called 100% natural or biodegradable, and there is a slightly increased chance of staining due to the dye.
The photo shows rose petals, but another flower which is popularly preserved using glycerine is the hydrangea (hydrangea petals make excellent wedding table confetti and so need to pass close inspection).