Untreated sweet pea flowers have a very short vase life due to the extreme sensitivity of the flowers to ethylene gas. Ethylene, C2H4, is present in atmosphere in small amounts. Sources include car exhaust fumes, bonfires and ripe fruit. Wounding a plant stimulates the release of ethylene within the tissues of the affected plant, and ethylene also plays a major role in the shedding of deciduous leaves in the autumn and the falling of ripe fruit.
The effect of ethylene on flowers is to cause them to wither and fall prematurely, but the sensitivity of individual species varies dramatically. Sweet peas and Dianthus spp. are among the worst affected, and must be treated with an ethylene inhibitor to extend the vase life.
The most commonly used ethylene inhibitor is Silver thiosulphate (STS). The chemical itself is unstable and quickly breaks down, so must either be used as a proprietary stabilized form or prepared freshly on a regular basis. To prepare a usable solution of Silver thiosulphate, it is necessary to mix stock solutions of Silver nitrate and Sodium thiosulphate. The resulting solution must be kept cool and dark, as must the Silver nitrate solution.
The most widely used proprietary product is Chrysal AVB from the Dutch company, Pokon Chrysal. The concentrate has a good shelf life, but should be kept cool and out of direct sunlight. Once diluted for use, the solution can be re-used for up to a week in cool weather but much less in summer. Spent solutions throw a brown precipitate of metallic silver and must be disposed of in accordance with waste regulations.
Pulsing is the term used for treating the flowers with a chemical solution for a strictly limited period. In this case, the freshly cut stems are stood in buckets of Silver thiosulphate solution for two to four hours. Excessively protracted treatment is detrimental, besides being wasteful. They are then moved to a holding solution which generally contains a suitable fungicide. If the object is to advance the development of the flowers, then the holding solution will also contain nutrients, but it is more usual for the grower to wish to retard development to obtain the longest possible storage life, in which case the holding solution will not contain any nutrients.
Once the flowers have been transferred to the holding solution, they should be kept in a cold store at around 2° Celsius. Under these conditions, vase life will be improved by 24 hours of storage, and will not be significantly reduced after 7 days.
There are a number of scientific articles online which examine the effects of STS on the vase life of sweet peas and other flowers. Links are provided below giving the country in which the research was carried out, year of publication, and the crop studied. The articles are in .pdf format, so Adobe Reader or similar will be required.
© 2006 - 2009 Mark Rowland