A tropical herbarium
At Dokmai Garden we have received young students with the school task of creating a private herbarium. A herbarium is a collection of dried plants used for education and research. The benefit is that you can study fruits and flowers any time of the year, even when out of season. University herbaria in the west may contain millions of specimens collected from all over the world. Species described new to science have to be collected and kept in herbaria, that collection or type specimen being the definition of the species. When in doubt about the identification of another plant sample, say a medicinal plant, you can compare your plant sample with that of the type specimen. Sometimes comparisons of the genome (the DNA) of that of the type specimen and your sample is needed.
Setting up a herbarium in the tropics, either private or scientific, is much harder than in temperate areas. High humidity at high temperatures may cause fungal rot, and termites and other insects may quickly turn the herbarium into powder. Still, building up a private herbarium is an old and reliable pedagogic way of learning the plants, applied long before the modern scientists by herbalists, textile dyers and alchemists. By keeping the plants you do not have to rely on your memory only. Another option is to create a living collection (a botanical garden such as Dokmai Garden here in Chiang Mai) and combine it with photographs or illustrations. However, that requires a lot of land and staff for maintenance, while a herbarium can be kept in a small flat.
The first problem when creating a herbarium is to dry the plants. Insufficient drying will result in decay. The sample has to be absolutely dry so that the sample does not bend down when held in one end. To achieve that you need to press the plant between water absorbing papers (such as newspaper). If a professional plant press is unavailable then several kg of thick books will do the job. Initially you need to change papers frequently (twice a day) so that the moisture is removed. The drying process vary depending on the moisture content of the sample and the surrounding atmosphere, typically 1-2 weeks. If a plant tissue is too thick and fleshy to be pressed, it can be sliced.
The dried tissues should not be kept in plastic bags which seals moisture, but be kept aerated, preferably mounted (glued) on stiffer paper which are kept in folded paper covers. To avoid bending which causes fragmentation of the collection, stacks of herbarium specimens can be kept on trays. Keep the stacks on shelves in cabinets and wardrobes, preferably in a room with constant air-conditioning which lowers the moisture content and slows down the speed of insect attacks. Trays and cabinets are wooden in temperate areas, but in the tropics wood constitute food for termites and so metal is better.
Frequent control is essential to discover insect attacks, and freezing is the best non-chemical treatment. Mercury salts are often used in tropical herbaria since they make the herbarium toxic to all insects, but in such a case a good ventilation is essential to prevent mercury poisoning of people. For orchids with their fleshy roots and pseudobulbs, a preservation in 80% ethyl alcohol is an option, but that demands frequent monitoring of the jars to adjust liquid levels.
Rare collections always need back-ups, and so a collection for e.g. the Chiang Mai University herbarium could be shared with other herbaria in temperate regions. At Dokmai Garden we only focus on living plants, but we encourage collectors, illustrators and photographers to work here using alternative ways of preservation and documentation.