Most cultivated plant products used in construction are by-products from the production of grain. Production of grain is often based on the extensive use of synthetic fertilizers and pesticides. Plants that have a high resistance to moulds and insect attack include hemp, elephant grass and linseed. Nevertheless, in linseed cultivation it is common to use 0.7 kg of pesticides per hectare - of which about 20% is lost to the atmosphere (Lippiatt, 2007). Alternative methods are possible, including the various types of organic farming, though in general they give lower yields. Ripe grain is usually harvested in late summer. Cutting of wheat and rye to be used for roof covering must be carried out without breaking or splitting the stalks.
Many of the useful, wild species grow in water - reeds, ribbon grass and pond sedge. These plants are perennial, sprouting in spring, growing slowly through the summer and withering during winter. On a land area of 1000 m2 0.5-3 tons of material can be produced. Harvesting either by boat or from the ice occurs in winter. Some of the species can be described as semi-cultivated, since the areas around are cleared to maintain good conditions for growth, and harvesting is done selectively. Some 'wild' plants are fully cultivated, for example nettles, which can give yields off almost 5 kg of biomass per square metre.
Moss grows ten times more than forests in volume per unit area. When harvesting moss, care must be taken not to destroy its system of pores. The best technical quality is achieved if it is pulled up in pieces.
Harvesting peat is best done during the summer when peat bogs are at their driest. Summers with high rainfall can cause problems during harvesting as well as in the quality of the final product. Whereas peat was formerly worked with a special range of manual techniques and tools, there are now large machines that shave 3-5 cm off the surface of the peat, cut it into blocks, stack it and dry it.
When large areas with wild species are harvested the local ecology of the area must be handled with particular care, especially species such as moss and peat. Marshes are very sensitive ecosystems.
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