Plant decoctions are plant-based sprays made by placing the bark, shoots, leaves or flowers of plants with medicinal properties in cold water, bringing this gently to the boil and if necessary then leaving it to simmer, typically for 20 to 60 minutes. Decoctions involve longer, hotter, and more extractive macerations than for teas and infusions. Decoctions are usually made from woodier plants which, if simply soaked in cold water, would otherwise struggle to release whatever useful substances they contain: calcium and tannins from oak bark for the oak bark decoction, salicin from willow and silica from common horsetail (Equisetum arvense) for example, the latter also considered one of nine biodynamic preparations. Once cool, the concentrate can be filtered off and stored. When needed it is diluted in water, typically at between 1:5 to 1:20. Other plants from which decoctions can be made include elder, tansy, stinging nettle and wormwood.