Hobson’s Conduit, a new river built in 1610 to bring clean water to the centre of Cambridge from the chalk hills to the south, forms the western boundary of the Botanic Garden. Some of this clear chalky water is diverted into the Botanic Garden to form the Stream that feeds the Lake.
A flowing stream is a rare sight in eastern England and presents an opportunity to grow a great variety of species in the water and along the banks. In early spring, yellow flowers dominate, with rafts of native Kingcup (Caltha palustris) appearing to float mid-stream and the American Skunk Cabbage (Lysichiton americanum), lining the banks with its remarkable flower of a waxy yellow sheath wrapped around the central, columnar spadix.
Later, Hosta species, elegant stands of Iris sibirica, the ornamental rhubarb Rheum palmatum, Primula and feathery Astilbe transform the scene. Lythrum, both native and cultivar provide vivid, magenta, late colour. There is a bed of magnificent, barely-contained, horsetail, Equisetum telmateia, up to one metre high, near the Lake.
Among the horsetails are masses of a parasitic plant, the toothwort Lathraea clandestina. Its brilliant-purple, hooded flowers at ground level are scattered along the stream and the borders of the Lake in damp spots throughout April. It has no chlorophyll at all and obtains its nutrients from the roots of willows and poplars.