The Limestone Mound opposite the Fen Display demonstrates some of the huge array of British wild plants that can be found on chalk and limestone (calcareous) soils, conditions which predominate particularly in our Eastern region. One of the most spectacular sites in May and June, is the tumbling display of suplhur-yellow rock-roses (Helianthemum) combining with the lovely, deep-blue flowers of Buglossoides purpurocaerulia, rare in the UK. This plant thrives here and spreads by tip rooting, like a bramble, an unusual characteristic.
On top of the mound is a massed planting of native shrubs and trees typical of chalk and limestone areas. Amongst them is a large and thriving Sorbus bristoliensis. The Botanic Garden has an almost complete representation of these rare and beautiful Sorbus species in the Gilbert-Carter Memorial Area.
A chalk grassland habitat is represented on the eastern face of the mound and features rare annuals such as Candytuft (Iberis amara) with white flowers. There is a nice population of an as yet unidentified Euphrasia, a hemiparasitic plant with small white flowers and lilac lip.
A specialised display focuses on Breckland plants from the borders of Cambridgeshire, Suffolk and Norfolk, a unique region of blown sand overlying chalk. The soils of this area are poor, and before the use of fertilisers to artificially enrich land became widespread in modern agriculture, fertility was restored by allowing fields to lie fallow every few years. These ‘brecks’ (fallow fields) support unique and now-threatened plants such as the catchfly (Silene conica) and the Breckland speedwell, Veronica triphyllos, which is very rare and known at only one or two sites in the wild.
The western slope of the mound has plants typical of the woodlands which develop on boulder clay containing large quantities of chalk, such as the ancient woods of Hayley Wood and Buff Wood in west Cambridgeshire. The graceful Wood Melick (Melica uniflora albescens, a white-bracted mutant) clothes the mound in soft grass in late spring. Plants of shady limestone are found on the northern side of the mound, such as the magnificent spires of the Large Bellflower (Campanula latifolia) under a Wild Cherry (Prunus avium).