The Limestone Rock Garden surrounds the north-eastern arm of the Lake. Construction of the Rock Garden took four years (1954-58) with each rock, some weighing up to three tonnes, winched into place with a tripod and block-lifting system. The horticultural staff then levelled and positioned the stones using crowbars to create a range of planting niches – angles that shade, nooks that shelter, and crevices that collect water – in which to grow plants from rocky regions around the world.
These specialised rock plants are laid out geographically. On the south side of the Lake, among the detached rocks, are plants of South African and Australasian origin. Proceeding anticlockwise are Asian plants, then European and finally North American species stretching up to the highest point. The North American section has an Opuntia (pad cactus) which survives and flowers even in this exposed habitat.
Rock garden plants are usually very colourful and are characterised by a high density of flowers on rather small plants. This reflects the low frequency of pollinators that occur in desolate, rocky and usually dry areas. The Limestone Rock Garden has a long flowering season but is most spectacular in May and June.
To the west of the Limestone Rock Garden is a small shady, sandstone Rock Garden that can be reached via the stepping stones across the Lake. The emphasis here is on shade and moisture-loving mountain plants, including some few calcifuge (lime-hating) plants: members of the heather family Ericaceae such as Rhododendron and Erica survive, although the chalky ground water seeping into the Sandstone Rock Garden reduces their life expectancy. Bulbs such as Trillium) and Dog’s Tooth Violet (Erythronium dens-canis) flower here beneath Prunus serrulata ‘Alboplena’, a neatly-shaped tree which in May has each limb thickly clothed in brilliant-white, double flowers.