The Avon Gorge is a 1.5 mile (2.5km) long gorge cut by the river Avon through a ridge of predominantly Carboniferous limestone.
The mainly south to west aspect and south easterly dip of the strata provide a sheltered microclimate of sun-baked niches suitable for plants at the northern end of their natural range such as: round-headed leek Allium sphaerocephalon and Bristol rock-cress Arabis scabra.
Forming part of ancient scrub and grassland communities found growing on carboniferous limestone, plants such as; western spiked speedwell Veronica spicata subsp. hybrida, dwarf sedge Carex humilis, fingered sedge Carex digitata, hutchinsia Hornungia petraea, autumn squill Scilla autumnalis, rock stonecrop, Sedum fosterianum and pale St. John’s-wort Hypericum montanum are some of the rare or nationally scarce plants found in this biodiverse environment.
Many are threatened in the wild by encroaching scrub, introduced species and engineering works. Honewort Trinia glauca has suffered particularly badly from habitat loss due to scrub encroachment. Seed collected from one of the last remaining populations has been grown at the garden and is displayed with the objective of establishing a viable ex-situ population. Two rare endemic whitebeams, Sorbus bristoliensis and Sorbus wilmottiana are grown. A number of other endemic whitebeams have recently been discovered and described and are currently been propagated for eventual planting in this display.