Australian National Botanic Gardens

The Gardens’ Environment

Map reference: Latitude 35° 17' S., Longitude 149° 6' E

Topography. The Australian National Botanic Gardens are located on the lower eastern slopes of Black Mountain overlooking Canberra. Elevations range from 576 m at the entrance to 670 m at the centre of the western boundary; which compare with elevations of approximately 565 m for the surrounding plains and 812 m at the top of the mountain itself. The area of the Gardens slopes generally uniformly to the east at approximately 7% (or 4°) with an increase in slope at the more elevated sections. Two drainage lines, both of which flow in an easterly direction into Sullivan's Creek, dissect the area. The Rainforest Gully has been established on the southern drainage line.

Geology. Black Mountain and its surrounds are composed of sandstone deposited under the sea during the Ordovician period (500 million years ago). It is part of the basement rock of the Canberra region and has been exposed as a residual mountain by uplift and differential erosion. During formation of the mountain colluvial material (moved by gravity) and alluvial material (moved by water) eroded from the sides of the mountain has been deposited around the base as detrital fans. It is on this colluvial and alluvial material that the Gardens are located.

Geomorphology. Soils are generally grey clay loams over heavy yellow clays. The loam is bleached and sets hard on drying. The clay is densely packed and has restricted drainage. There is a distinct boundary between the loam and the clay horizon. Rocks are very common through the profile, reflecting the colluvial nature of the parent material.

Soils. The soils are of inherent low fertility with deficiencies in nitrogen, phosphorus, calcium and a range of trace elements. Structure is generally poor to moderate. Erosion can be a problem where the soil is not protected by vegetation. Intensive horticultural use requires some fertilisation and protection of soil structure to overcome these limitations.

Climate. The mean annual rainfall is 655 mm in the Gardens. The distribution of this is irregular and the effectiveness of summer rain is reduced by high evaporation rates and rapid run-off on the slopes of Black Mountain. The afternoon relative humidity ranges from 35% in January to 64% in June.

Frosts occur in winter, particularly in open clearings, and may persist throughout the day on the southern slopes of Black Mountain. Fog and mist sometimes shroud the mountain top.