Fox Covert is of quite recent origin, being planted in the late nineteenth century with beech. Many of these trees have become tall and thin and the gales of 1987 and 1990 caused several to come down.
The resulting upturned root boles show how shallow -rooted beech trees are and also how close to the surface the underlying chalk is. Beech woodlands generally cast a deep shade where little grows beneath, except for the pale flower spikes of the white helleborine. The gales have created more open areas where wild privet, sycamore, ash and beech are quickly regenerating. Some fallen trees have been left on the ground as they provide a valuable micro-habitat for many invertebrates and fungi.
The woodland is frequented by deer and many species of birds.
Therfield Heath is a large reserve (169 hectares) and is one of the most extensive remaining areas of chalk grassland in eastern England.
Church Hill is renowned for its large colony of rare pasque flowers which bloom in early spring. Grassland herbs include bastard toadflax, lesser meadow rue and spotted cat's-ear. Other typical chalk grassland flowers include horseshoe vetch, squinancywort and the nationally rare field fleawort, together with wild candytuft and five species of orchid.
During winter the reserve attracts visiting birds such as fieldfare and redwing, while meadow pipits and skylarks are present all year. In summer several species of butterfly can be seen including the brown argus and the largest colony of chalkhill blues in Hertfordshire.
Therfield Heath is a Site of Special Scientific Interest and also a Local Nature Reserve.
