We are not sure exactly how old Great Groves is. It is classified as Ancient semi-natural woodland in a survey carried out by English Nature in 1988 and in the inventory of Hertfordshire woods made by the Hertfordshire and Middlesex Wildlife Trust in 1978. This simply means that the site is believed to have been continuously wooded since about AD1600, the year when reliable maps first became readily available. It does not mean that the trees are all about 400 years old. though many of the hornbeams probably are, nor does it mean that the trees have grown up haphazardly from the wild wood. Many are growing in lines which suggests that they have been planted. "Semi-natural" implies that the wood has been managed by man and not just left to its own devices.
The earliest map of Great Groves that we have seen in the Hertford County Archives is dated 1790. The wood's boundaries are almost identical with those of today except for an area that was cut down to build a mansion, "Fanshaws" in 1896 and a strip that was modified when the Hertford to Kings Cross railway was built in the 1890s. This supports the belief that Great Groves is an ASNW.
Other pieces of evidence point towards ''ancientness''. The name ''grove(s)'' is often associated with ancient woods. The wood is surrounded on three out of four sides by a sinuous bank and ditch system and in places the banks are topped by huge old hornbeam stubbs. Their branches were once ''laid'' to make a stock-proof barrier against horses or cattle in the surrounding fields. Many of the hornbeam stools within the wood are very large and they must have taken several hundred years to reach this size. The plant community includes several species which are strongly associated with ancient woods in Hertfordshire. These include yellow pimpernel (Lysimachia nemorum) wood anemone (Anemone nemorosa) wood sorrel (Oxalis acetosella), yellow archangel (Lamiastrum galeobdolon), violet helleborine (Epipactis purpurata) wood spurge (Euphorbia amygdaloides) and wild service trees (Sorbus torminalis).
Twenty five species which are typical components of botanically rich ancient woodland communities in Hertfordshire grow in Great Groves. The presence of 20 or more often indicates "ancientness".
Finally, there are no signs of old habitation in Great Groves and no areas of disturbed ground. If the soil had been ploughed within historical times there might have been signs of ridge and furrow but none have been seen.