Henllys Bog Nature Reserve is a small, rather remote and difficult to locate site in Torfaen, a few miles northwest of Newport, containing a permanently wet, long grass marshland area, sustained by springs and surrounded by a thin belt of woodland. The place has a quiet, rural location, close to the steep slopes of Mynydd Henllys to the west, and is notable for the interesting variety of plants that grow here, including two rare orchids, the marsh fragrant orchid and the marsh helleborine, this latter especially pretty. Other unusual and/or colourful species in bloom include bogbean, marsh valerian, common butterwort and purple loosestrife in the spring, and meadow thistle, Dyer's greenweed, sundew, bog pimpernel and devil's-bit scabious in the summer. The two orchids flower at the same time, during June and July.
The bog, also a site of special scientific interest, is reportedly the best surviving lowland mire in the county, and is home to at least 80 species of plants, in the wetland, and the enclosing woods. A mire is characterised by an accumulation of organic matter which decomposes only partly, due to limited oxygen, forming peat. Most such areas have historically been drained, to allow the land to be used for agriculture, and hence have become uncommon.
Henllys Bog is reached by a narrow country lane southwest of Henllys, branching west off Henllys Lane and south at the first junction, passing through one farm (Pensarn) then right at a minor junction, over a stream and up a little hill to the entrance to another farm, Pandy Mawr, where the road ends. Parking is on the left, just before the gated farm entrance. The land slopes down to the east, towards the patch of woods which encloses the bog. The party signed walking route (700 feet) to the reserve is south along the edge of the adjacent field, through a gate, then southeast, descending slightly, round to the south side of the wood, where a faded sign marks the entrance. A very short section of boardwalk extends into the boggy area, which can usually explored further by careful walking, though at some times of the year the marsh may hold too much water. The orchids seem to be concentrated around the boardwalk, though the plant species are in general distributed quite evenly. The surrounding woodland contains mostly alder, with beech in the drier locations, plus such plants as sanicle, angelica and tomentil. The east edge of the reserve includes a short section of a brook, Nant y Pandy, which flows south into a much larger wooded area.