This section is from the book "British Wild Flowers - In Their Natural Haunts Vol2-4", by A. R. Horwood. Also available from Amazon: A British Wild Flowers In Their Natural Haunts.
This common plant has been found in Preglacial beds in Suffolk, Interglacial beds in Sussex, and in Neolithic beds in Edinburgh. At the present time it is at home in the North Temperate Zone in Arctic Europe, North Africa, Siberia, West Asia as far as the Himalayas. In the United States, America, it is an introduction. It is generally distributed in Great Britain, but it is absent in the counties of Cardigan, South Lincs, Stirling, S. Perth, Elgin, Westerness, Mid and N. Ebudes, West Sutherland, and the Northern Isles. In Yorkshire it grows at a height of 1000 ft.
The Perforate St. John's Wort is as familiar a plant along the roadside as Herb Robert, the Yellow Vetchling, or Tufted Vetch, or Hedge Parsley, Cleavers, and Wood Basil, which commonly grow with it. It is generally found near hedges or banks, and the highway is quite gay with clumps of its yellow bloom from July to September.
Many rounded or slightly angular stems arise from the same root in this as in other species, giving it a clustered appearance. They are erect, tall, branched above, the branches being opposite, and like the stem 2-edged.