Landscape architect: If the screen between these two areas is to be a hedge, that hedge must be of either hemlocks or Leyland cypress trees. However, Leyland cypress trees cannot be grown this far north. So if the screen is to be a hedge, it will be a hemlock hedge.
The pattern of reasoning in the landscape architect's argument is most similar to the statement: 'If there is to be an entrance on the north side of the building, it will have to be approached by a ramp. However, a ramp would become impossibly slippery in winter, so there will be no entrance on the north side.'