Complex sentence: A key landscape element is the driveway, which is lined with an avenue of 51 elms down each side; these are mature from the gates through to the 28th tree, and semi-mature after that indicating two stages of planting.
Split into simpler sentences: A key landscape element is the driveway, which is lined with an avenue of 51 Elms down each side. They are mature from the gates through to the 28th tree, and semi-mature after that indicating two stages of planting.