Indeed flat. That's why we are called during centuries already the Low Countries. (i.e. Holland and Belgium: our highest "peaks" in the east of the country are about 600 and a few meters).
About the border itself: I applicate (or try to) the principles of the Dutch plantsman Piet Oudolf and the Americans James Van Sweden and Wolfgang Oehme. (=a majority of perennials, strong hardiness, needing not too much staking, not too much improvement of the cultivars so they are still near their ancestors the plants in free nature and have a good resistance against diseases and pests).