Non-gardeners just shake their heads when I tell them one of the highlights of a recent pre-Christmas trip to New York (apart from the no-hassle flight on Porter Air…)  was a brisk walk along the elevated park that runs from Gansevoort in the Meat Packing District to just beyond Chelsea Market. I admit, the grass-lined walkway wasn’t bustling, and the wind whipped the bitter cold air into an icy, body-numbing cold, but it made the subsequent lunch at a cosy little Soho bistro all the more warming. And, winter isn’t such a bad time to look at a garden. The ornamental grasses obligingly bent to the wind and you could clearly see the way designer Piet Oudolf wove the plants into the patterns made by the old rail lines.