Gardening in the Sky: Growth on the High Line

In our new Gardening in the Sky blog series, Horticulture Manager Andi Pettis shares the history of the High Line's trees and their dramatic transformations.

Before the High Line became a park, the design team of architecture firm Diller, Scofidio + Renfro and landscape architects James Corner Field Operations famously asked themselves, what will grow here? That question (long since answered, see a list of all the species and varieties of plants growing on the High Line) has since become, how much will it grow? When the young birches and burr oaks and American hollies, all very large trees when growing in their natural environments, were planted on the High Line, we wondered, how tall will they grow?

The question of just how much the High Line's trees have grown since the park opened in June 2009 is answered in the images above. The image on top was taken in October of 2009 at the Tiffany and Co. Foundation Overlook at Gansevoort Street. The one on the bottom was taken just a few days ago, in the summer of 2015, in almost exactly the same spot. The amount of growth that has taken place in the last five years is dramatic, to say the least. Not only have the trees grown several feet taller, their canopies have filled out so much that their foliage nearly obscures the yellow brick building that is clearly visible in the image taken in 2009.

All landscapes, designed or naturally occurring, are constantly changing systems. In an establishing forest, for instance, as larger tree species grow and fill out the woodland canopy, other species inevitably succumb to competition for resources such as light and water. In a garden or park setting like the High Line, this process, known as successional growth, is carefully planned for and managed by arborists and horticulturists. As the tree canopy filled in on the High Line, we began to plan which trees must be thinned out as they mature in order to maintain a healthy environment for all the plants in the gardens. Over the past year or two gardeners have selectively removed a sassafras that wasn't getting enough sun, or an entire stem of a birch tree that was encroaching on its neighbor. This year, nature did some of the work for us.

Take another look at the image on the bottom. Believe it or not, just weeks before this photograph was taken, gardeners removed at least nine trees from this area. The removals weren't all planned. These were trees that had been severely damaged or even killed by one of the harshest winters we've seen in years; the winter of 2015 actually helped us along in the management process by naturally culling some of the weaker plants. Whether the removals were planned or not, the gardens have become so lush that many visitors will never realize that entire trees were removed.

Even so, we are planning to replace some favorite specimens. Several of our eastern redbuds, or Cercis canadensis, had to be removed, but what would spring on the High Line be without the spectacular magenta flowers that line the branches of Cercis canadensis 'Appalachian Red'? The horticulture staff has spent the last several weeks sourcing replacements, and we hope to begin planting soon. We may even take this opportunity to introduce some new varieties into the gardens, begging the question, what will grow here next?

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