In the last Gardening in the Sky
post I discussed how human activity often defines urban plant communities. To some extent, the reverse is also true: the wild plant communities that originally grew on the High Line inform both the design and maintenance of our current gardens. The High Line's garden designer, Piet Oudolf, drew inspiration from the forms and compositions of the original self-seeded landscape.
Over the last eight years we've watched the plantings shift as a result of plant competition and site conditions. Some perennials that were originally planted never thrived here and were eventually given up, while others seeded in and were deemed appropriate for the garden.
As I mentioned previously, a seemingly trivial site alteration can alter species composition. This impact can be seen on the Diller - von Furstenburg Sundeck, where many plants use the fence running along the back of the bog to lean out into the sunlight. Thanks to the fence, the back of the bog shows much higher diversity than the front, where cattails (Typha laxmannii) dominate.
We have experimented with many plants in the bog that could not compete among the dense stands of cattails (Typha laxmannii). Rather than replant season after season we are adjusting our palette and allowing tough survivors like white turtlehead (Chelone glabra) and swamp milkweed (Asclepias incarnata ssp. pulchra) to spread. Because it is an appropriate plant for a native wetland planting, we also left the joe pye weed (Eupatorium maculatum) that seeded in on its own from another area of the garden.
Another self-seeder that has found its niche in the bog is violet. In the original design creeping mazus (Mazus reptans) was planted as a groundcover, but it had difficulty establishing. Violets seeded in, forming a dense mat. Trailing delicately over the edges of the bog, they function much like the mazus was intended to. On the Hudson River Overlook, violets fill a temporary niche. In the gaps created during a recent renovation, violets have crept in, covering the bare soil and helping retain moisture. As the newly planted perennials grow they will close up these gaps and the violet's niche will mostly disappear. This process of succession is very similar to what we've seen in the self-seeded landscape.
Sometimes in order to honor the design, we need to alter conditions. For example, we lost much of the little bluestem (Schizachyrium scoparium) grass where colonies of sumacs (Rhus sp.) had created dense shade. To keep this grass in the design, we had to remove some of the sumacs and create open sunny patches. In other cases, we've had to increase or decrease irrigation to keep a plant happy in a desired location.
As human activity disrupts ecosystems and throws together plants from disparate geographical locations, maintenance has become the primary distinction between garden and wilderness. Wild urban landscapes offer important maintenance lessons for gardeners about plant competition, niches, and succession. Gardeners evaluate whether self-seeders serve an ecological or aesthetic purpose in the garden or whether it is worth altering site conditions to keep a desired plant happy. Like wild plant communities, gardens change over time and the naturalistic gardener learns from the plants as she steers these changes. On the High Line we work to maintain a sense of wildness without losing control of the design.
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