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Gardening in the Sky: Predicting Success

By Erin Eck | December 1, 2017

Failure is a big part of gardening, especially in the unusual conditions we have on the High Line. Growing plants in 15- 18″ inches of constructed soil on a bridge that passes over streets and buildings presents unique challenges. By observing plants in the wild we can form an idea of how they will perform in our garden. However, just because a plant thrives in our region does not mean it will thrive here in the park. Soil, topography, sunlight and hydrology all affect how a plant grows – how big it gets, how long it lives, and how it competes with its neighbors. If we pay close attention to these conditions, we can get a sense of whether we can give a species what it needs. We’ve learned that no matter how beautiful a plant might be and how well it might fit aesthetically in the garden, it may not be worth planting if we have to alter existing conditions to keep it happy.

Soil nutrient levels, pH, and depth are some of the first things to consider when deciding whether to introduce a new species. In the poor soils along roadsides and train tracks, you’ll often see swathes of native little bluestem grass (
Schizachyrium scoparium) standing perfectly upright into the winter. In our soils, which are regularly irrigated and rich in nutrients and organic matter, the little bluestem tends to flop, making for an unattractive autumn display. To account for the difference between our conditions and those of this grass’s wild habitat, we’ve switched to the ‘Standing Ovation’ cultivar which has a stiffer habit.

Black huckleberry (Gaylussacia baccata) has many ornamental features that it make it a desirable garden plant.

The delicious berries and scarlet fall foliage of black huckleberry (Gaylussacia baccata) make this shrub, which is common to many parts of our region, highly desirable for the garden. Because they tolerate shallow soils and harsh winds, one might expect these shrubs to do well on the High Line. However, while huckleberries form large thickets in the sandy, acidic soils of the Pine Barrens or on the windy balds of the Catskill Mountains, they are not always easy to please in the garden. Many have died and the remaining ones have not spread as they would in the wild. We believe this may be because our soil is not acidic enough. While we do apply acidifying fertilizers around these shrubs, this is only a temporary solution that must be repeated regularly. At the end of the day, we’ll have to decide what lengths we’re willing to go to maintain these plantings and at what point it makes more sense to select a different shrub that is better suited to the park.

In some areas of our wetland planting – casually referred to as the bog – we plan to alter conditions in order to create niches for species like Virginia marsh St. John’s wort (Triadenum virginicum) and handsome Harry (Rhexia virginica). These smaller perennials were crowded out by graceful cattail (Typha laxmannii) and giant horsetail (Equisetum hymale). While both St. John’s wort (Triadenum virginicum) and handsome Harry (Rhexia virginica) grow well in the rich soils of our “bog”, these conditions favor the larger more aggressive plants. To help the smaller plants compete we will create niches where they have the advantage. We’ll add sand in some areas to reduce nutrient levels. We may also propagate and plant sphagnum moss, which has the capacity to lower pH. By mimicking their tough natural habitats, we hope to give our smaller wetland perennials the upper hand and control the spread of some of the larger species.

The handsome Harry (Rhexia virginica) and Virginia marsh St. John’s wort (Triadenum virginicum) on the right were crowded out by plants like giant horsetail (Equisetum hyemale). The horsetail stems shown here will reach a height of about 3′.

By paying attention to microclimates, soil and hydrology in wild habitats we can make predictions about how plants will perform in the garden context. Going out into the wild provides gardeners with inspiration for new plantings and information about the conditions each species needs to thrive. In the next post, we’ll discuss another fundamental link between the garden and the wild – seed collecting.

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TD Bank is the Presenting Green Sponsor of the High Line.