Pruning is one of the most important ways to promote tree and shrub health, particularly since a bad cut can affect a tree for years to come. As Marek Pundzak, staff arborist for Friends of the High Line, says: "Before a single cut is made, every tree assessment needs to start with a thorough inspection and understanding of not only the tree in question but the impact it has on the environment around it."
As the gardens on the High Line mature, the horticulture team is developing a comprehensive pruning strategy to maximize the longevity of our trees and shrubs. The positioning of the High Line thirty feet in the air causes us to adjust many of our gardening practices, and pruning is no exception. Several site conditions, including crowding, wind, and soil depth, impact our pruning techniques in the park.
Trees that were planted as slender saplings have doubled and tripled in size since the park's opening. As their canopies grow together, these trees have created cool shady woodlands beloved by our human and avian visitors alike. Gardeners carefully thin the canopies in these areas to prevent branches from rubbing together. Continuous rubbing can damage the outer tissues of the bark and open up opportunities for infection. Good airflow also reduces the spread of disease, so it is important that growth not become too dense.
When pruning in such tight quarters, gardeners must work with the natural habit, or form, of the tree. Otherwise, the end result may be a shape that looks glaringly artificial within the naturalistic context of our gardens' design. Working with a tree's natural habit is especially tricky in the narrow beds where branches extend into the path. Gardeners limb up these trees, carefully shaping them to keep branches above eye level.
The wind gusting off the Hudson River can be severe, and its impact is magnified by the park's elevation. If weight is unevenly distributed through a tree's canopy, the wind can twist the tree, causing the trunk to develop deep cracks. We prune to make sure the canopy is balanced. Where it is not desirable to remove branches, cabling is another strategy we employ. Cabling creates an artificial support system that mimics the natural flexibility of the branches and helps support weak branch unions. Bolts are drilled into the main trunk and a branch and then cable is run between them, so weight is redistributed.
The average soil depth in the park is only 18 inches. Because of this limited soil depth, a tree's root system may not develop as extensively as in wild conditions. A reduced root system means that the tree cannot store as much energy or draw up as much water. In effect, the tree cannot support as much canopy. By thinning canopy, we can reduce pressure on root systems. A reduced root system means reduced growth because, with less sugar stored, less growth is pushed out each season. In this way, the tree may "self bonsai" by staying at a more diminutive size based on the resources available to it.
Pundzak points out that "trees are survival experts" and can adapt to many harsh conditions. We give them the best odds by learning the needs of each species. By making the right cuts at the right times of year, we keep the balance between aesthetics and health.
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