Todd Forrest is the Arthur Ross Vice President for Horticulture & Living Collections at The New York Botanical Garden.

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This is the fourth installment of a five-part series on caring for NYBG’s magnificent trees.

With so many trees a century old or older growing across the landscape, Garden arborists have been implementing a wide array of tree-care techniques specifically developed for mature trees. In 1999 we began removing turfgrass and installing mulch beneath the canopies of the pines, spruces, and firs in the Arthur and Janet Ross Conifer Arboretum in an effort to alleviate soil compaction from lawn mowers, reduce competition for water and nutrients, and to improve soil health. These “mulched rings” allow soil pH and fertility to be adjusted on a tree-by-tree basis and reduce conflicts between the needs of turf and trees. In some cases, the soil that sustains the trees is so poor that arborists must resort to more dramatic restoration methods. By 2007 the soil beneath the historic grove of centuries-old oaks and sweetgums in front of the LuEsther T. Mertz Library Building had become so compacted and depleted of nutrients that Garden staff had to perform “vertical mulching,” a process that includes using compressed air or a soil auger to drill eight-inch deep holes that are backfilled with a mixture of topsoil and compost. Nearly 15 years after the vertical mulching, the soil is once again light, rich in organic material, and teeming with earthworms.

NYBG arborists have also been systematically working through the landscape assessing trees, pruning dead, diseased, weak, and crossing branches, and, where necessary, installing cables and braces. Pruning trees does more than make them more beautiful—it removes weak branches that are often the entry point for insects and allows light to penetrate the canopy. Cabling and propping are used as a last resort when large branches or even whole trunks may not be able to support themselves. Experiments with polypropylene cables, which are flexible and do not require drilling holes into branches, have proven successful with small trees such as the Tanyosho pines in the Ross Conifer Arboretum. Larger trees require traditional galvanized steel cable.

Frequent severe storms, inconsistent precipitation, and an ever-growing suite of introduced pests threaten the health and beauty of the Garden’s most venerable trees. By providing thoughtful and constant care, we help our irreplaceable mature trees persevere in spite of the myriad stresses they face.

The next installment in the series focuses on plant health. This article originally appeared in the Summer 2021 edition of Garden News.