Be careful: The garden wants to have its own way, to blur the lines and refocus those pictures you planned so carefully and then planted.
Anyone who has ever tried to create a garden — especially a naturalistic one — learns a few things very quickly. Most emphatically, that change is the only constant. But also that the skills of observation and anticipation are the gardener’s most essential horticultural tools in any effort to stay ahead of it.
Try working in this loose, nature-inspired manner in an urban setting — in man-made beds 30 feet above street level, filled with a mere 18-inch layer of soil — and you have the High Line. Those added constraints only intensify the challenge.
This month marks the 15th anniversary of the opening of the first section of that 1.5-mile-long park, built on an elevated rail line on the West Side of Manhattan. And by now, Piet Oudolf’s planting design is one of the best-known naturalistic gardens anywhere, a sort of emblem for the style from the leader of the movement.
In addition to some seven million people a year who visit the High Line, it has attracted another devoted following: a fan base that includes warblers and other migratory birds, 33 native bee species and various butterflies, including painted ladies. The other day, one laid her eggs on a swath of pussy toes (Antennaria neglecta).
Such moments of recognition and connection — and the whole immersive gestalt of the woodland- and grassland-inspired gardens — may create the impression that the landscape was created by nature. But, of course, that’s not the case.
There are 10 horticulturists up there trying to foresee the plants’ every move. They work to respect Mr. Oudolf’s artistic and ecological intentions without being unrealistic about how insistent the forces of natural succession can be — even within the confines of a 30-foot-wide planting bed.
“You have to know plants, and you have to have a good understanding of how the plants move or change over time,” said Yuki Kaneko, the senior horticulture manager, who has worked at the High Line for nine years.
It helps “to be able to enjoy that process, instead of a goal-oriented one,” Ms. Kaneko said, and to be “willing to participate in that process on a daily basis.” Her words could be part of a job description for prospective gardeners of any naturalistic landscape.
These are “not set it and forget it gardens,” said Richard Hayden, the High Line’s senior director of horticulture, who joined the team two years ago.
The gardener needs to be “open to the evolution, and realizing that it is never going to be done,” he added. “You’re always looking for the solution of the moment and what’s going to make things be successful going forward.”
Looking to Nature for Patterns and Guidance
To manage the High Line gardens, the horticulturists look to the same place that Mr. Oudolf did when conceiving his design: examples set by the natural world.
In the Gansevoort Woodland, a section near the southern end of the park, John Gunderson, the senior horticultural supervisor, works to stay ahead of changing light conditions and their effect on the understory layer’s plantings.
A canopy of gray birches (Betula populifolia) that were eight feet tall when installed are now pushing 30 feet. Layered beneath them are flowering dogwoods (Cornus florida), redbuds (Cercis canadensis) and serviceberries (Amelanchier laevis), with a complex ground-cover layer of autumn moor grass (Sesleria autumnalis), Virginia bluebells (Mertensia virginica), betony (Stachys monieri Hummelo), asters and various ferns.
To let some light reach the ground and reduce root competition in the shallow soil, occasional trees and shrubs have been removed. Others are pruned artfully every other year.
The herbaceous layer needs regular editing, too. That means “taking things away in a naturalistic way that actually reads as if it’s been going on, on its own,” said Mr. Gunderson, who joined the staff in 2011.
His intention: “To make something look like it’s happened over years, when in fact it’s just constantly being manipulated a little bit, always keeping that vision of patterns you would find in nature.”
When he is translating those patterns to the garden setting, he may “force them to be a little bit more dramatic, but they’re rooted definitely in trips to the woods,” he said.
“When I’m traveling, we love to pull over in front of a field and be mesmerized by the patterning that’s happening without anyone’s help,” he continued. “And that’s always what I bring back here — not to try to replicate it, but it’s a way of just understanding how things can occur in nature without any human intervention.”
But remember, Mr. Hayden said, that the scale of any garden is far smaller than that of nature. Take a cue from Mr. Oudolf and narrow your palette to a few key plants grown in multiples. “Piet’s not trying to get too many plants into any one area,” he said. “And it’s easier on the eye if you think of it in terms of repetition, and create drifts.”
Making the drifts large, massing more than 10 plants in each organic-shaped grouping, Mr. Gunderson said, “keeps the gardens feeling calm and the plants legible.”
When planning such layouts, he first does some research on how a particular species establishes itself in the wild.
“Does it form a dense mass, or are there just a few spread throughout?” he said. “After I have an idea of how it establishes and moves naturally, I will plant to replicate its native setting — always in uneven numbers, and always avoiding even spacing.”
Another suggestion: Let those drifts of a similar plant extend across both sides of a path. That “makes you feel like the garden’s always been there,” Mr. Hayden said, “and you’ve just plopped the path down in the middle of it.”
The Importance of Well-Timed Cutbacks
The advice to skip the wholesale cutting back of our gardens in fall — and “leave the leaves” to support the overwintering of beneficial invertebrates — has received wide attention lately, and much of the High Line is managed this way.
Another kind of perennial cutbacks, though, is key to keeping the herbaceous plantings in scale. These cutbacks are done not in fall, but during active growth, from around May to July.
The timing and frequency varies by plant and placement — whether the plant is growing in sun or shade. Even plants in the same genus may stretch more in lower light; keeping them bushier and upright by pruning takes extra effort.
In the woodland beds, Mr. Gunderson may cut back the asters three times between early May and the beginning of August, removing the top third each time. By contrast, in a sunnier, grassland-style area of the park, the asters have the company of tall surrounding grasses to support them, as they would in wild prairie and meadow communities. Those asters require only the more traditional Chelsea Chop (named for the Chelsea Flower Show in England, which the haircut’s timing roughly coincides with there). Just one cutback about halfway down is performed, around late May.
Goldenrods (Solidago), Coreopsis, Rudbeckia and Nepeta are among the other regularly chopped plants. Because there are large drifts to contend with, hedge trimmers are often the tool of choice, except close to the pathways, where hand pruning with shears leaves behind less obvious traces of the cuts.
Sometimes certain weeds are cut back rather than pulled, to save time and reduce soil disturbance that might expose more weed seeds to light. This tactic is used mostly with annuals and biennials, Mr. Hayden said, which won’t return unless there’s a supply of seed to fuel another generation.
To maintain the balance of the garden, even species that are part of the intended design need editing, if they are inclined to self-sow into new spots or too great a concentration. Mr. Gunderson cuts down and removes some Virginia bluebell stalks and leaves in May, for instance, as the foliage begins to brown. Yes, there can be too much of a good thing.
Other times, there is not enough, as when some of the original plants specified in Mr. Oudolf’s plans, including sneezeweed (Helenium autumnale) and Mexican feathergrass (Nassella tenuissima), wouldn’t take hold.
Beyond the horticulture team’s vigilant, ongoing editing, there is another way the High Line is being made more legible to visitors: with signage, something the gardens did not originally have. As part of the 15th anniversary, 35 of what Mr. Hayden called Mr. Oudolf’s “iconic Piet plants” are now introduced to visitors this way.
It probably comes as no surprise that the complex park landscape is managed organically.
“The goal is to create a plant community that takes care of themselves,” Ms. Kaneko said.
Last summer, she came upon “beautiful lacewing eggs” beneath a leaf on a plant growing alongside an aphid-infested swamp milkweed (Asclepias incarnata). “The Asclepias was there not to be just admired by us,” she said, “but to provide food to aphids, which will support another insect’s life. A garden should be a place that has ecological function.”
Resiliency, she added, “is the ethos of the place — the resiliency of nature, the city and of people.”
Margaret Roach is the creator of the website and podcast A Way to Garden, and a book of the same name.
If you have a gardening question, email it to Margaret Roach at gardenqanda@nytimes.com, and she may address it in a future column.