In season, lilacs are an extravagance of color and fragrance — especially when you have something like 437 plants, representing 138 different species and varieties, as the New York Botanical Garden does in its Burn Family Lilac Collection.
After they finish blooming, though, lilacs can present an extravagantly messy aftermath, nudging the gardener to intervene in the name of tidiness.
Get out the shears (hint: the long-reach version with a telescoping handle is especially helpful for such an assignment). Sharpen your powers of observation as you head out for duty, too, said Melissa Finley, the botanical garden’s Thain curator of woody plants. Deadheading may be the obvious task, but there are subtler clues to discover about fine-tuning your shrubs’ performance — or maybe extending your lilac season and its color palette.
Ms. Finley is always applying that kind of careful eye to the garden’s historic collection, an entire lilac-forward world on five acres first planted in 1949 and renovated in 2016.
Some of what she’s looking for: Which older plants have grown leggy and need to begin a rejuvenation cycle starting late next winter? Are there damaged stems or signs of last season’s handiwork by pests such as lilac borer?
And which plants simply are not performing as well as they once did and might need replacing, now that the new-normal climate patterns have taken hold? With winters not as consistently cold, some of the earliest-blooming varieties in particular may be way off schedule or experience injury to their opening buds in aberrant late-winter temperature swings.
“Lilacs are really a great poster child for this type of conversation,” Ms. Finley said. She explained why: They have long been used for tracking phenology — nature’s calendar of recurring seasonal events driven by environmental conditions — because besides flowering early, lilac phenology is wholly dependent on temperature, not day length or a combination of those factors.
“So they’re much more sensitive to any sort of shift in temperature,” she said. “What the lilac wants is a nice cold winter that emerges gradually into spring.”
Early bloomers, such as the hyacinth lilacs (Syringa x hyacinthiflora) — hybrids between the common lilac (S. vulgaris) and Asian S. oblata, which flower about 10 days before the vulgaris ones do — could especially be affected by temperature changes.
This explains why for a couple of winters before this most recent one, Ms. Finley noticed some lilac flowers open at Christmas, meaning any spent flowers on those plants wouldn’t bloom again in lilac season. A later-blooming cultivar might make a better choice for their spots in the collection, she imagines, “plants that are better attuned to our climate, or our anticipated climate,” she said.
A Diversity of Scale and Color
The collection awoke this year in mid-April with such hyacinthiflora hybrids as Vesper Song and Excel; by month’s end, the common lilacs were in high gear. The bloom sequence will continue until around mid-May, Ms. Finley predicts, as the Asian species and their selections take over, such as the cut-leaf lilac (Syringa x laciniata) and compact, mound-shaped S. meyeri Palibin.
Depending on which taxonomic reference you adhere to, the genus Syringa includes either 12 species or as many as about 20. Most are from Asia, but two are from Southeastern Europe, including the common lilac, S. vulgaris, from which the majority of the 1,600-plus known cultivars get part of their genetics. (Side note: What are called “French lilacs” aren’t native to France, but are double-flowered vulgaris cultivars originally bred in nurseries there.)
Two Asian species, Syringa reticulata and S. pekinensis, are tree forms; the rest are of shrubby stature, in varied sizes.
Without considering one of those true tree types, the scale range is still quite diverse, but for many gardens, shrubs reaching 15 feet or more may not be the best choice. Compact cultivars Ms. Finley recommends besides Palibin include Miss Kim, Prairie Petite and Little Lady.
The International Lilac Society officially recognizes seven flower colors, but it’s more nuanced than that in real life, Ms. Finley said.
“Is it early in the flower’s life, or is it about to end? It’s going to look totally different,” she explained, adding that soil pH is another variable affecting color. The botanical garden’s slightly acidic conditions promote “really nice blues, which not everybody can get,” she added. A widely admired favorite among them: President Lincoln.
Would a pink cultivar (Maiden’s Blush, for example) enliven the scene in your garden at lilac time, or perhaps one in the palest yellow such as Primrose, or vivid reddish-purple Congo?
Deadheading and Pruning
Lilacs, which bloom on the previous season’s growth, start the bud-setting process pretty quickly after flowering, so Ms. Finley recommends that spring’s deadheading and light pruning happen within about two weeks after the flowers fade.
The deadheading process is straightforward: Simply cut back to the next pair of leaves, she said.
No time to do it? Don’t worry. Though it’s “horticulturally impactful,” she said — meaning your plants will look better — it’s not essential to plant health. An incentive, though: You may be rewarded with better flowering next year, as the plants won’t squander energy in seed production. (Another potential motivation: Prune earlier, during peak flowering, and your remuneration will be in the form of bouquets.)
A spring pruning step not to skip: Identify and remove branches that are rubbing against each other, along with dead or damaged ones, or those impacted by pests, she said, such as any with pinholes near the base created by lilac borers.
Plants requiring major pruning to bring too-tall shrubs back to scale and vigor should be noted now, but the work saved for late winter, when a multiyear rejuvenation can begin. After eight or 10 years, Ms. Finley said, lilac stems simply aren’t as productive. Some of the oldest, tallest ones — maybe a quarter of them, or as much as a third if you’re feeling daring — can be cut to the ground each late winter for three years to renovate the plant, with an eye to creating a nice open shape.
As much as you might wish it to be so, a lilac can’t be sheared into scale by heading back its branches partway.
“When you make that big heading cut it’s going to create this proliferation of twigs that just grow in every direction,” Ms. Finley said. “It won’t do what you wanted to do, which was keep it shorter.” The density of new shoots may also contribute to additional disease problems, like powdery mildew.
Even with the embarrassment of Syringa riches before her, Ms. Finley can point to some standouts — including Lilac Sunday, a 1997 introduction from the Arnold Arboretum at Harvard University. It doesn’t produce flower clusters just at the tips of its branches in typical lilac style; rather they form along the length of the stem, creating the illusion of an inflorescence that can measure some two feet long.
“So it’s one big branch basically of this kind of magenta-pink,” Ms. Finley said. “They’re very, very beautiful.”
Another cultivar that always gets people talking: the bicolored Sensation, whose unique flowers are purple, edged in white.
One cultivar Ms. Finley still hopes to add to the collection, Rochester, is distinctive in another way. It has what are called radial double flowers, with as many as an astonishing 25 or so lobes, or petals, on each little floret within the larger flower trusses.
“Certainly we want to show the breadth of possibility of what is a lilac,” she said, “to challenge people and say, ‘This is also a lilac — look how unusual it is compared to what’s in your garden.’”
Margaret Roach is the creator of the website and podcast A Way to Garden, and a book of the same name.
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