Why Boxwood, a Perennial Favorite, Needs a New Approach

It’s time for boxwood-loving gardeners to learn the acronym BMP – best management practices – and get started with the program. Boxwood needs our thoughtful attention and care to do its job as the indispensable landscape element it has become since the first boxwoods were planted in the United States in the mid-17th century.

It’s hard to think of another plant that gives such structure to a design all year round as boxwood, defining spaces with its evergreen presence, while showing little interest in hungry deer – another big plus.

But in the past decade, the fungal pathogen Calonectria pseudonaviculata causing boxwood blight has affected this mainstay and important nursery crop. The disease — first identified in the United States in 2011 and registered in at least 30 states and the District of Columbia since then — has sparked a wave of research into its control and the possibility of breeding resistance in boxwood genetics.

For Andrea J. Filippone, an architect and designer of landscapes and interiors who owns AJF design, the challenge underscores a feeling she had years before the advent of the disease. The conventional wisdom of boxwood maintenance — starting with regular, drastic shearing to within an inch of the plant’s life — didn’t seem to fit her with boxwood’s needs.

“This factory has historically been badly treated,” said Ms. Filippone, who became president of the in March American Boxwood Society, an organization of enthusiasts, from growers to home growers. “We’re babying this plant to death. Plants are overwatered, over-fertilized and have only a small scab of leaves to photosynthesize with.”

She is not alone in her assessment.

“It’s actually our cultural decisions that created the sheer number of susceptible plants,” said Margery Daughtrey, a plant pathologist and senior extension officer at Cornell University’s School of Integrative Plant Science.

Mrs. Daughtrey pointed to conditions we usually create that cause problems: wall-like patches of hedges, for example, are monocultures of boxwood. And there’s nothing more hospitable to mold than small spaces, where moisture can build up when air and light are shut out.

Especially vulnerable are the low hedges that define traditional herb gardens or formal rose gardens, she said. They are an easy target for disease, if rain or water splashes mold spores from the soil onto the plants.

The bottom line: If we appreciate the value of boxwood, we need to change our ways. That means choosing the most resistant varieties for new plantings and aligning our care regimes for new and existing plants with a more sustainable approach – those best management practices.

These guidelines, from looser pruning to regular mulching and more, should become what Bennett Saunders, of Saunders Brothers, called “a new mentality of the boxwood gardener.” His Virginia family business, a wholesaler that dates back more than a century, has made boxwood his “signature crop” since about 1950, he said. Today, that means focusing on breeding for greater resistance to fire blight and box leaf miner, an insect pest.

Saunders Brothers has been actively selecting cultivars for fire blight resistance since 2011, when the nursery had about 150 varieties in its collection to evaluate. The gardeners there started making crosses of the more resistant varieties, and now the resulting 5,000 unique seedlings must be screened for resistance to diseases and pests. The process continues, but so far two cultivars have been introduced: NewGen Independence and NewGen Freedom.

“We’ve tested hundreds of varieties and we haven’t found one that is completely resistant to fire blight,” Saunders says. “We’re finding better plants – against blight and leafminer.”

At the same time, the Agricultural Research Service of the US Department of Agriculture and a number of universities – in Oregon, Texas, Tennessee, Virginia, Maryland, Connecticut and New York – are also looking for solutions. The stakes are high: A 2020 USDA report estimates that more than 11 million boxwood plants are sold annually, with a market value of $126 million.

A gardener may not notice that there is anything wrong with boxwoods until obvious symptoms such as rapid defoliation occur. But the first tip of blight is on the foliage: brown spots with dark edges, sometimes surrounded by yellow halos. This is not the brown or brown discoloration that results from winter injuries, de-icing salt damage or drought. Leaves affected by fire blight usually fall off, revealing black, striated lesions on the stems.

The Connecticut Agricultural Experiment Station reports that Buxus sempervirens, of the most commonly grown boxwood species, is most susceptible to fire blight, followed by B. microphylla crosses with sempervirens and then B. microphylla. B. sinica is the least susceptible. However, the report cautioned: “While we can draw general conclusions, there is a lot of variation within species and that needs to be recognized.”

Including variety shape, “Cultivar architecture is a huge determinant of blight,” said Mr. saunders.

And the way plants perform in different regional conditions varies. No single set of recommendations will fit every yard, making breeding for regional problems a different necessity.

Boxwoods are especially conducive to fire blight for extended periods when the foliage remains wet or the humidity is high, especially at temperatures between 60 and 77 degrees. The fungus can become temporarily inactive during hot, dry spells or winter cold. A climate like Atlanta’s can have favorable conditions for fire blight for many months of an average year. And extra rainy growing seasons, such as in 2018 in the eastern United States, could trigger epidemic-level outbreaks.

The sticky pathogen can be spread by tools and other equipment, on clothing, or by moving infected plants and plant debris. Spores once introduced can live in the soil for years. And relatives of boxwood family Pachysandra and sweet box (Sarcococca) can also be hosts for the fungus.

Mrs. Filippone never saw herself as a boxwood species until 1992, when she bought an 18th-century dairy farm in Pottersville, NJ, that came with deer, not cows.

How could she create the defining, evergreen elements of the period-style formal landscape that she longed for and that her architecture-schooled eye envisioned? Thirty or 40 visiting deer, she knew, would make short work of yew hedges.

The answer: boxwood.

Today, the place where she and her husband, Eric T. Fleisher, of F2 Environmental Design, live and garden is called Jardin de Buis, or boxwood garden. Their property, where they operate a small boxwood nursery, is a popular destination on Garden Conservancy Open Days (June 12 this year

Ms. Filippone has long appreciated and flaunted “the nuances of boxwood,” the range of shapes, and even variations in leaf color and texture between cultivars.

But as much as she loves certain ones to create the perfect globe, mound or pillar, certain favorites can’t withstand today’s disease pressures. So she takes a break from using it, especially some dwarf types.

Likewise, at Saunders Brothers, former classics like dwarf English boxwood (Buxus sempervirens Suffruticosa) are no longer for sale.

“You don’t want to make all your boxwood choices regarding fire blight,” Ms Daughtrey said. “But it’s such an eliminator that we may have to choose for a while based on whether they’re more or less likely to make fire blight.”

There is no cure for blight — even chemical fungicide sprays are only preventative — but there is a toolkit of cultural tactics that gives gardeners the best chance of success.

“Fungicides are not drugs; they’re protectors,’ said Mrs Daughtrey. Affected plant parts should be removed and destroyed before attempting to work with fungicides. Always pack all debris — don’t compost it — and apply a thin layer of fresh mulch to cover fallen leaves, reducing inoculum.

As with leafminers, which are more commonly seen on cultivars commonly grown in northern areas, fire blight should be controlled. (In 2021, another pest, the boxwood moth, entered the United States on nursery plants from Canada, but it’s too early to know how its presence will develop, or what the impact will be.)

Our main targets are light and air. Studies have shown that leaving a boxwood shaggier reduces fire blight. Again, low hedges are particularly susceptible, so a little pruning to encourage air movement under the plants is helpful. Cleaning tools regularly with a disinfectant also helps prevent transmission.

“If we could even eliminate that low-hedge type of plantings, everyone else would do better,” said Ms Daughtrey, who has worked with colleagues to gather more information on BoxwoodHealth.org† “The bigger a plant is and the further from the ground, the more air it captures and the better it looks.”

Some tactics are simple (and reminiscent of tomato best practices): Don’t over-water; do not work between plants when the foliage is wet.

And don’t forget to mulch: Research by Chuanxue Hong, a professor of plant pathology at Virginia Tech, has shown that a layer of mulch is up to 97 percent effective at blocking spores that would otherwise splash onto plants.

“A leaf that fell from an infected plant the year before can’t come back and chase it again once it’s buried,” said Ms Daughtrey, who is looking forward to a rapid test for developing blight — and to see the result of joint efforts to build a better boxwood today.

Margaret Roach is the creator of the website and podcast A way to gardenand a book of the same name.

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SOURCE : www.nytimes.com

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