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 informed attention and care to fulfill its work as an indispensable landscape element since the first boxwood was planted in the United States in the mid-17th century.

Few plants add structure to a design all year round like boxwood, defining spaces with its evergreen presence while attracting little attention from hungry deer – another huge plus.

But in the last decade or so, the fungal pathogen Calonectria pseudonaviculata, which causes box blight has tarnished this garden mainstay and important nursery plant. The disease — first identified in the United States in 2011 and since recorded in at least 30 states and the District of Columbia — has sparked a wave of research into how to control it and the possibility of breeding resistance into 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 disease hit. The conventional wisdom of boxwood care—beginning with regular, drastic shearing to within an inch after the plant’s lifespan—did not seem to her to meet the boxwood’s needs.

“This plant has had a history of abuse,” said Ms. Filippone, who in March became president of the American Boxwood Society, an organization of enthusiasts from gardeners to home growers. “We baby this plant to death. Plants are over-watered, over-fertilized and only a small crust of leaves is allowed to photosynthesize.”

She is not alone in her assessment.

“Actually, it is our cultural choices that have produced the large numbers of susceptible plants,” said Margery Daughtrey, a plant pathologist and senior extension associate in Cornell University’s School of Integrative Plant Science.

Ms. Daughtrey pointed out conditions we usually create that cause trouble: Wall hedges, for example, are boxwood monocultures. And nothing is more hospitable to fungi than tight spaces where moisture can build up when air and light are excluded.

The low hedges that define traditional herb gardens or formal rose gardens are particularly vulnerable, she said. They are easy targets for disease when rain or watering sprays fungal spores from the soil onto the plants.

Bottom Line: If we can appreciate the value of boxwood, we need to change our ways. That means selecting the hardiest varieties for new plantings and tailoring our care programs for new and existing plants to a more sustainable approach – those best management practices.

These guidelines, from looser pruning to regular mulching and more, must become what Bennett Saunders of Saunders Brothers called “a new way of thinking about the boxwood gardener.” His Virginia-based family business, a more than century-old tree nursery, has been making boxwood its “signature crop” since about the 1950s, he said. Today that means focusing on breeding for increased resistance to rot and boxwood leaf miner, an insect pest.

Saunders Brothers has been actively selecting cultivars for rot resistance since 2011, when the nursery had about 150 cultivars in their collection for evaluation. Gardeners there began making crosses of the hardier varieties, and now the resulting 5,000 unique seedlings need to be screened for disease and pest resistance. The process is ongoing, but two strains have been introduced so far: NewGen Independence and NewGen Freedom.

“We’ve tested hundreds of varieties and have yet to find one that’s completely resistant to rot,” Mr Saunders said. “We are increasingly finding better plants – against blight and leaf miners.”

At the same time, the US Department of Agriculture’s Agricultural Research Service 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 realize something is wrong with boxwood until obvious symptoms such as rapid defoliation appear. But the first hint of rot is on the foliage: brown spots with dark edges, sometimes surrounded by yellow halos. This is not the tanning or browning that comes from winter damage, road salt damage, or drought. Leaves affected by blight usually fall off, showing black, streaky lesions on the stems.

The Connecticut Agricultural Experiment Station reports that of the most commonly grown boxwood species, Buxus sempervirens is the most susceptible to rot, followed by B. microphylla crosses with Sempervirens and then B. microphylla. B. sinica is the least susceptible. However, the report warned: “While we can draw general conclusions, there are many differences within species and this needs to be recognised.”

Including a cultivar’s shape: “Cultivar architecture is a big determinant of rot,” said Mr. Saunders.

And the way plants function in different regional conditions is different. No set of recommendations suits every garden, making breeding for regional concerns another necessity.

Boxwood is particularly prone to rot during extended periods when foliage remains wet or humidity is high, especially when temperatures are between 140 and 175 degrees. The fungus can become temporarily dormant during hot, dry periods or winter cold. A climate like that of Atlanta can have favorable conditions for the blight for many months of an average year. And especially rainy growing seasons, like 2018 in the eastern United States, can cause epidemic outbreaks.

The sticky pathogen can be spread through tools and other implements, on clothing, or by moving infected plants and plant debris. Once introduced, spores can live in the soil for years. And the boxwood family relatives Pachysandra and Sweet Box (Sarcococca) can also be hosts for the fungus.

Ms. Filippone never thought of herself as a boxwood type until 1992 when she bought an 18th-century dairy farm in Pottersville, NJ, which had deer instead of cows.

How could she create the defining, evergreen elements of the historical-style formal landscape that she longed for and her architect-trained eye envisioned? She knew that 30 or 40 visiting deer would tend yew (Taxus) hedges for a short time.

The answer: boxwood.

Today the place where she and her husband Eric T. Fleisher of F2 Environmental Design live and garden is called the Jardin de Buis or Boxwood Garden. Their property, where they run a small box tree nursery, is a popular spot for the Garden Conservancy’s open house (June 12 this year).

Ms. Filippone has long appreciated and displayed “the nuances of boxwood,” its variety 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 don’t stand up to today’s disease pressures. So she takes a break from using them, especially some of the dwarf types.

Likewise, earlier classics such as English dwarf boxwood (Buxus sempervirens suffruticosa) are no longer offered for sale at Saunders Brothers.

“You don’t want to make all your boxwood decisions rot-related,” Ms Daughtrey said. “But it’s such an eliminator that maybe for a while we should decide whether they increase or decrease the likelihood of a plague.”

There is no cure for rot—even chemical fungicide sprays are only preventative—but there is a toolbox of cultural tactics that will give gardeners the best chance of success.

“Fungicides are not medicine; they are protectors,” said Ms. Daughtrey. Infested plant parts must be removed and destroyed before attempting treatment with fungicides. Always bag up all waste – don’t compost it – and apply a thin layer of fresh mulch to cover fallen leaves and reduce inoculum.

As with the leaf miner, which is more common in varieties commonly grown in northern areas, rot needs to be controlled. (In 2021, another pest, the boxwood moth, arrived in the United States from Canada on nursery plants, but it’s too early to know how its presence will unfold or what impact it will have.)

Our main goals are light and air. Studies have shown that abandoning a shaggy boxwood reduces rot. Again, low hedges are particularly vulnerable, so it helps to prune back a bit to encourage air movement under the plants. Cleaning tools frequently with a disinfectant also helps prevent transmission.

“If we could eliminate even just that type of low hedge planting, everyone else would do better,” said Ms Daughtrey, who has worked with colleagues to compile more information at BoxwoodHealth.org. “The taller a plant is and the further it is off the ground, the more air it traps and the better it looks.”

Some tactics are simple (and reminiscent of tomato best practices): don’t water overhead; Don’t 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, showed that a layer of mulch is up to 97 percent effective at blocking spores that would otherwise splatter onto plants.

“A leaf that might have fallen off an infected plant the year before can’t come back and visit it again once it’s buried,” said Ms Daughtrey, who is looking forward to a rapid rot test being developed – and to it The result of a joint effort to build a better boxwood today.

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

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