The future of boxwood

The popularity of boxwood has exploded in the last 15 years. And why not? It’s deer-resistant, drought-tolerant, versatile, and easy to grow. To call it “proven” is an understatement – it’s been a staple in American landscapes since the mid-1600s. To top it off, it only suffers from one major pest: boxwood leaf miner. As the housing market trends up, more landscapers and consumers are looking for large boxwood for instant impact.

But the supply hasn’t been able to keep up with the demand. There are a few reasons for the shortage. Several large boxwood-providing nurseries went out of business during the recession, and many of the nurseries that stayed afloat cut back on their plantings. Then, in 2011 a new disease appeared that ruthlessly attacks several of the most popular types of boxwood.

Growers face an uphill battle to meet the demand for healthy plants.

Supply and demand

Saunders Brothers is nestled in Piney River, Va., under the shadow of the Blue Ridge Mountains. The business began as a partnership between five brothers in 1915. Today, third-generation brothers, Tom, Bennett, Jim, and Robert with their dad, Paul, operate the wholesale nursery, orchard and farm market.

The wholesale nursery is known for boxwood, but also produces a wide variety of woody shrubs.

Tom Saunders, the container nursery manager at Saunders Brothers, said the current demand for healthy boxwood far outstrips the supply. It’s a welcome change from the recession years, but because of the time it takes to grow a salable boxwood, few growers are in a position to take advantage.

“We threw a lot of product away in 2008,” he says. “There was just no demand. But it’s exciting to see it back now.”

The closure of several nurseries has left a hole the surviving growers are trying to fill. Nurseries are ramping up production, but boxwood doesn’t grow overnight.

When customers are unable to find boxwood in their desired size, many end up buying smaller plants. That presents its own problems.

Waverly Farm is a Maryland-based wholesale B&B nursery that has developed a reputation for larger, heftier material.

Although it can be tempting to start selling at smaller grades, “that action would negatively affect our future crop,” says Jessica Ahrweiler, sales and marketing manager for Waverly Farm. “If we succumbed to selling smaller stock now, we would have a few gap years in our future crop.”

At Waverly Farm, the shortage has hit hardest for the mounding types of boxwood, such as ‘Green Velvet,’ ‘Chicagoland Green,’ and ‘Green Mound.’

When customers call the nursery asking for those cultivars, Ahrweiler redirects them to newer hybrids or lesser-known options that would make a good substitution for the in-demand but unavailable cultivars.

Growers, landscapers and retailers all anticipate the shortage to continue. Peter Mezitt is the president of Weston Nurseries, a Massachusetts company that operates two garden centers that sell to homeowners and landscapers and provides landscape design services. He expects the spring to bring more supply challenges.

“Fulfilling stock after the spring was tricky last year,” Mezitt says. “We were not able to obtain a lot of the 18- to 24-inch and 2- to 2.5-foot sizes that we normally would stock. We did go with smaller sizes than we normally would.”

A Maryland-based design build firm, has the benefit of being able to design with what’s available. Kevin McHale, president of McHale Landscape, says smaller sizes are easier to acquire, but getting your hands on large stock requires expanding your supplier network.

“We are still able to find 24- and 30-inch material, but we need to broaden our search to a wider group of nurseries to find them,” he says.

The blight

The biggest threat to boxwood, and a major contributor to the current shortage, is boxwood blight. Two closely-related fungi, Calonectria pseudonaviculata and C. henricotiae, cause the disease on Buxus, Pachysandra and Sarcococca. The disease was first identified in the U.S. in 2011 and has since been detected in 23 states in both nursery and landscape settings.

Symptoms include leaf spots, rapid defoliation, distinctive black cankers on stems, and severe dieback. Plants become weakened by the fungus and susceptible to other fatal diseases. Fungus spores are sticky and cling to anything exposed to an infected plant, including gardening tools, gloves, shoes, pets and lawnmowers. Spores can last more than 10 years in soil and dropped leaves.

When North Carolina was hit with the blight, a large source of boxwood disappeared. In July 2016, Pennsylvania imposed a “preventative” boxwood quarantine; all boxwood coming into the state must be certified clean of blight. Currently, Maryland is taking steps to create a Boxwood Compliance Agreement. Jerry Faulring, owner of Waverly Farm, has been on the forefront and expects to receive certification this spring.

Ahrweiler believes it is important for growers to comply with this agreement and for other states to adapt similar compliance agreements.

“If the blight is able to continue spreading, it may be that the shortage will continue for a long time,” she says. “However, if proper steps are taken to manage the spread of blight, I believe boxwood numbers can increase again.”

McHale says the blight is a challenge specifically to boxwood in poorly drained shaded areas. Conversely, his crews have had some success in sunny exposures with well-drained soil.

“We have had relative success combining pruning and a systemic soil drench,” he says. “However the pathogen stays in the soil and can be very difficult to control.”

Scientists at work

The disease continues to spread, with new findings in Illinois just this year. But there are reasons for hope for the future. More than $700,000 in federal funding will go to boxwood blight research through the Horticulture Title of the 2017 Farm Bill, an increase from 2016’s $486,000.

“The federal government is taking this issue seriously,” says Jill Calabro, science and research programs director for AmericanHort.

Thanks to this funding, researchers will study diagnostic tools, control and mitigation strategies. Calabro says there is even a special project studying the impact of compost and mulch on boxwood blight infections and spread.

As part of the funded research, Jim LaMondia at the Connecticut Agricultural Experiment Station is studying fungicides with the potential to control boxwood blight. Pyraclostrobin and propiconazole demonstrated good preventive control, and propiconazole shows promise for early curative control. LaMondia’s research also shows most triazole fungicides exhibit preventive control of blight, as does preventive sprays of chlorothalonil.

Marc Cubeta’s lab at North Carolina State University is conducting heat therapy experiments to examine the response of boxwood varieties to hot water exposure during propagation. Several varieties are being screened, and studies are ongoing to determine whether the pathogen can be eliminated from infected cuttings in this manner.

AmericanHort published a best management practices checklist for the disease at bit.ly/boxblightBMP. More resources are available at boxwoodblight.org.

A preventive plan

Tom Saunders believes the extra steps his family nursery has taken to prevent boxwood blight has driven up demand for Saunders-grown boxwood.

“I think demand for our plants is higher because of the protocols we have here for keeping out boxwood blight,” Saunders says. “These things echo to the customer that we take boxwood blight seriously, and we’re going to do everything we can to keep it out of here.”

This includes preventive sprays of chlorothalonil on two-week intervals to protect the crop that makes up one-third of its production. Lindsay Day, propagation manager and assistant grower, estimates that Saunders Brothers propagates about 75 percent of its plants in-house, including flowering shrubs, azaleas, hollies, and of course, boxwood.

While propagating the plants in-house helps the nursery control the variables, it does open the doors to other risks.

“Propagation is the perfect conducive environment for a whole plethora of diseases,” Day says.

Day says disease pressure is usually at its highest during the second week of propagation, the week after the plants have been stuck. She uses a chemical rotation recommendation from Virginia Tech University for boxwood blight, botrytis and other fungal pathogens.

“We use three different ones on boxwood,” she said. “We try to rotate chemical classes, so it’s not so much about product as it is about the different modes of action. Most of the fungicides used for boxwood blight are broad-spectrum and knock out everything.”

Growers looking to develop their own system should rely on extension agents and their local university. Saunders Brothers developed its system after attending several talks from experts like Ann Chase of Chase Agricultural Consulting and Margery Daughtrey, senior extension associate with Long Island Horticultural Research & Extension Center, and working closely with Virginia Tech University’s plant and disease clinic.

In addition to those propagation measures, the Virginia grower added several production and shipping protocols to minimize the chance of getting, or spreading, blight. Severely restricting traffic in boxwood production areas was the first step.

“We don’t allow plants to come in from outside,” Saunders says. “We’re afraid we’ll bring the disease in from another nursery. We’ve eliminated customer traffic. Even buyers or researchers have to wear TyVek pants and boot covers just in case they’re carrying the [pathogen] on them.”

Saunders Brothers also limited its digs to twice a week, instead of five times a week. That change lessened the number of times staff enters each field and limits the chance that the disease spreads. All container boxwood orders are pulled only once daily. This minimizes the number of times Saunders personnel enter boxwood production houses.

While all of these measures have helped keep the nursery blight-free, in the long-term, Saunders believes the key to stopping boxwood blight will be the development and usage of resistant cultivars.

“There are going to be some varieties that we – the industry – just need to quit growing,” Saunders says. “It’s a problem that we’re all going to have to live with one day, but for the time being we can put up some walls to keep it out as long as we can.”

The 2012 research done by Cal-Poly San Luis Obispo’s Kelly Ivors, while she was working at North Carolina State University, is still the most-cited work on variety susceptibility. B. sempervirens types were more susceptible in general, according to Ivors’ work, with B. sempervirens ‘Suffruticosa’ (English boxwood) and B. sempervirens ‘American’ (common or American boxwood) especially susceptible.

By contrast, the microphylla, insularis, harlandii, and koreana types are the most resistant to the disease. The "Greens" (Velvet, Mountain, Mound, Gem) are in the middle. However, the architecture of the plant and growing environment are also playing major roles. For instance, taller, more open semperviren like ‘Dee Runk’ or ‘Fastigiata’ do well, while very tightly sheared microphylla grown close to the ground become more susceptible.

“If a nurseryman looks at the natural structure of an unpruned plant, the more dense the plant, usually the more susceptible it is to the disease,” Saunders explains. “In pruning, we make a denser plant. If a plant is left natural, and is more open, it will be less prone to blight.”

Quest for alternatives

The most commonly used boxwood cultivars are chosen for good reason; they are already landscape-proven. Ahrweiler sees a silver lining to the shortage – giving growers a chance to explore cultivars that were previously overlooked.

“When buyers cannot find the big name cultivars anywhere, they are more inclined to try new ones,” she says.

Peter Mezitt at Weston Nurseries has been adapting to the shortage. When sourcing plants, he’s avoided the ‘Suffruticosa’ boxwood and brought in more koreana including ‘Justin Brouwers,’ a type with a similar dwarf habit to ‘Suffruticosa.’ He also tries to minimize risk by purchasing only from growers that have propagated their own boxwood from cuttings.

Mezitt has also encouraged his customers to try several alternatives that work well in the Northeast, like Ilex x crenata hollies.

“We sold a lot of ‘Steeds’ holly last year as an alternative to the pyramidal shaped ‘Green Mountain’ boxwood,” he says. “Other plants that can take some shade include Ilex x Meserveae hollies such as Blue Maid or Blue Princess. Other than that, there are not a lot of substitutes for boxwood to fill the need for smaller size, more formal plants that can tolerate some shade. The other alternatives are also not as deer resistant, which is very important for many customers in our area. Yews are another choice for a smaller growing evergreen shrub that can tolerate shade, but they are the most prone to deer damage.”

Instead of the big-name mounding varieties that are out of stock, Ahrweiler recommends growers try to push more upright cultivars like ‘Northern Charm’ or ‘North Star,’ which have excellent disease resistance and maintain a low, dense, thick hedge with little pruning. She says the upright B. sempervirens such as ‘Pyramidalis,’ ‘Dee Runk,’ and ‘Fastigiata’ have shown good disease resistance as well. ‘Dee Runk’ was one of the first upright cultivars and quickly became the go-to upright cultivar everyone knows and trusts, she adds. At this January’s MANTS show, Ahrweiler tried an experiment with these three boxwood cultivars to show the importance of name recognition.

“I lined up one of each, all the same height, and labeled them: ‘Can you ID me?’ No one was able to ID them correctly, and the majority of people thought they were all the same plant,” Ahrweiler says. “Even I could barely tell them apart after lining them up side by side.”

Her experiment illustrates how easy it would be to substitute ‘Pyramidalis’ or ‘Fastigiata’ for ‘Dee Runk.’

Plant breeding presents a more long-term solution. Saunders Brothers plans to introduce new varieties to the marketplace that are resistant to blight and leaf miner. As a member of the Syn-RG partnership of growers, new plant introducers and retailers, expect extensive trials before an introduction (Editor’s note: See Sept. 2016 issue of NM, bit.ly/Handpicked4You).

“Building the numbers and trialing takes time,” he says. “I know everybody is yearning for something new, but until it’s proven and we’re comfortable, we’re going to just hold on to it.”