Bark and wood: reunited at last. For decades we have relied on pine bark (or fir bark on the West Coast) substrates to produce most of our container-grown outdoor nursery crops. For a variety of reasons, the use of wood materials as components in bark and peat substrates has increased over the past 15 years. As we discuss more about bark and wood as substrate materials, let’s first discuss some terms associated with different materials regarding substrates. Bark, the outside protective layer of trees, is different structurally and chemically than sapwood (functioning wood closest to the bark) and heartwood (nonfunctioning wood in the center of trees) as illustrated in Figure 1. Bark, a by-product of the forestry industry, has been the backbone of nursery substrates for decades and it remains as the primary material of choice. The inner portion of a tree, the wood, is often referred to even more specifically as “white wood” when describing the brightly colored pieces that can be seen in substrates. Many folks have gone to great efforts to limit the amount of white wood present in bark mixes as a result of growers refusing bark shipments upon delivery or not being pleased when some wood can be seen in bark inventories (Fig. 2A-C). Bark processors/producers across the U.S. work tirelessly to get as much of the white wood as possible out during the processing, aging and handling of the bark substrates, but to remove it all is close to impossible. Some processors have invested in specialized extraction devices to remove strips of inner bark (cambium), limbs, larger chunks of wood or other foreign debris (Fig. 2D) while other processors even have hand-picking stations where larger pieces of white wood are removed.

White wood and fertility

We have been taught to believe that fresh wood, even in small quantities, can cause fertility issues and rapidly decompose in containers leading to poor crop growth. The basis for this caution stems from decades of evidence that when incorporated into mineral/field soils, wood will rob nitrogen from the surrounding soil as it is being decomposed. One big difference in field production and container production with soilless substrates is the amount, type and frequency of fertility that is applied to container crops and the ability growers have to compensate additional nitrogen if/when needed. There is little scientific evidence to suggest that a small percent of white wood in bark substrates will do anything to negatively affect fertility and crop growth. What we have learned over the course of many years of research on bark and wood substrates is that perception is not always reality (maybe rarely so) regarding white wood in bark being bad. Bottom line, the presence of white particles in bark should not be concerning to growers as it relates to substrate fertility, toxicity or decomposition. If anything, the white wood found in bark may have an effect on the physical properties of the overall mix which could present some issues. If, for example, large pieces of white wood are present in bark substrates (Fig. 2C) this could cause water to channel through the container and greatly reduce the container water holding capacity.

In 2013, researchers at North Carolina State University conducted a survey of pine bark substrates from suppliers across North Carolina. A total of 24 products were analyzed for physical, chemical and hydrological properties and of those 12 were identified as the most common products used by nursery growers. After analyzing the physical properties of those 12 samples, the white wood was removed (handpicked with tweezers) and the volume and mass of wood from each sample was determined (Fig. 3A). The physical properties of those 12 wood-free bark samples were analyzed again to determine if any changes had occurred in the physical properties. What we discovered is that the percent (volume) of white wood found in those 12 commercial bark substrates ranged from 22% to 2.5%. We concluded that the main effect seen on physical properties (air and water) was not correlated to the percent wood in the bark but instead it was the size and shape of the wood particles that made the biggest difference in substrate properties. Figure 3B shows the difference in wood particle sizes that were extracted from some of the bark samples. A closer look at six of those bark samples we analyzed can be seen in Figure 4. Sample 1, which had 22% white wood, showed a significant change in air space and water holding when the wood was removed. Sample 3, which had 14% wood, showed very little change in air and water properties when the wood was removed compared to Sample 4, which had half as much wood (7.5%) yet showed significantly higher reduction in air space (-8.0%) and increase in water holding. Bottom line, the presence of white wood in bark only affects air, water, and drainage properties if the size of the wood is very different than the bark.

Bark:wood mixes

All of the white wood discussed so far is wood that is unintentionally added to pine bark substrates. Now, let’s focus on the practice of adding an engineered wood component to bark mixes as a means of stretching bark supplies, lowering costs or other potential benefits. Researchers in Mississippi first tested the idea of growing nursery crops in bark and fresh wood combinations in the early 1980s, but the research never changed industry practice. Fast forward to 2004 and the concept was brought back to life as researchers began an aggressive campaign to investigate the potential of incorporating hammer-milled pine wood in bark and peat-based substrates. Since then, we have a better understanding of how to properly use wood substrates with bark, with peat, or even alone at 100%. There are several commercial wood products on the market produced in a variety of ways. The greenhouse industry has been more aggressive in adopting new mixes that contain wood products due to the significant cost savings when substituting substrate components like perlite. For the nursery industry, aged pine bark remains abundant and cheap so the switch to wood has not been as advantageous. But that has not stopped some growers from wanting to adopt bark:wood mixes nor have researchers stopped investigating how best wood can be used in nursery production now and even more so in the future.

Understanding pH

Recent trials have evaluated bark:wood blends as it relates to crafting specific substrates to have unique air and water properties suitable for large containers or potential for fruit crop production in containers. We have also been working to better understand and predict how the pH of bark and wood blends can be adjusted and maintained. We conducted pH trials in the summer of 2019 at the NCSU Horticultural Substrates Laboratory. Our objective was to evaluate the pre-plant substrate pH modification of aged pine bark when blended with hammer-milled pine tree substrate (PTS) at varying percentages and at increasing rates of dolomitic limestone additions. To produce the PTS, logs of freshly harvested 12-year-old de-limbed loblolly pine (Pinus taeda L.) trees were shredded in a wood shredder before being further processed in a hammer mill. Two commercial wood fiber products, one disc-refined and one extruded material were also evaluated in this trial. All three wood products were blended with aged pine bark at ratios of 20%, 40%, 60%, 80%, and 100% for a total of 15 substrate blends. Each blend was wetted to 60% moisture content and subsamples were amended with 100 mesh dolomitic limestone at rates of 0, 4, 8 and 12 pounds/yard3. Samples were incubated in a controlled climate chamber and sampled for pH determination on 0, 1, 3, 5, 7, 10, 14, 21 and 28 days after lime addition. In this article we will only present and discuss data from the PTS blends (Fig. 5).

The pH of 100% pine bark was the lowest, while the pH of 100% PTS with no lime addition started at 5.8 and increased over the four-week study to 6.2 (Fig. 6). What is not shown are all lime rates in the 100% pine bark. The PTS treatment with 12 pounds of lime had its pH rise to almost 8.0 by the end of the incubated study. Overall, the pH trends were as expected in that the pH increased as the percent PTS increased (and conversely the percent pine bark decreased). The addition of PTS to pine bark even without lime may establish a pre-plant substrate pH range suitable for some crops. The high pH of fresh pine wood can be attributed to several factors including tree species, age of the tree, location of wood within the tree, site location and soil type trees are grown on and season of tree harvest. The pH comes from the acidic nature of the tree components, in particular the organic acids (acetic, formic, oxalic) and polyphenols present in the wood. Over the past 16 years the pH of fresh PTS has been reported to range from 4.4 to 6.0, a range mostly attributed to the season of tree harvest and age of the tree. It should be noted that the PTS used in this trial was fresh. If the PTS was aged, the pH may change and therefore would change the bark mix pH as well once amended. It is also unknown how different bark sources or supplies may alter the initial pH prior to or immediately after potting or the effect of that fertilizer, irrigation water or plants have on pH shift during crop production.

As a side note, the addition of wood products (PTS or commercial materials) has been noted many times to have an effect on rooting and rootball development of numerous woody and herbaceous crop species (Fig. 7). Research trials aimed at studying and quantifying root growth and development in Horhizotrons have shown that as wood percentage increases, so does the speed of root growth of some propagation material (cuttings) and larger plant material being stepped up to larger containers (Fig. 7B). Much is left to learn about this phenomenon but it appears to be linked to the humidity of the substrate environment that wood fiber materials create as well as the physical nature of the thin wood fibers providing ease-of-passage of roots through the substrate to the container wall.

Remember that some percent of unintentional white wood in pine bark substrates does not pose negative production challenges. And the intentional addition of engineered/processed pine wood to bark offers a range of physical properties that could be crafted to certain crops or container sizes. Keep in mind that substrate pH is naturally higher with wood than bark, so if mixes are made using ratios of bark and wood do not assume that “normal or traditional” lime rates are needed or acceptable.

Brian Jackson is an Associate Professor and Director of the Horticultural Substrates Laboratory at NC State University. Brian can be reached at Brian_Jackson@ncsu.edu.

The ever increasing and seemingly unstoppable population of the spotted lanternfly (SLF) (Lycorma delicatula) is significantly embedded in Delaware, Maryland, New York, New Jersey, Pennsylvania and Virginia. While many authorities make an almost continual reference to tree of heaven (Ailanthus altissima) as the preferred food plant for this insect, I would submit an adjustment to the word “preferred.” Field observations in Southeast Pennsylvania shows Ailanthus is common and is a frequent host to the SLF, but the plant is not necessarily plentiful in some areas and population densities can vary from nearly nothing (Bucks County) to extensive (Montgomery, Chester and Delaware counties). However, the spotted lanternfly is quite adaptable in finding new horizons on which to feed. Adaptation has led them to silver maple (Acer saccharinum), which in many quarters is more plentiful than Ailanthus. Oddly, the spotted lanternfly does not rely on other maples and generally leave red maple (Acer rubrum), sugar maple (Acer saccharum) and Japanese maple (Acer palmatum) alone. They do seem to like Acer platanoides, an invasive exotic. SLF has no interest in anything in the Fabaceae family (Cercis, Gleditsia, Gymnocladus and Cladrastis). They have no interest in the Asian flowering cherries nor the Northeast native Prunus serotina. However, the pest does have a strong affinity for things in the Vitaceae family and in lower Southeast Pennsylvania there is a cornucopia of things to go after in that family. It seems almost negligent to discuss Ailanthus and the spotted lanternfly and leave out the members of the grape family. Muscadine grapes (Vitis rotundifolia) reach a northern limit in Virginia but then parade all the way down into central Florida. Vineyard owners tell me that SLF is a major pest in their operations going after wine grapes. American porcelain vine (Ampelopsis cordata), a North American native, is an SLF favorite. But its cousin, the invasive Chinese porcelain vine, (Ampelopsis brevipedunculata) covers extensive areas in the Northeast, Southeast and portions of the Midwest and is the mother lode when it comes to feeding SLF. Casual observation would suggest that porcelain vine is an entrenched, significant component of almost any natural area with varying levels of disturbance. For agricultural authorities to suggest wholesale war on tree of heaven and leave out porcelain vine is woefully insufficient.

Another plant keen on the SLF’s want list is Aralia spinosa, as SLF will flock to the new shoots in the spring while in the instar stage. This native plant occupies a vast area of the Northeast, Southeast and Midwest and is common in the Philadelphia metro area. It is easily spread by seed and is stoloniferous, which forms large colonies, much like tree of heaven. Spotted lanternfly was once dependent on Asian species of plants but has significantly altered its feeding habits to take advantage of some native plants and introduced ornamentals. They like cultivated roses, especially Knock Out Rose, but leave native roses alone, including Rosa virginiana, R.caroliniana and R. nitida. Perhaps this is because those roses have distinct foliar odors that the cultivated roses do not have. SLF instars also have an affinity for the mulberry family (Morus sp). Another clear host is Salix alba ‘Sericea’. It is my observation that aside from porcelain vine, the silver willow is by far the most frequented plant for the instar stage of the SLF. It is not clear if other willows are also on the menu. A rather surprising find is that SLF has preferences for our native black walnuts (Juglans nigra) but not butternut (Juglans cinerea). It is clear however that the insect is adapting fast to what's at hand.

Photos by Bill Barnes

What is abundantly evident is that the SLF instars are looking for plant tissue that is in soft new growth. They prefer new, green, smooth bark and generally have no interest in rough bark or bark that is formidable in any way. They avoid many plants that harbor toxins, except for Styrax japonicus which has considerable amounts of saponins in their tissues. Perhaps this is not by chance. The instars change color from black with white spots to bright red with white spots. As adults they are very colorful with mixtures of red, blue, white, black and gray. Vivid coloration in insects often is a clue to an inherent toxicity. Perhaps what is driving them to certain plants is the acquisition of the plant poisons that they can ingest and incorporate to protect them from predators. This seems more than anecdotal as assassin bugs will readily approach them only to be repelled after reaching a close encounter. Cicada killer wasps have an interest in SLF adults, and they do kill some of them but not on a grand scale. This indicates that potential predators might be limited possibly because they are so vastly outnumbered.

Spotted lanternfly is a hitchhiker with considerable evidence of its egg laying abilities on smooth metal equipment such as the under carriages of trucks, buses, automobiles and, more than likely, railroad equipment. The extent of this is probably greatly underestimated. Amtrak regularly travels from Philadelphia to points west. While waiting for an Amtrak train, one such volunteer traveler flew into my wife's purse as she was headed to Pittsburgh. Had I not spotted it and killed it; this hitchhiker would have easily taken a ride to Pittsburgh. This was in the fall and it could have easily laid eggs and set up shop in Pittsburgh. This capability of the insect to utilize rapid transit of all kinds makes it virtually unstoppable.

The case for control

Are insecticides a possible solution? Maybe for a localized occurrence, but as a practical matter they are largely ineffective — except in agricultural situations. There are too many insects in too many areas not reachable by pesticides, nor is there money available to go after the insect in such a wholesale fashion. There are limits to modeling as a tool considering how widespread the insect is. Areas that are not cultivated nor accessible by any control measures such as egg laying on the underside of roof overhangs, natural areas, hedgerows and state and local parks act as reservoirs for continual repopulation. Killing Ailanthus is a reasonable proposal, but not practical because it will be an almost impossible task and will have almost no effect on the eventual spread of the insect. Killing off Ailanthus for SLF control will only increase the propensity to find new plant species to utilize. If the other plant hosts, such as Ampelopsis, Morus and Salix, are not controlled as well, there is little hope of success in controlling the SLF.

Where is the money to combat this pest and who is going to pay for it? Along the I-476 corridor between the Mid County interchange (I-276) and I-95 are thousands of Ailanthus along a distance of 15 miles through Montgomery, Chester and Delaware counties. Moving south to Delaware and east to New Jersey there are more likely tens of thousands to hundreds of thousands of these trees, along with tens of thousands of acres of porcelain vine which often occur in conjunction to the tree of heaven. The extent of control by any means is a small percentage of what is really out there. Due to propensity for the egg laying on smooth metal surfaces it is impossible to monitor all the railroad cars, trucks, buses, private trailers and other vehicles.

Money spent on non-effective measures should be redirected. The only recourse is to hit this insect with a disease, be it viral or bacterial. Perhaps Bacillus thuringiensisis a possibility. An introduced predator could well cause collateral damage on native insects. Emerald ash borer (Agrilus planipennis) will eventually burn itself out because its food supply of native ash will disappear, and it has not adapted to anything else. But spotted lanternfly does not hesitate to alter its feeding habits and it is here to stay. Germ warfare is the preferred option for control. All the other measures are essentially wasting time and money and will not be effective now or ever.

About the author: Bill Barnes is a horticultural consultant and has been in the nursery business for 45 years. He’s an active member of the International Plant Propagators Society, an IPPS Fellow and received the IPPS Award of Merit. Bill also teaches at the Barnes Arboretum at St. Joseph University in Philadelphia. He introduced Cercidiphyllum japonicum Tidal Wave and Morus alba Green Wave. bill@barnhortservices.com

Opinions expressed are those of the author and do not necessarily represent the views of GIE Media, Inc.

Whether it’s on a dusty softball field, in a corporate boardroom or on a gravel-lined nursery plot, Lyndsi Oestmann remains consistently devoted to the task at hand. Her purposeful and sincere work ethic was forged when she was a child, watching her dad play professional baseball. It was reinforced as she played competitive softball, persisted into her college studies and eventually became etched in her career. Lyndsi owns and operates Loma Vista Nursery in Ottawa, Kansas, a business that was founded by her father, Mark Clear in 1991.

Lyndsi spent nearly the first decade of her life traveling to ballparks to watch her father play. From some of her first memories, he instilled in her not just the importance of hard work, but the necessity of it in every aspect of life.

“Dad always taught me that if I wanted something, I had to work hard for it,” says Lyndsi. “It’s something he’s always done, too.”

When Lyndsi started playing softball at age 12, Mark told her a lesson he learned from a former coach, “You can expect to play like you practice.” It’s a lesson she’s never forgotten and a standard that continues to drive her.

“We’re a very competitive family, and when dad was my coach, we always practiced like it was the ninth inning of the World Series,” she says. “It’s the same in business. You need to always do your best, not just when we’re pulling orders for the customer that will scrutinize the plants the most. We want to do that every time, every day.”

Sports has certainly influenced Lyndsi’s outlook and how she manages employees and the business. When she started softball, she was playing 150 games a year. Mark noticed her natural leadership qualities with the team.

“I was her coach and probably harder on her than most of the other kids,” he says. “Instead of moping or pouting about it, she looked on the positive side and worked until she got to be one of the better players in the area. She always dug in, no matter what it was, and she was always the team leader. It’s no different now as she’s running the nursery.”

Mark and the family moved from California to Olathe, Kansas, and started the nursery when Lyndsi was 10. Lyndsi and her brother performed several duties at the nursery and even planted the first crop of trees. Lyndsi enjoyed working at the nursery, but she had different career aspirations. She attended Kansas State and received a marketing and international business degree with a minor in Spanish and planned to travel the world. After graduation, she landed a job at PepsiCo Inc. and got a taste of corporate America. She dove in, learning all she could about managing people and process improvement. While at PepsiCo, she learned about accountability and had the good fortune of being cross trained within several departments.

She wasn’t traveling the world as originally planned, but unbeknownst to her, Lyndsi was soaking up valuable lessons that she’d eventually take back to the family business.

Mark bought property in Ottawa to expand Loma Vista and he needed someone to run the Olathe operation, which had been converted into a landscape distribution center. He asked Lyndsi to return home and manage the distribution division.

“I really liked being part of corporate America, but when dad offered me this opportunity, I thought, ‘I can do this because I’ve had this great training and I’ve been managing people at Pepsi and learning how to manage processes,’” she says.

Lyndsi brought a fresh perspective when she returned to the nursery.

“Dad and I make an awesome team because his passion for the business has always been on the production side, and I came in and immediately focused on sales, customer service and marketing,” she says.

Executing a game plan

As she gained a foothold in the distribution operation at age 23, she looked to some of her landscape contractor customers as mentors.

“I tried to learn some of their best practices and see how we could apply those to our own operation. I looked at companies with really strong cultures and ones that had good employee retention,” she says.

She asked customers what they needed from the Loma Vista team, had face-to-face meetings with customers, conducted focus groups and sent out surveys.

“She came in and found areas that needed improvement and executed those changes. She did it by building a good team. She’s got the team concept down pat,” Mark says.

Four years ago, Loma Vista sold the distribution division to SiteOne, one of the largest landscape distribution companies in the nation. The deal essentially reduced the size of the company by half, which gave Lyndsi the opportunity to work on the production side of the business.

“It was a great move for our company,” she says.

Now the company was solely focused on growing and there were some staff changes, which presented another chance for Lyndsi to learn new skills.

Her first lesson was in approaching change. The company’s controller was leaving, and Lyndsi took over the finance and accounting duties of that position.

“I was ready to change everything. We were doing a good job, but I felt like we could be so much more efficient,” she says. “As I started talking to one of our key team members about all of these changes, they had one of those deer-in-the-headlights looks. I tended to approach change like just grab the bull by the horns and just do it. But I’ve learned that I need to slow down a bit and respect how others react to change. It’s important to have lots of communication during changes, really overcommunication in this case, to have complete transparency and buy-in. Because nothing works without buy-in.”

Providing for people

Mark taught Lyndsi a lesson that she never forgot and one that resonates with every decision she makes about the company.

“Dad has always said that people are the most important asset of a company,” she says.

Investing in the Loma Vista employees has been her top priority. Applying some of her experiences from PepsiCo, nursery team members are cross trained to understand how each department affects the others. There’s accountability throughout the system.

“It used to be that one person knew how to do everything, and if that person wasn’t at work, there’d be a lot of people standing around not knowing what to do. So, we developed a playbook for our company with strong documentation of our processes and a formalized planning system,” she says.

Another vital part of training at Loma Vista is taking part in nursery conferences and visiting other nurseries.

“I want everyone on our production team to have lots of contacts in the industry,” she says. “If they’re facing a challenge, they can pick up the phone and call on a fellow grower. Or they can go on a nursery tour and bring back an idea for us to execute. Making those contacts and going to those events has totally changed the dynamic of our team.”

Lyndsi values every person that’s related to the nursery, from each employee and their families to the customers and the community.

“My mission in business is to help the company become the best it can be,” she says. “The company is a lot bigger than just a nursery and its employees. The health of a lot of families relies on Loma Vista, and I take that really seriously.”

That blends impeccably with her mission in life.

“I want to always strive to be the best version of myself, to act with purpose and outwardly express people’s value to everyone I encounter,” she says.

Caitlin Hupp, a territory sales manager at Loma Vista, says she admires Lyndsi’s ability to see the potential in people and help guide them to their best role.

“There’s so much about Lyndsi that makes her a great leader. She’s very conscious of everyone having a work/life balance and she gets to know the families of everyone here,” Caitlin says. “I admire that she’s worked in every role in the company, so she can speak from experience no matter the job. She doesn’t hire square pegs for square peg roles. She sees the potential in people, lets them experience the company culture and helps find a good fit for them.”

Reacting with purpose

As the nursery was navigating the Great Recession, Lyndsi made a profound observation during a budget meeting with her dad, an operations manager and the accounting manager.

“I said, ‘Hey, we’re four people and we’re trying to move the direction of 150 people. We need everyone in the company to hear these numbers. We need everyone to understand what’s going on so the four of us aren’t spinning our wheels.’”

The company became totally transparent and began sharing all financial information in company meetings, including how the nursery is performing compared to budget and how it’s doing compared to industry benchmarks.

“It has made a huge difference. Having buy-in from everyone — not just your top managers, but your middle managers, supervisors and down to the individual level — it’s difficult to get 100% buy-in, but we’re working on it,” she says. “One person, or in our case four people, can’t make the company successful. It is the effort of every single person here because every person is here for a reason.”

The employees appreciate the transparency.

“When Lyndsi implemented the open-book mentality with our budget and finances, that let us know she believed in our capabilities and our ability to problem solve,” Caitlin says. “She gives us the power to get involved and make decisions about the business and the direction we’re going, and to provide solutions to obstacles.”

During the last recession, Lyndsi also realized the nursery’s customer base needed more diversity.

“We were deeply tied to the commercial construction market, so we took a hit during that time,” she says. “We knew right away that we had to put some measures in place, which took a lot of time and planning, to diversify our customer base. We put a lot of work into our product mix, our production timing, how we tag products, how we deliver products and how our sales team operates. I feel a lot more comfortable having a diverse customer base as we face another recession.”

With the recent COVID-19 concerns, Lyndsi realized the importance of being nimble, which also means having a well-developed plan in place.

“We work off of really solid plans — plan A, B, C, all the way to Z if needed. You have to be able to make changes in a timely matter and plan for the things that you can control,” she says. “In the past I have spent a lot of sleepless nights worrying about things that are out of my control. That’s futile. Now we plan for possible scenarios and execute the plan based on variables we can control.”

Loma Vista’s customers appreciate the team’s focused planning and the calculated reactions.

“My company started in 1991, the same year as Loma Vista, and we were their first credit customer,” says Marty Seiler, principal of Epic Landscape Productions in Olathe, Kansas. “Lyndsi has worked hard to lead Loma Vista and take it a step forward. Sometimes in family businesses, the second generation rides the coattails of the first generation. That is not the case with Loma Vista. She’s very professional, well studied and very knowledgeable of the industry. There’s a high level of confidence from father to daughter.

“She demonstrates a passion and love for the industry. She’s personable and easy to talk to. As we’ve gone through the recent COVID-19 issues, we’ve been able to work through some of those issues together.”

© Ruud Morijn | Adobe Stock

Boxwood is a workhorse you can always count on: Tough, adaptable, evergreen, deer-resistant and beautiful. Sheared or shaggy and available in many sizes and forms, there are few gardens where boxwood doesn’t fit right in. No wonder U.S. boxwood sales are around $126 million annually, a significant source of revenue for growers and nurseries.

But the plant’s fate has come into question since boxwood blight reached the U.S. in 2011. The disease has now spread to 28 states, posing a significant threat to established boxwood plantings in landscapes and historical gardens as well as nursery inventory. Researchers and growers have quickly responded to better understand this pathogen, determine the susceptibility of various boxwood varieties already on the market and develop more resistant cultivars.

A new paper, “Ranking Resistance of Buxus Cultivars to Boxwood Blight — An Integrated Analysis,” was published in the June 2020 Journal of Environmental Horticulture. The paper documents a study conducted by researchers from the U.S. Department of Agriculture, Agricultural Research Services and Rutgers University who assembled data from prior studies as well as new data to create the most comprehensive, statistically consistent ranking to date of Buxus cultivars to boxwood blight.

A comprehensive study

Researchers used 289 samples from the National Boxwood Collection at the U.S. National Arboretum, one of the most complete collections of boxwood in the world with 700 varieties.

“The National Arboretum is one of the primary groups involved in breeding resistant boxwood, we’ve been screening for resistance for several years,” says Dr. Margaret Pooler, a supervisory research geneticist with the U.S. National Arboretum and USDA’s Agricultural Research Service.

From those initial 289 samples, the researchers chose the 65 most resistant varieties for further study and statistically merged them with data of prior studies of boxwood resistance.

“Instead of publishing yet another disease resistance comparison, we decided to compare our data to that of other studies and put them together to create a bigger study that compiles all the data. We undertook this study because there were several groups who had completed studies to screen for resistance among cultivars but the results of these studies were inconsistent. So we took a statistical approach to tease out what was causing this variability. We looked at all the studies we could find that had data, or we obtained unpublished data from other researchers to conduct a meta-analysis. We wanted to figure out why the results were different and compiled the prior research with our own, so all the data is summarized in one place,” she says.

Chuanxue (Chuan) Hong, professor of plant pathology at Virginia Tech, who summarized some of the unpublished data for the study says, “I think it’s a very important paper because it provides information that can help growers and those in the nursery industry supply less susceptible boxwood plants to their customers. If we can lower the risk of boxwood blight by using more resistant cultivars and practicing good plant maintenance, everyone will have better success. The nursery industry will have fewer returns and everyone downstream will be more successful, and this has a national impact in reducing boxwood blight. Boxwood is important to the American landscape, it is irreplaceable. It’s important to carry the boxwood heritage forward.”

Key findings

The report provides a susceptibility estimate for 131 varieties of boxwood. While no variety showed complete resistance to boxwood blight, some were less susceptible to the disease than others. The ten least susceptible boxwood listed in the study are:

• B. sinica var. insularis 60705 (Korean littleleaf boxwood)
• B. microphylla ‘Little Missy’
• B. microphylla var. japonica ‘Winter Gem’
• B. SB17 (parent plant of NewGen Freedom)
• B. microphylla ‘Compacta’
• B. microphylla var. japonica ‘Green Beauty’
• B. sempervirens 43877
• B. microphylla ‘Northern Emerald’
• B. microphylla var. japonica 4227
• B. sinica var. insularis ‘Wee Willie’

The full article, including the ranking of 131 boxwood varieties, is available at http://bit.ly/boxwood-ranking.

“Our key findings are that there is a lot of variability among boxwood types to the blight and there is nothing that is truly resistant, at least among the plants we have in the U.S. The different plants we looked at have varying degrees of susceptibility. So right now, everything that we know of will get blight if you hit it hard enough with the pathogen,” Pooler says.

Bennett Saunders, general manager of Saunders Genetics, a boxwood grower that is evaluating boxwood varieties for blight, says, “The sempervirens cultivars are very susceptible and we have to get away from those cultivars. Generally, the microphyllas are the most resistant to blight.”

In reviewing this new paper, he says, “The Sheridan series of boxwood are scattered in the middle as far as boxwood blight.” These were developed at Sheridan Nurseries in Ontario in the 1950s and are the crosses resulting in B. sinica var. insularis and include the varieties ‘Green Gem’, ‘Green Velvet’, ‘Green Mountain’ and others. Sheridan cultivars are the most-grown boxwood in the country, and ‘Green Velvet’ might be the most grown boxwood in the U.S. and it has moderate susceptibility to blight.”

The study did not evaluate how a plant’s form or architecture factors into blight susceptibility. Saunders provides his field observations on this aspect: “If a plant is more upright and open it tends to be more resistant because of better airflow. In most cases the disease originates at the bottom of the plant due to water splashing spores onto the plant and then the disease has to grow up through the plant. One upright sempervirens that we’ve seen a little less blight on is ‘Dee Runk.’ The disease seems to have trouble climbing that plant but under intense pressure it will succumb.”

A way forward

Will a truly blight-resistant boxwood ever be developed? That remains unclear for a number of reasons, Saunders says.

“There are landscapes full of susceptible cultivars carrying the disease,” he says.

But there are options to reduce favorable blight conditions.

“If we reduce the numbers of susceptible cultivars in our gardens, replant with more resistant cultivars and use best management practices, we will minimize the chances of ever seeing blight. Maybe the plants don’t have to be totally resistant if we diligently follow best management practices, some of which are very basic, such as mulching boxwood. Chuan Hong has determined that mulching can reduce blight lesions by as much as 96%,” he says.

Saunders notes that more testing is warranted since “some of the plants in the study only had one test done on them and those might benefit from having more testing done.”

Hong says the green industry will benefit from additional trials.

“Those in the industry should push for further evaluations, not only of newer cultivars since the last studies were done, but also of how the most resistant cultivars from this paper perform in different locations around the country. This additional data will further substantiate the existing studies on susceptibility or resistance of the cultivars and help determine whether the cultivars perform differently in different environments,” Hong says.

Pooler expects the evaluations to continue.

“We definitely will evaluate new cultivars that have come out since this study. We don’t know what a truly resistant boxwood looks like. That’s what we would call a negative control and would be one extreme of our study,” she says. “We have plenty of plants to use at the susceptible end but having resistant cultivars as the gold standard; that is still pretty hard to come by. We’re waiting for what comes out from breeders in the U.S. and Europe to see how they stand up to strains of the pathogen. Our number-one objective is to breed new plants and we actively seek out and obtain new cultivars.”

The U.S. National Arboretum has made hundreds of crosses and have thousands of plants in poly houses, Pooler says.

“Some look promising in terms of ornamental characteristics. It’s a matter of getting them tested in the lab and to see how they do in the landscape. We’re doing genetic studies of resistance that will open doors to anyone who wants to breed new cultivars if we can get a handle on the genetics,” she adds.

A collaborative research project has also been established to combat boxwood blight under a Specialty Crop Research Initiative (SCRI) grant from the U.S. Department of Agriculture.

“This project is a collaboration between all the key players in government, industry and academia. It includes pathologists, breeders and even economists, so it’s taking a multi-prong approach,” Pooler says.

“It’s an exciting project,” says Pooler. “We’re hoping it will help us find some breakthroughs in finding resistant boxwood cultivars.”

Martha Swiss is a garden writer and designer in Pittsburgh, Pennsylvania; https://plantsomejoy.com