Testing different lighting setups in your greenhouse has its advantages, but there are also some factors to be aware of in order to ensure accurate data collection and observations.
With just over two-thirds of commercial greenhouse managers surveyed indicating plans to replace, expand or invest in new supplemental lighting technologies in the next three years (67%), it naturally begs the question: How do you evaluate the many different options on the market today?
The University of New Hampshire Cooperative Extension recommends that growers set up small-scale trials throughout greenhouses to evaluate which species and cultivars are more profitable under supplemental lighting.
AJ Both, extension specialist at Rutgers University, has been undertaking advanced horticultural research projects since the late 1980s. His advice for commercial growers looking to conduct their own research on plant response to different types of lighting systems is simple Find an expert.
“It can be helpful to consult with an expert who understands plant lighting, because plant lighting is quite different from lighting applications for human environments,” he says.
“I think that conducting trials make a lot of sense when specific benefits are expected,” Both adds, noting that “there is some published research data that can be helpful, but the number of plant species and their cultivars is large.
“Chances are, conducting trials is the only way to determine what the optimum light environment is for a specific plant species,” he says.
Choose your sensor wisely.
Both notes that it is crucially important for those conducting supplemental lighting research to recognize the fact that different plants and cultivars respond quite differently than our own eyes to various sources of light.
Therefore, one cannot simply rely on human observations. Finding the right sensor to capture the right data set is key in any supplemental lighting trial.
“Don’t use a foot candle or lux meter, but instead use a quantum sensor,” he says. “A quantum sensor produces a reading that informs about the intensity of light that plants can use for photosynthesis.”
Once you zero in on the right sensor suite for your operation, a good place to start is using the sensors to obtain a solid understanding of the level of uniformity of light distribution over the entire growing area, Both says.
“Researchers typically achieve this through so-called light maps, which measure the distribution of light across a horizontal plane located some distance below the fixtures,” he says. “Measurements taken are then distributed across the measurement plane using a uniform grid, and simple statistical tools can then be used to calculate light uniformity across the entire area.”
Know your ideal spectrum.
Different plant cultivars respond independently to different spectrums of light. Some prefer red and blue spectrums during vegetative growth, while some need a healthy dose of infrared during flowering.
“If it is important to know something about the spectrum, or the color distribution, of light in the greenhouse, you can use a spectroradiometer,” Both says, noting the use of these instruments is not entirely common in commercial greenhouse applications.
“Ask for assistance from local researchers, or even from a lighting manufacturer who has experience in this area,” he says.
Identify variance across your canopy.
One of the most tangible benefits of conducting your own supplemental lighting trials is the ability to collect data in the same environment where the system will eventually be placed. This is often more valuable than research that takes place in a clinical setting where conditions are often more controlled than in a typical commercial greenhouse.
“When collecting these light readings, it is important to understand that in most cases light measurements will exhibit both spatial and temporal variation,” Both says. “In other words, time and location of the measurements taken are important.”
In order to diminish the influence of variation during the trialing phase, light measurements should be conducted over longer time periods (days, weeks, months) so as to reduce, or “smooth out” instantaneous variations, according to Both.
“As much as possible, sensors should be located where there is minimal shading taking place from the plant canopy, or even from stationary or moving overhead equipment like spray booms,” he says.
Both also notes that, when measurements are being conducted over long periods of time, sensor location adjustments are usually a necessary evil to ensure the data being collected is free of shadows and other obstructions throughout the process.
“This is also necessary when the canopy starts growing out and covering a sensor,” he adds.
Another issue that can pop up when trialing supplement lighting systems is the physical location of the experiment itself. Because the number of light sensors used during these evaluations is “typically small,” Both advises that “it becomes important to consider a location that is representative of a larger growing area.”
10 tips for closing down a greenhouse for the winter
Departments - Tech Solutions
A downturn in the economy could mean more empty greenhouses dotting the countryside this winter. Here’s some advice on making sure your investment stays safe and sound until spring season.
In snow country, a few well-positioned posts under the ridge will help to support a heavy snowload.
Photos courtesy of John W. Bartok Jr.
It’s a tough reality to face after a long period of upheaval: the economic effect of the COVID-19 virus may unfortunately force you to shutter some greenhouses this winter.
The impact of severe weather and the prolonged exposure to the elements can cause significant damage to a closed greenhouse, most of which are designed by code to maintain a 50° F minimum temperature.
Here are some pointers to help minimize damage to the structure and equipment:
Covering shutters and vents will reduce infiltration and keep the wind out
Visually inspect the exterior of the greenhouse to see that it will keep the snow and rain out. Replace cracked or broken glass. Tape tears and seal leaks in the poly cover. Check to see that the plastic is attached securely and that any holes are taped. On air inflated greenhouses, increase the inflation pressure slightly when snow or strong winds are predicted by opening the blower’s intake valve. This will reduce the rippling effect. Weatherstrip and fasten doors so they can’t be opened by the wind.
To support wind and snow loads, check that the frame is secure. Tighten truss connections, frame bolts, straps and bracing. In heavy snow country, add a row of wood 2 x 4’s under the ridge on hoophouses to support heavy wet snow that would normally melt and slide off when the house is heated.
Decide whether to provide minimum heat or shut the heating system down. If you have a boiler system, it may be better to keep the heat on and set the thermostat at 35° F. This eliminates the need for draining the system and blowing out the pipes. It may also prevent seals, gaskets and valves from drying out. A plastic shelter over propane and fuel oil tanks can provide protection, or adding methanol to oil will keep the fuel more viscous. For hot air furnaces typically used in hoophouses, they can be shut down without problems. Turn off the oil supply valve.
Check vents and shutters, ensuring they are closed tight. Inspect roof vents and roll-up sidewalls for leaks or gaps that would allow wind and snow to enter. Fan shutters should be covered with a sheet of plastic or insulation board. Cover evaporative cooling pads and drain tanks, piping and valves.
In southern climates, a closed greenhouse may overheat on a bright, sunny winter day. This may be fine for reducing insects and diseases, but it can also damage plastic piping, deteriorate shade material or warp plastic benches. Vents or fans may have to be operated on these days.
Energy/shade screens should be open. The sun will warm the greenhouse and soil on clear days and keep the frost penetration to a minimum. It will also provide protection to the screen material to reduce deterioration. If you are running heat then close the screen (unless snow is predicted).
Electric power and controls may still be needed when the greenhouse is buttoned up. The power and smoke/fire alarms should be checked frequently to insure they are working. Heat may be needed to melt and remove a heavy snow load.
Remove pesticides, fertilizer and other chemicals that could be affected by cold temperature. Check Safety Data Sheets (SDS) for storage requirements. Many chemicals have minimum/maximum storage temperatures and light exposure requirements.
Provide rodent control. Mice, chipmunks and squirrels may enjoy the mild climate of a vacant greenhouse. Set traps and baits.
Get ready for Spring. The winter is a good time to do needed maintenance. Remove all debris left from the previous crop, dispose of all used containers and powerwash benches. Pick a warm day during slack time to service and repair equipment. Check operation of heating and cooling equipment, calibrate controls, tighten up doors and vents and make needed structural repairs.
John is an agricultural engineer, an emeritus extension professor at the University of Connecticut and a regular contributor to Greenhouse Management. He is an author, consultant and certified technical service provider doing greenhouse energy audits for USDA grant programs in New England. jbartok@rcn.com
Growing plants & people
2020 Horticultural Industry Leadership Awards - Advertorial
At Millcreek Gardens, George Pealer, along with his late wife Lynda, spent a career growing top-quality plants and trying to empower those around them to reach their potential.
Fred Higginbotham III, growing operations manager, left, George Pealer, owner and general manager, Doug Heindel, sales, business development, and marketing manager, and Megan Armstrong, assistant general manager and business office manager
Growing up with his two brothers in Mount Vernon, Ohio, near Columbus, gardening was a chance for George Pealer to spend time with his dad.
“It wasn’t a huge garden, but that wasn’t the point,” George says. “He spent a few hours a week out there and he let me tag along.” When he got to high school, the Pealers moved to Bexley — a larger Columbus suburb — where George worked at Connell’s Flowers. At the time, it was one of the largest florists in Ohio.
“It was a bit of shock, but it was probably the best thing for me, moving away from a small town,” he says. “Connell’s had a greenhouse and a plant retail business, so I was able to work in the greenhouses and help out in the flower shop and receive flower shipments. It really got me interested in growing flowers.”
Today, George runs Millcreek Gardens, an annual, perennial and herb operation in Ostrander, Ohio — within driving distance of where he grew up. George founded the business in 1978 with his late wife Lynda.
In his career, George has served on the board of directors of the Ohio Nursery and Landscape Association and as the president of the Perennial Plant Association while Millcreek became one of the first nurseries in the Ohio Valley region to sell herbs on a wholesale basis. Today, he is still at the business every day, trying to help it grow and help every employee succeed.
“He’s at the forefront,” says Fred Higginbotham, Millcreek’s growing operations manager. “He will just come up and say, ‘Tell me where I can pitch in; tell me where I can help out.’ No job is too big or too small for George. People see that commitment he has.”
The defining relationship
In George’s last quarter at Ohio State, he met his future wife, Lynda, in a greenhouse management course. They started dating soon after.
“She was a very strong woman,” George says. “She went to Purdue University and had a degree in microbiology, and when she graduated, she worked at a medical center in Indianapolis doing cancer research. She realized after a year or two that she didn’t want to spend her life in a lab. She had been married, had children and got divorced.” George says Lynda came to Columbus with an interest in horticulture and knew Ohio State was a good opportunity for her to begin doing the kind of work she wanted. Ultimately, she got a horticulture degree from Ohio State.
The two married in 1977 and founded Millcreek Gardens a year later in February. Before starting the business, the two traveled together, visiting operations such as White Flower Farm, a nursery in Connecticut, and Gilbertie’s Organics, an herb farm in Connecticut, and thought the operations set a blueprint for them to found a business together.
At the beginning, George returned to work at Connell’s for three more years as the business got going. At Millcreek, they combined two passions — George’s for perennials and Lynda’s for herbs — into one combined vision. They settled down in Ostrander, located just outside of Columbus, and for a long time, Lynda grew the herbs day in and day out. When she took a step back from growing, she still helped behind the scenes.
“The thing that always impressed me about Lynda was that she loved herbs, and loved to cook with them, but couldn’t find them anywhere,” George says. “But we saw them [on our trip] and she thought it would be great to do them here. She was really a pioneer in our area for growing herbs in pots like you see now. It’s such an integral part of our company now. People know us for our perennials and our herbs.”
Lynda died on Christmas Eve in 2018, leaving behind George, six children and 12 grandchildren. She made an impact on everyone she met.
“She had extremely high standards,” says Nathan Pealer, George and Lynda’s son, who works in real estate. “She pushed everybody to be better. And since she had such high standards, everybody tried their best around her, be it her family or someone at work. She held herself to those same high standards, too.”
“She was only here for a couple of years when I started here,” says Higginbotham, “But I’d never seen a man have as much love for his wife as George had for her.”.
A potrait of George and his late wife, Lynda
Helping others grow
Higginbotham first visited the company in the early 2000s during an open house, recalling thathe was “blown away” by the facility even back then. After interning at Millcreek one summer and graduating from Ohio State the next spring, he joined the company in 2005. He started out as an assistant grower, became a head grower, and then was finally promoted to his current role as growing operations manager about five years ago.
From the time he first visited Millcreek, Higginbotham saw that George went out of his way to help him however he could.
“Throughout the years, the thing I can stay about George is that he’s the nicest guy ever,” Higginbotham says. “He treats everybody from seasonal employees to somebody who’s been here for 25 years exactly the same, always with a smile on his face. It’s one thing that sets George apart.” Higginbotham says that it is not uncommon for George to hop in and help with shipping, putting stickers on pots or bringing out cold Gatorades for the workers in the greenhouse.
Higginbotham adds that the culture George has created is the major reason he has stayed at the business and cannot imagine himself leaving any time soon.
“It’s about the people,” he says. “We have people that have been here 20, 25 years plus. Like any good organization, the good starts at the top and works its way down. And while George is very involved, he lets a lot of us on the management team have the freedom to do what we do and do our jobs. I’ve always appreciated that there’s a lot of trust involved here.”
Another employee George helped empower is Megan Armstrong, the company’s assistant general manager and business office manager. For the first part of her career, Armstrong was a grower working with gallon-size perennials and, in 2004, was named the Perennial Plant Association’s Young Professional of the Year. In 2012, she was promoted to assistant general manager, taking on responsibilities outside of growing like budgeting, staffing and overall company management.
According to Armstrong, it was a change she wanted, and one that George encouraged her to seek out.
“One of the biggest things is the trust George places in people,” she says. “He may challenge you for an idea, like developing a new product line for the slow season, but he’s not going to pigeonhole you. He wants your input, wants your ideas and is willing to go for it.”
Both Armstrong and Higginbotham both say that, amid the coronavirus pandemic, George has been essential in keeping the company connected while also prioritizing employee safety while working and trying to keep business as normal as possible.
For George, at the end of the day, empowering employees is part of the ethos he and Lynda set out to create when they founded Millcreek. To him, along with quality and profitability, values like integrity, leadership and teamwork are part of Millcreek’s DNA. Ultimately, a significant part of his legacy is helping people find their passion, just as his dad, his Ohio State professors, and his first employer did for him.
“Our mission statement is ‘growing high quality plants, people and relationships,’” George says. “For us, that says it all.”
How to properly estimate lighting costs
Lighting Report - Supplemental Lighting
Estimating lighting costs, and knowing how much light to use, are key to turning a profit when using supplemental lighting.
For growers considering supplemental lighting, cost is a major factor to consider, said Dr. Marc van Iersel, a professor at the University of Georgia and co-founder of Candidus, a supplemental lighting control company.
Recently, he hosted a webinar with GLASE titled, ‘Lighting approaches to maximize profits.’ It is available in full here.
Determining cost
GLASE, in coordination with the Cornell College of Agriculture and science, created a lighting cost calculator for supplemental lighting need.
“What this calculator does is show how much supplemental light you need over the course of a year in order to make sure your crop receives 17 mols of light. Seventeen mols is a common amount for growing lettuce,” van Iersel said in the webinar.
Using lettuce production as an example, the calculator outlines two different scenarios for growers depending on if they are growing in a poly greenhouse with an estimated 62% light transmittance rate or in a glass greenhouse with an estimated 70% transmittance rate. While the color-coded maps — which detail how much supplemental light is needed each year for a specific area — are similar, there are differences in certain parts of the U.S., including Texas, Oklahoma, Georgia and South Carolina.
He added that growers in glass greenhouses need less supplemental light because they receive more natural sunlight due to the higher transmittance rate.
“Once you know how much supplemental light you need to provide and you know how efficient your lighting system is, plus the cost of electricity, it becomes really easy to calculate cost,” van Iersel said.
The amount of mols needed does vary by crop. Tomatoes, for instance, have a DLI target of 25 mols. As a result, the map looks different even with the same transmittance rate.
“Tomatoes require more light,” van Iersel said. “It’s a lot more light as compared to lettuce.”
In the coming months, van Iersel said an online calculator for growers to determine costs for their specific zip code, crop and lighting type will become available. Neil Mattson, a researcher at Cornell, has also released a tool to help growers determine their lamp seeds. It is available at hortlamp.com
According to Marc van Iersel, a professor at the University of Georgia, light transmittance rates differ between poly and glass greenhouses.
According to van Iersel, the DLI has been used in greenhouse production for roughly 30 years and is primarily used for lighting recommendations.
“I think the daily light integral and using recommendations based on it is really useful,” he said. “But one thing we’ve been able to show is not all daily light integrals are calculated equally. How you provide that light over the course of the day also matters.”
Van Iersel said this is because of natural light variation. The amount of light and its variations depending largely on a geographic region.
In the webinar, van Iersel compared Elmira, New York, and Phoenix, Arizona. Elmira’s varied greatly due to the changing weather based on the season. In the winter towards the end of the year, for instance, much less light (as measured in mols) was created naturally. So, a grower in Elmira would need more supplemental light. By comparison, Phoenix still saw a dip in natural light towards the end of the year. However, because Phoenix does not have the same winter climate as Elmira, it doesn’t drop off nearly as much. It also varied much less day-to-day over the course of a year.
“In Phoenix, because it is typically sunny all year, there is just must less variation,” he said. “But you still have less light in the winter.”
For growers, that means understanding how the daily light integral is applied in their specific region. In the webinar, van Iersel showed a rudbeckia study where the same DLI – in this case, 12 mols – was applied over four different time frames (12 hours, 15 hours, 18 hours and 21 hours) vs. a control group with no supplemental light. The results showed, generally speaking, that plant growth improved as the photoperiod expanded.
“What we found is that as we go from a 12-hour photo period to a 21-hour photo period, we got a 33% increase in above-ground growth,” he said. “We also got a 22% increase in root dry weight.”
As for profit, van Iersel said the key is to use this data and find the right balance of spending on lighting in relation to pricing.
Take-home message
One other note van Iersel offered is that current research suggests that stability is best when applying light. That means it is better to apply the same amount of light vs. a variation during a growing cycle. An overlooked tool in helping that is using different greenhouse control systems to keep light levels consistent.
He also said it is essential to work with a lighting company that can help estimate various costs and develop a specific plan.
“If you’re working with a company that doesn’t have those resources available, you’re probably working with a lighting company that doesn’t know what they are doing,” van Iersel said.
Methods for mitigating toxicity in fresh wood substrates
Features - Growing Media
This is the second article in a two-part series highlighting the potential occurrence of wood substrate-induced phytotoxicity as well as methods for mitigating those risks.
Fig.1. Crops can be successfully grown in 100% hammer-milled wood substrates once the wood toxins are mitigated and if sufficient fertility and irrigation regimes are supplied.
Photos Courtesy of Brian E. Jackson
Reduced plant growth in fresh wood-based substrates has been attributed to several factors including fertility, irrigation, and most commonly toxins (phytotoxicity).
Evidence of this condition has been published and observed for decades when growing in substrates that contain certain types and percentage of fresh (non-composted) and untreated wood materials.
The wood chemicals (secondary metabolites) responsible for inhibiting seed germination and stunting herbaceous plant root and shoot growth, are vital to the health and defense of trees while standing, but they need to be removed or rendered harmless before being used as a substrate to grow plants. These wood chemicals, known as extractives, are a group of compounds (either volatile or soluble) that vary in type and concentration depending on tree species and origin (region of growth).
Extractives are found in higher concentrations in older trees (thicker bark and more heartwood) than in younger trees (thinner bark) and the amounts in any given tree can fluctuate with different seasons of the year. Additionally, the amount of potentially harmful extractives is higher in tree bark and heartwood compared to softwood.
With these as well as other factors being known, tree species, tree age, harvest season, and the presence of bark should be considered when selecting or acquiring wood feedstocks to produce substrate materials.
Fig. 2. Common machines used to make wood substrate products include hammer mills (A), single or twin-screw extruders (B), and twin-disc refiners (not shown) which produce varying amounts of heat (C-D).
Strategies for removal
Wood extractives, which research has shown to be less harmful to woody plants (rooted nursery liners for example) compared to herbaceous plants (seeds, plugs, transplants), can be successfully removed or mitigated by one of several strategies.
Better yet, once toxins are removed, growing quality plants in 100% wood substrates is possible as seen in Fig. 1.
Note that there are many variables to each of the following procedures that cannot be fully addressed in this article, but more information will be released in the future as research continues.
Fig. 3: Aging of harvested pine logs (A), chipped logs (B), processed wood substrate outdoors in piles (C), or processed wood substrate in totes (D) are all potential methods of mitigating fresh/green wood toxins.
Wood processing method
After feedstock selection, the type and method of processing that wood chips undergo is a critical step in removing harmful extractives.
There is roughly a half dozen commercial machine types used (globally) to produce wood substrate products but three are most common: hammer mills, single or twin-screw extruders, and twin disc refiners. Each of these technologies function differently to reduce/separate wood chips into fibers or smaller wood chip particles.
Each of the methods also differ in their initial set-up cost, operating costs, annual maintenance costs, the range of size and moisture content of wood chips they are able to process, and ability to produce different particle sizes and shapes.
The primary features of the different machines that are responsible for the reduction or elimination of harmful wood extractives are heat generation, pressure, friction, and/or dry or wet processing. Heat and pressure (together or separate) are known to drive off (volatilize) wood extractives into the air. Exposure to a water or chemical solution bath for some period of residency time has also been shown to effectively solubilize certain extractives from wood chips.
Either by one, or both of these factors, wood chips can be refined to smaller particle sizes suitable for substrates and at the same time have their inherent chemicals removed in the process. Let’s take a look how the three machine types differ.
Hammer mills, the cheapest and most common of the methods, are used universally for processing many types of organic materials. Hammer mills operate by pulverizing particles with swinging hammers (or knives) until the material is reduced in size enough to pass through a perforated screen (Fig. 2A).
Wood chips typically need to be dry (under 40% moisture content) when processed in a hammer mill to prevent clogging. During the operation of the mill there is some heat generated, but the amount and extent is mostly proven negligible in significantly volatilizing wood chemicals. There is, however, some minimal chemical volatilization occurring as a direct result of the reduced (smaller) particle sizes being exposed to the air and dried during processing.
The heat that is generated in a mill depends on the moisture of the wood chips being processed, rpms and operational speed of the machine, and time of year of processing. Twin-screw extruders (Fig. 2B) have opposing screws that apply mechanical energy to wood chips which crushes, shears, and squeezes them to separate the wood fibers. During the process the pressure can rise to 500 kilopascals (~75 pounds per square inch – PSI) and the temperature can reach 210 – 230 °F (100 – 110 °C). After passing through the screws, a sudden “relaxation” of friction and pressure creates a steam explosion which results in further tearing of the wood fibers.
The friction heat and pressure effectively volatilize many wood extractives, thus mitigating the phytotoxicity of the end product (substrate) even when used immediately (Fig. 2C-D).
The twin-disc refiner is a large disc mill operating at high speed and pressure (+/- 4000 kilopascals = ~580 PSI) with internal temperatures reaching 220 – 400 °F (110 – 200 °C). This technique often has a water bath phase where wood chips are pre-soaked in a pressurized heated vessel for several minutes before being forced through the rotating discs. In this scenario, there is a solubilization and volatilization phase of the process which are effective at removing harmful wood extractives.
Lastly, the practice of co-refining wood chips with some percentage or peat, bark, or other material has been shown to be effective in further reducing (by dilution) wood phytotoxicity in the substrate material produced. Co-refining wood with other materials can be done with any of the three machine types mentioned above.
Fig. 4: Drying wood products after processing (A), binding toxins with charcoal (B), soaking or steaming (C), or leaching containers (D) are all methods of mitigating toxins prior to or during crop production.
Photos Courtesy of Brian E. Jackson
Substrate preconditioning
These methods are aimed at hammer-mill processed wood products as a result of the lack of sufficient heat/pressure generated in the mill.
Extruded or disc-refined commercial wood substrates have not been shown to need additional corrective measures to reduce toxins.
For hammer-milled wood substrates, aging has been shown to be the best and most effective method of reducing green wood toxins. Aging of wood can occur at different stages including the storage of harvested trees for a period of months prior to processing (Fig. 3A), aging of wood chips (under shelter preferred; Fig. 3B), aging of processed wood substrate outdoors or under shelter (Fig. 3C), or processed substrate aged in totes (Fig. 3D).
Aging, often called “seasoning”, of wood chips in piles for two weeks has been shown to be as effective in reducing toxins as aging whole logs for six months. The season of the year that aging occurs also will affect the duration needed to effectively mitigate the wood toxins due to temperature and rainfall fluctuations.
Aging of processed substrate in totes has received the most research over the past decade with the general recommendation that 4-6 weeks be allowed for the process. Longer periods of time do not negatively affect the quality of the end product.
However, aging large piles of fresh pine wood (sawdust or processed fiber) similar to pine bark substrates has not been perfected at this time and seems to be less consistent, more time consuming, and more variable than other forms of aging.
Preliminary research suggests that processed fresh pine wood substrates can also be dried at high temperatures very quickly to reduce (volatilize) harmful chemicals (Fig. 4A), although the exact temperature and duration needed are not yet fully understood.
Industrial dryers/drying can add an additional cost to the substrates, but based on how cheap hammer milled substrates can be produced, this added expense still makes this a viable economic option.
Other practices that have been explored with varying levels of effectiveness include the addition of charcoal, brown coal/lignite, or pulverized biochar products to wood substrates to bind the toxins, preventing them from affecting plant growth (Fig. 4B). Substrate steaming (pasteurization) in soil carts (Fig. 4C) and substrate washing or soaking in cold and hot water have also been investigated but the results are varied and inconsistent.
Additional strategies explored in reducing wood toxicity include the inclusion of peat, bark, or coconut coir (non-toxic substrate materials) to wood, thus reducing the wood percentage in the final substrate (Fig. 5A). This has been the most practiced strategy over the past decade since hammer milled pine substrates were first investigated (2004 – 2008) to be used at 100%. Since then, the most common wood product inclusion rate has been 20–40%, thus reducing the threat of green wood toxins. Research continues on this topic and is needed before more hammer milled wood products are used reliably and consistently.
Fig. 5: Blending wood with peat (A), increasing fertility (B), reducing wood volume and increasing leaching/drainage (C) are some production strategies tested to grow plants in 100% wood (D).
Photos Courtesy of Brian E. Jackson
Production practices
If fresh hammer milled wood materials (or pine sawdust, though not recommended at this time) are used in substrates without any preconditioning, there are practices that could help reduce plant stunting and other growth defects.
The most proven effective way to reduce wood toxins in containers is to leach them with water. Leaching containers with clear water frequently for a week (or as often as possible without imposing more detriment to the plants) has been shown to be effective in removing the toxins from the container (Fig. 4D).
Leaching of substrate-filled containers with over-head irrigation prior to planting has also been successful for some growers. Experiments have been conducted on various container sizes, leaching frequencies, drainage rates, and limited or stratified wood layers and volumes (Fig. 5C) to better understand how to solve production problems with wood substrates if they occur. Secondary to leaching, doses of high fertility have in some cases been shown to overcome phytotoxicity of young herbaceous crops including germinated seedlings (Fig. 5B).
However, there are conflicting reports as to the role that fertilizer can possibly play in mitigating toxins, as it may be the leaching of the applied fertigation that is actually reducing the toxins. A combination of leaching and increased fertility can mitigate any negative growth, allowing for healthy plants to be produced in substrates with very high wood percentages (Fig. 5D).
As we continue to learn more about practical and cost-effective methods of reliably removing toxins from fresh hammer milled wood, as well as some sawdust and other wood industry by-products, it is likely that these cheap materials will be used more frequently as substrate components. As a result, these developments will drive the increased production of these cheaper wood products across the US (and in parts of Europe) which should keep costs low as a result of limited transportation distance and manufacturer/supplier competition.
And in the meantime, it should be noted that there are several commercial wood fiber products on the market that are toxin-free and available for immediate use.
The author is an Associate Professor and Director of the Horticultural Substrates Laboratory at NC State University. Jackson can be reached at Brian_Jackson@ncsu.edu.
John W. Bartok Jr.
Chris Manning
