A closer look at components

art: © Vadzim | Adobe stock

Greenhouse Management editor Kelli Rodda spoke with Ryan Dickson, assistant professor of greenhouse and controlled-environment agriculture at the University of Arkansas, about ongoing research into sawdust as a growing media component for greenhouse crops. The investigation is in its early stages, and Dickson says there’s still a lot to learn. But the initial findings are positive. He’s collaborating with greenhouse growers in the Southeast who are sourcing sawdust locally. The crops being studied include bedding plants and poinsettias. We’ll bring you more about the research as Dickson and his team complete certain phases.

Greenhouse Management: What was the precipice for starting this research?

Ryan Dickson: Although the use of sawdust as a substrate component is not new, it’s had a recent resurgence because of a few factors. One is price. Growers are looking for cheaper substrate options and sawdust can be very inexpensive compared to more traditional materials. There have also been some recent shortages and quality issues with peat moss.

GM: Which growing media components can sawdust potentially replace?

RD: In some instances, sawdust could partially replace common materials like perlite, peat moss, coir and bark. The type of material and amount which can be replaced by sawdust will depend on the sawdust particle sizes as well as chemical and biological stability of the of the wood.

GM: What are some production or plant health issues related to the use of sawdust that you're currently observing?

RD: One of the main issues related to sawdust as a substrate is potential to reduce plant growth. I should also mention there are a number of crops which appear to tolerate wood products in the root zone quite well. However, in certain scenarios, it is clear the sawdust is causing a nutrient deficiency and stunting growth, which can sometimes be easily corrected. Other times, the symptoms are more subtle and hidden. For example, experienced growers look at a crop and know that it should be further along than it is. Sometimes the top growth can look just fine, like its leaf color and flowering, for example, but the canopy is a little bit smaller than you’d normally expect. But if you turn the pot over and look at the roots, you see poor root development and roots not extending all the way down to the bottom. Wood products hold less plant-available water than peat moss. That impacts irrigation —you have to irrigate more frequently and in smaller amounts, basically.

We know sawdust needs to be "aged" for a certain amount of time, to improve its quality before use. The aging process is important in terms of stabilizing the sawdust and mitigating issues like nutrient tie-up, wettability and the leaching of wood extractives (e.g. fresh wood toxins) into the root zone. And the way you age the sawdust affects the stability and cost of the material. For example, the cheapest option is to simply stockpile sawdust and let it rest for several months. Periodic turning, irrigating and other management practices may speed up the aging process, but add cost and complexity — in a way you are now getting into the substrate manufacturing business.

GM: Which crops are better suited for using sawdust as a growing media component? Which ones are more sensitive?

RD: Based on our experience, poinsettia, calibrachoa and petunia appear to be a little more sensitive to wood in the root zone. We are still learning why these crops are sensitive. We’ve seen fewer impacts of wood on crops like verbena, impatients and marigolds. However, another grower may have experienced something entirely different, just because of the variability in wood products, the aging process and production practices between grower locations.

But there's a caveat here. The sensitivity of the crops also depends on the quality of the sawdust, and that can change over time. One of the big issues with sawdust is that it's a non-manufactured material, it's a byproduct. So there are a lot of factors that are out of the grower’s control. For example, the age of the tree, if there are other tree species getting mixed in with the sawdust, the particle sizes that were generated during the processing of the wood, and the potential for release of phytotoxins from wood, which can impact the sensitivity. One year, you can have really good results and the next year can be entirely different. We're trying to evaluate what we can do both on the front end and the back end, before and after planting, to improve the consistency of these kinds of variable materials. I want to emphasize again that growers are having success with this product, but as always there is room for improvement.

GM: What’s the typical proportion of sawdust in a growing media formulation?

RD: The proportion of sawdust we've seen is typically 30% or less, but this depends on the grower and the crop. It is best to start with a low amount and trial on a small scale at first.

GM: What’s the difference in the crop(s) as the sawdust component increases from 10% up to 30%?

RD: There’s a potential increase in the wood immobilizing nitrogen and other nutrients like phosphorus and sulfur — basically limiting the availability of those nutrients for plants. It can also tend to increase pH a little bit. If you've got high pH-sensitive crops, they might be some of the first to show issues with higher levels of wood. As I mentioned before, there is this potential for wood products to leach phytotoxins into the root zone, which may include preservatives used in the lumber industry, various organic compounds such as phenols and terpenes found naturally in wood, and potentially toxic amounts of micronutrient metals such as manganese.

GM: Is sawdust a regional product?

RD: Yellow pine tree species, such as loblolly pine, are commonly grown for different forest industries throughout the Southeast. Loblolly pine is often considered one of the more suitable pine species for use in substrates. If you go to the Northeast or different parts of the country, you may encounter more white and red pine tree species, as well as some hardwood species. In the Pacific Northwest you might have spruce, for example. And so, those materials are still available, but the tree species and the type of tree will be different.